The Solar Dynamics Observatory by malj


									Solar Dynamics
“…to understand the nature and source
of the solar variations that affect
life and society.”

                         Report of the Science Definition Team
      Solar Dynamics Observatory Science Definition Team

David Hathaway              John W. Harvey                 K. D. Leka
Chairman                    National Solar Observatory     Colorado Research Division
Code SD50                   P.O. Box 26732                 Northwest Research Assoc.
NASA/MSFC                   Tucson, AZ 85726               3380 Mitchell Lane
Huntsville, AL 35812                                       Boulder, CO 80301

Spiro Antiochos             Donald M. Hassler              David Rust
Code 7675                   Southwest Research Institute   Applied Physics Laboratory
Naval Research Laboratory   1050 Walnut St., Suite 426     Johns Hopkins University
Washington, DC 20375        Boulder, Colorado 80302        Laurel, MD 20723

Thomas Bogdan               J. Todd Hoeksema               Philip Scherrer
High Altitude Observatory   Code S                         HEPL Annex B211
P. O. Box 3000              NASA/Headquarters              Stanford University
Boulder, CO 80307           Washington, DC 20546           Stanford, CA 94305

Joseph Davila               Jeffrey Kuhn                   Rainer Schwenn
Code 682                    Institute for Astronomy        Max-Planck-Institut für Aeronomie
NASA/GSFC                   University of Hawaii           Max Planck Str. 2
Greenbelt, MD 20771         2680 Woodlawn Drive            Katlenburg-Lindau
                            Honolulu, HI 96822             D37191 GERMANY

Kenneth Dere                Barry LaBonte                  Leonard Strachan
Code 4163                   Institute for Astronomy        Harvard-Smithsonian
Naval Research Laboratory   University of Hawaii           Center for Astrophysics
Washington, DC 20375        2680 Woodlawn Drive            60 Garden Street
                            Honolulu, HI 96822             Cambridge, MA 02138

Bernhard Fleck              Judith Lean                    Alan Title
ESA Space Science Dept.     Code 7673L                     Lockheed Martin Corp.
c/o NASA/GSFC               Naval Research Laboratory      3251 Hanover Street
Code 682.3                  Washington, DC 20375           Palo Alto, CA 94304
Greenbelt, MD 20771

Richard Harrison            John Leibacher                 Roger Ulrich
CCLRC                       National Solar Observatory     Department of Astronomy
Chilton, Didcot             P.O. Box 26732                 UCLA, 9831 MSB
Oxfordshire OX11 0QX        Tucson, AZ 85726               Los Angeles, CA 90024

                            Barbara Thompson
                            Project Scientist
                            Code 682
                            Greenbelt, MD 20771
                                                      Table of Contents
1      Executive Summary .................................................................................................... 1
2      Overview ............................................................................................................................ 3
3      Current Scientific Understanding and Outstanding Questions ......... 8
    3.1     Solar Influences on Global Change and Space Weather ................................................ 8
       3.1.1     Irradiance Variations ............................................................................................... 8
       3.1.2     Energetic Particles from Flares and CMEs ........................................................... 13
       3.1.3     Coronal Structure and Solar Wind Variations ...................................................... 17
    3.2     Mechanisms of Solar Variability .................................................................................. 20
       3.2.1     The Solar Cycle..................................................................................................... 21
       3.2.2     Active Region Evolution....................................................................................... 24
       3.2.3     Small-Scale Magnetic Structure ........................................................................... 27
4      Required Observations ........................................................................................... 31
    4.1     Helioseismic Images ..................................................................................................... 31
    4.2     Longitudinal Magnetograms ......................................................................................... 32
    4.3     Atmospheric Images ..................................................................................................... 33
    4.4     EUV Spectral Irradiance ............................................................................................... 34
    4.5     Photometric Images ...................................................................................................... 35
    4.6     Vector Magnetograms ................................................................................................... 36
    4.7     UV/EUV Spectra .......................................................................................................... 37
    4.8     Coronagraphic Images .................................................................................................. 38
    4.9     Total Irradiance ............................................................................................................. 39
    4.10 Coronal Spectroscopy ................................................................................................... 40
    4.11 Heliometry .................................................................................................................... 41
5      Potential Instruments and Allocation of Resources .............................. 41
6      Mission Concept ......................................................................................................... 42
    6.1     Orbit Selection .............................................................................................................. 43
    6.2     Attitude Control System ............................................................................................... 44
    6.3     Data and Communication System ................................................................................. 45
    6.4     Spacecraft Power .......................................................................................................... 46
    6.5     Instrument Module ........................................................................................................ 46
    6.6     Ground System.............................................................................................................. 46
    6.7     Mission and Science Operations ................................................................................... 47
7      Concurrent Observations ...................................................................................... 48
    7.1     STEREO ....................................................................................................................... 48
    7.2     Solar-B .......................................................................................................................... 49
    7.3     Solar Probe .................................................................................................................... 49
    7.4     SORCE.......................................................................................................................... 50
    7.5     GOES/NPOESS ............................................................................................................ 51
    7.6     SOLIS ........................................................................................................................... 52
    7.7     ATST............................................................................................................................. 52
    7.8     Solar Sentinels .............................................................................................................. 53
    7.9     Solar Orbiter.................................................................................................................. 53
    7.10 FASR............................................................................................................................. 54
8      Acknowledgements ................................................................................................... 54
1    Executive Summary                                  advance in modeling, quantifying, and perhaps
                                                        eventually predicting solar variability over the
The Solar Dynamics Observatory (SDO) is a               relevant timescales. Variable solar outputs nec-
cornerstone mission within the Living With a            essarily impact parallel efforts aimed at under-
Star (LWS) program. SDO‟s mission is to un-             standing, modeling, and predicting the behavior
derstand the nature and source of the solar va-         of the Sun-Earth system. Accordingly, we also
riability that affects life and society. As such, its   speak to the scientific issues and problems that
principal functions are two-fold. First, it must        arise from inadequate knowledge of solar radia-
make accurate measurements of those solar pa-           tive, particulate, and magnetic plasma outputs.
rameters that are necessary to provide a deeper
physical understanding of the mechanisms that           Together, the solar and terrestrial science ques-
underlie the Sun‟s variability on timescales            tions and ongoing research activities point to
ranging from seconds to centuries. Second,              specific observables that must be supplied by
through remote sensing, it must monitor and             SDO for the successful operation of the LWS
record those aspects of the Sun‟s variable radia-       science program. This document identifies a set
tive, particulate, and magnetic plasma outputs          of required observables that best addresses the
that have the greatest impact on the terrestrial        dual objectives of SDO. In so doing, it provides
environment and the surrounding heliosphere.            the basic arguments and the supporting evidence
                                                        that leads to the identification of the essential
Our Sun is an active star. This activity impacts        observations. The nature of the required mea-
planet Earth and human society in numerous              surements in turn drives the choice of a geosyn-
ways. Terrestrial climate, ozone concentrations         chronous (GEO) orbit for the spacecraft. A po-
in the stratosphere, and atmospheric drag on sa-        tential suite of instruments is described which is
tellites all respond to variations in the Sun‟s ra-     in principle capable of acquiring the required
diative output. Astronauts, airline passengers,         observables, while at the same time satisfying
and satellite electronics are all imperiled by the      the logistical constraints imposed on the SDO.
energetic particles produced in solar flares and
coronal mass ejections (CMEs). Electrical power         The overarching science questions to be ad-
to our homes and businesses, communications,            dressed by the SDO are:
and navigation systems can all be interrupted by
geomagnetic storms driven by blasts in the solar           What mechanisms drive the quasi-periodic
wind. SDO will study the mechanisms of solar                11-year cycle of solar activity?
variability – through a broad spectrum of tem-             How is active region magnetic flux synthe-
poral, spatial, and energetic scales – to provide           sized, concentrated, and dispersed across the
the tools and scientific understanding that will            solar surface?
enable us to improve the quality of forecasts of           How does magnetic reconnection on small
solar activity. SDO will also provide the mea-              scales reorganize the large-scale field topol-
surements that are critical as input to studies of          ogy and current systems? How significant is
the geospace environment at their point of origin           it in heating the corona and accelerating the
in the Sun-Earth system in order to quantify the            solar wind?
Sun‟s influence on global change and improve               Where do the observed variations in the
our characterizations and forecasts of space                Sun‟s total and spectral irradiance arise, and
weather.                                                    how do they relate to the magnetic activity
In this report we discuss several key science              What magnetic field configurations lead to
questions that derive from our present incom-               the CMEs, filament eruptions, and flares that
plete knowledge of the physical underpinnings               produce energetic particles and radiation?
of the Sun‟s variability. They are selected for the        Can the structure and dynamics of the solar
promise they hold in catalyzing a significant               wind near Earth be determined from the
    magnetic field configuration and atmospher-      listed below satisfy the data requirements stated
    ic structure near the solar surface?             above while remaining within the financial and
   When will activity occur, and is it possible     logistical limitations placed upon the SDO mis-
    to make accurate and reliable forecasts of       sion.
    space weather and climate?
                                                        Helioseismic/Magnetic Imager
The answers to these pressing science questions         Atmospheric Imaging Array
are to be found in direct observations of the re-       EUV Spectral Irradiance Monitor
levant solar activity and the interpretation of         Coronagraph
these data. To this end, the complement of SDO          Photometric Mapper
instruments should supply the following basic           UV/EUV Spectrometer
data types (ordered from solar interior outward,        Vector Magnetograph
not by priority).
                                                     The first three instruments are of highest priori-
   Full-disk dopplergrams of appropriate spa-       ty. SDO must include instruments such as these
    tial and temporal resolution, duration and       to fulfill its mission. All three provide data with
    continuity to permit accurate helioseismo-       proven value that are not likely to be supplied
    logical inferences of conditions in the solar    through other programs. The final four instru-
    interior.                                        ments are of high priority. The data they obtain
   Full-disk magnetograms capable of charac-        are needed for the SDO mission but they may be
    terizing the surface magnetic field and its      supplied in some other, albeit compromised,
    evolution, and monitoring the emergence          form by other programs, may be more specula-
    and processing of magnetic flux.                 tive in nature, or may tax the SDO resources. A
   Full-disk precise photometric images to ex-      Total Solar Irradiance Monitor is also consi-
    plore the temporal and spatial variability of    dered to be of highest priority, but two TSI
    the solar irradiance and determine couplings     Monitors are expected to fly on other platforms
    with the magnetic structures.                    concurrently with SDO.
   Full-disk filtergrams recorded simultaneous-
    ly in a variety of visible and EUV band-         The remainder of this document amplifies on the
    passes to assess the dynamics and energetics     considerations that went into this selection of
    of the solar atmosphere on global and active     questions, critical data, and instrument comple-
    region scales.                                   ment. It provides the traceability from the scien-
   Sun-as-a-star EUV spectral irradiance mea-       tific questions to the instrument requirements
    surements to monitor and record temporal         and mission design.
    variations of radiative outputs crucial for
    gauging ionospheric, mesospheric and ther-
    mospheric responses to solar forcing.
   Restricted field of view UV/EUV slit spectra
    to make precise diagnoses of plasma dynam-
    ics and energetics.
   White-light polarization brightness images
    of the solar corona to record and monitor co-
    ronal evolution and re-structuring important
    for generating geoeffective interplanetary

To provide a context and a guide for proposals
submitted in response to the forthcoming AO
associated with SDO, we have devised a poten-
tial suite of generic instruments. The instruments

                                                   observations, potential instruments, and a
2 Overview                                         mission design.

The primary goal of SDO is to understand
the nature and source of the solar variations
that affect life and society. This broad goal
leads to two objectives. One objective is to
understand the mechanisms of solar variabil-
ity as characterized by three processes that
operate on three different timescales – the
solar cycle (months to centuries), active re-
gion evolution (hours to months), and small-
scale magnetic element interactions (seconds
to hours). The second objective is to under-
stand the solar influences on global change
and space weather as characterized by three
different sources – irradiance variations,
energetic particles from flares and CMEs,
and plasma disturbances from solar wind
                                                   Figure 2.1. Recent solar cycle variability. Sunspot
structures.                                        number and solar 10.7 cm radio flux (top two panels)
                                                   are well-correlated indicators of solar variability.
With the exception of the slow evolutionary        Solar flares (third panel) and total solar irradiance
changes in solar structure over the last 4.5       (fourth panel) generally follow the solar cycle. Geo-
                                                   magnetic variability (bottom panel) has a component
billion years, all solar variability is magnetic
                                                   in phase with the cycle but also shows considerable
in origin. The solar cycle is a magnetic cycle     activity near solar minimum attributed to the effects
in which the Sun‟s magnetic poles reverse          of high-speed solar wind streams.
with a periodicity of approximately 11 years
and intense magnetic fields erupt through          What mechanisms drive the quasi-
the surface in sunspots whose numbers wax          periodic 11-year cycle of solar activity?
and wane with the cycle. Solar flares and
CMEs occur when magnetic fields are                The solar cycle controls the long-term beha-
stressed beyond their limits. The very struc-      vior of solar activity and the resultant mod-
ture of the corona and the solar wind is de-       ulation of the Sun‟s electromagnetic, parti-
termined by the structure of the magnetic          culate and magnetic plasma emissions that
field. The heating of the Sun‟s corona and         affect the Earth. Solar magnetic fields with
the acceleration of the solar wind are             their associated forces and electric currents
thought to be due to interactions between          are recognized as being responsible for the
small-scale magnetic elements. SDO will            Sun's activity, but the underlying processes
help us to understand the mechanisms of            which create and then dissipate these fields
solar variability by observing how the mag-        in an 11-year cycle are poorly understood.
netic field is generated and structured and        Although helioseismology has revealed
how this stored magnetic energy is released        flows and thermal structures related to the
into the heliosphere and geospace. Our cur-        magnetic variability, present theoretical
rent scientific understanding leads to a series    models based on these observations can only
of outstanding questions that must be ad-          broadly reproduce the observed magnetic
dressed by SDO. These questions lead to            evolution and are far from having predictive

capability. Historical records suggest that       dent as faculae in the photosphere, plage in
the strength of the cyclic magnetic variations    the chromosphere and large loop structures
may have been different from what is ob-          in the corona – alter electromagnetic radia-
served today and that there may have been         tion at all wavelengths. The evolution of ac-
associated terrestrial climate changes. Fur-      tive regions depends upon the structure of
thermore, sun-like stars are observed to have     the emerging magnetic flux, the local flow
a wider range of activity than is seen in the     patterns, and the magnetic connections in
Sun, suggesting that current solar behavior       the solar atmosphere.
could be misleadingly steady.
                                                  SDO will have the capability to study active
SDO will examine the processes that control       regions and their evolution by obtaining
the solar cycle. The SDO Magnetographs            measurements unavailable from other mis-
will measure the structure and evolution of       sions or observatories. The Helioseismo-
the magnetic field itself. The Helioseismo-       graph will measure the local flow patterns
graph will measure the relevant fluid flows       using observations with unique spatial reso-
at levels within the Sun from the surface to      lution and coverage. It will have the ability
the deep interior with unprecedented resolu-      to “see” active regions as they develop on
tion and coverage. By extending the volume        the far side of the Sun and will resolve struc-
of the solar interior accessible to helioseis-    tures just below the surface. It may also be
mic probing and increasing its sensitivity in     able to detect magnetic structures before
the crucial regions, SDO will provide data        they emerge at the surface, a capability
critical for understanding the 11-year activi-    shared with the Photometric Mapper. The
ty cycle. The Photometric Mapper will             Magnetographs and Atmospheric Imaging
measure temperature and radiance variations       Array will provide critical information on
on the solar surface that are associated with     the complexity of the magnetic structures in
the deep-seated drivers of the solar cycle        active regions. The resultant fluctuations
while the EUV Spectral Irradiance Monitor         recorded by the EUV Spectral Irradiance
measures the dramatic variations in the           Monitor will capture the disk-integrated ef-
EUV.                                              fect of the emergence, evolution and rotation
                                                  of all active regions.
How is active region magnetic flux syn-
thesized, concentrated, and then dis-             How does magnetic reconnection of solar
persed across the solar surface?                  magnetic fields on small spatial scales re-
                                                  late to coronal heating, solar wind accele-
The evolution of active regions controls the      ration and the transformation of the
behavior of solar activity on timescales from     large-scale field topology?
hours to months. Sunspots cause decreases
of a few tenths percent in total in solar irra-   The small-scale magnetic elements control
diance. Energetic particles are released          solar activity on short timescales. As these
through magnetic reconnection associated          elements erupt through the photosphere they
with the evolution of active regions. Al-         interact with each other and with larger ex-
though simple isolated regions rarely pro-        tant magnetic structures such as those asso-
duce flares or CMEs, complex sunspot              ciated with active regions. The magneto-
groups with complicated and stressed mag-         dynamic nature of small-scale magnetic flux
netic field elements frequently do produce        may be the basis for short-term solar varia-
eruptive events. Bright active regions – evi-     bility. They may provide the triggers for

eruptive events and their constant interac-       Sun‟s EUV radiation cause dramatic fluctua-
tions may be a key source of coronal heating      tions in the density of the Earth‟s outermost
and solar wind acceleration. They may also        atmospheric layers and the electron density
contribute to irradiance variations in the        in the ionosphere, affecting the control and
form of enhanced network emission.                operation of earth-orbiting spacecraft, and
                                                  communication and navigation systems.
SDO will make critical observations of the        Magnetic features – sunspots, active regions,
small-scale magnetic elements, their interac-     network – that alter the temperature and
tions, and the resulting transformation of the    composition of the solar atmosphere are
large-scale field topology. The Magneto-          primary sources of solar irradiance variabili-
graphs will have sufficient spatial and tem-      ty.
poral resolution and coverage to follow the
evolution of these elements. The Helioseis-       SDO will make direct measurements of the
mograph will determine the nature of the          solar irradiance, map the sources of the irra-
photospheric and sub-photospheric flows           diance variations, and provide observations
that control their motions. The Atmospheric       of the physical characteristics of these
Imaging Array will follow the magnetic            sources. The SDO EUV Spectral Irradiance
connections within the atmosphere by simul-       Monitor will provide the first continuous,
taneously providing images of coronal loops       high time resolution measurements of the
at a series of different temperatures. The        EUV irradiance variations that are critically
EUV Imaging Spectrometer will determine           important for changes in the Earth‟s upper
the physical conditions associated with these     atmosphere and ionosphere. The measure-
features including temperature and bulk ve-       ments will be made on a timescale of
locity. The Coronagraph will provide infor-       seconds while the SDO mission provides
mation on the large-scale field topology and      coverage over the solar cycle. The Photome-
the associated solar wind.                        tric Mapper will provide unique imaging
                                                  capability in bolometric intensity and in vis-
Where do the observed variations in the           ible and IR bandpasses that allow for the
Sun’s total and spectral irradiance arise,        positive identification of the solar sources of
and how do they relate to the magnetic            the total irradiance variations. The Atmos-
activity cycles?                                  pheric Imaging Array will provide coinci-
                                                  dent images of the full solar disk made in
The Sun‟s electromagnetic radiation is the        EUV radiation at selected emission tempera-
primary energy input to the Earth. This radi-     tures so as to identify the sources of the ob-
ation varies at all wavelengths, and on all       served EUV variations. SDO will also pro-
timescales observed thus far. Total solar ir-     vide measurements to aid in our understand-
radiance varied by about 0.1% during recent       ing of the sources of the irradiance varia-
11-year sunspot cycles. While these varia-        tions. The EUV Imaging Spectrometer will
tions are thought to be too small to have a       examine the physical conditions of key fea-
dominant impact on climate, there is never-       tures. Magnetograms will provide informa-
theless considerable evidence in both con-        tion on the magnetic nature of the features
temporary and paleo-climate records of ap-        and their interactions while Helioseismic
parent solar-related variability. Variations in   images provide information on related sub-
the solar UV are larger and have a direct and     surface structures and flows.
significant effect on stratospheric ozone
concentrations. Still larger variations in the

What magnetic field configurations lead          reorganize populations of energetic particles
to the CMEs, filament eruptions, and             in the Earth‟s magnetosphere, radiation
flares that produce energetic particles?         belts, and ionosphere. Storm-induced elec-
                                                 tric currents can flow through power lines,
The Sun emits energetic particles during so-     overpower circuit breakers and transformers,
lar flares and CMEs. The energetic particle      and ultimately disrupt power distribution.
flux in a large proton flare can be lethal to    Sudden changes in the electron density and
an astronaut outside the protective envelope     structure of the ionosphere can hamper
of Earth‟s magnetosphere. Significant par-       global positioning systems and radio com-
ticle fluxes penetrate to aircraft altitudes     munications. During maxima of the solar
where they pose a health risk to passengers      cycle CMEs are the primary contributors to
and crew. Showers of these energetic par-        these solar wind disturbances. Near minima
ticles degrade and destroy electronic com-       the disturbances are dominated by variations
ponents on commercial, military, and re-         in the structure of the solar wind itself. Solar
search satellites. They also cause short-term    wind structures also modulate the flux of
depletions of the ozone layer, especially in     galactic cosmic rays, allowing a larger flux
polar regions. The solar eruptions that pro-     of cosmic rays at the Earth during periods of
duce large fluxes of energetic particles are     lower solar activity. The resultant cosmo-
also magnetic in nature, frequently resulting    genic isotopes – 14C and 10Be – when stored
from the reorganization of magnetic fields in    in tree rings and ice cores produce unique
the outer solar atmosphere.                      archives of long-term solar variability that
                                                 are crucial to untangling solar influences on
SDO will provide continuous and improved         Earth‟s past climate. There is even specula-
observations of the solar conditions that lead   tion that galactic cosmic rays may alter cli-
to eruptive events such as flares and CMEs.      mate by creating cloud condensation nuclei.
Magnetographs will provide images of the
underlying magnetic field. The Atmospheric       SDO will observe the structure and dynam-
Imaging Array will reveal the resulting          ics of both the corona and the source region
structures in the chromosphere and corona at     of the solar wind. The Coronagraph will ob-
the spatial and temporal resolution of the       serve both CMEs and solar wind structures.
TRACE imager but with the full-disk cover-       The Magnetographs and Atmospheric Imag-
age of the SOHO/EIT instrument. The Co-          ing Array will reveal the related magnetic
ronagraph will detail the structure and evo-     structures closer to the surface. The EUV
lution of the corona and CMEs while the          Imaging Spectrometer will examine the
EUV Imaging Spectrometer examines the            physical conditions that accompany the ac-
physical conditions at key positions asso-       celeration of the solar wind, the heating of
ciated with the energetic particle events.       the corona, and the initiation of CMEs. Co-
                                                 ronal spectroscopy would be an important
Can the structure and dynamics of the            tool for detailed measurements of velocity,
solar wind be determined from the mag-           composition, and temperature.
netic field configuration and atmospheric
structure near the solar surface?                When will activity occur, and is it possible
                                                 to make accurate and reliable forecasts of
Disturbances in the solar wind rack the          space weather and climate?
Earth‟s magnetosphere and drive geomag-
netic storms. These storms enhance and

Within the LWS Program SDO must pro-               The Helioseismic/Magnetic Imager would
vide the observations that are crucial for un-     extend the capabilities of the SOHO/MDI
derstanding and predicting solar variability       instrument by going to higher spatial and
on all timescales. SDO must provide the da-        temporal resolution with continuous full-
ta that serves as inputs to, or boundary con-      disk coverage. The Atmospheric Imaging
ditions for, the other systems within the          Array would be similar to the SOHO/EIT
LWS program. The extent to which space             and TRACE instruments but with several
weather and climate can be predicted will be       independent telescopes providing simulta-
recognized only after we gain a better un-         neous observations with the spatial resolu-
derstanding of the sources and nature of so-       tion of TRACE, an order of magnitude in-
lar variability by addressing the preceding        crease in temporal resolution and the full-
questions.                                         disk coverage of EIT. The EUV Spectral
                                                   Irradiance Monitor would provide long-
SDO Science. In the following section              term, continuous measurements of the solar
(Section 3) we outline our current under-          irradiance from 1 nm to 120 nm. The Coro-
standing of solar variability itself – irra-       nagraph would be similar to the SO-
diance variations, energetic particle emis-        HO/LASCO instrument but with far better
sion by flares and CMEs, and solar wind            spatial and temporal resolution along with
variations – along with the mechanisms that        extended spatial coverage from 1.1 RSun to
drive this variability – the solar cycle, active   15 RSun. The Photometric Mapper would
region evolution, and small-scale magnetic         be unique to SDO. It would provide photo-
element interactions.                              metric and bolometric images of the solar
                                                   radiance. The UV/EUV Imaging Spectro-
SDO Instruments. The outstanding scien-            meter would be a high resolution, high time
tific questions that arise can be addressed by     cadence imaging spectrometer designed to
a set of observations as described in Section      complement the Atmospheric Imaging Array
4. A potential set of instruments to acquire       and Magnetographs. The Vector Magneto-
these observations on SDO includes:                graph would provide both the strength and
                                                   direction of the magnetic flux across the full
       Helioseismic/Magnetic Imager               solar disk at 5-minute intervals.
       Atmospheric Imaging Array
       EUV Spectral Irradiance Monitor            SDO Mission Concept. The high resolu-
       Coronagraph                                tion, rapid cadence, and continuous cover-
       Photometric Mapper                         age required for the observations lead to an
       UV/EUV Imaging Spectrometer                SDO Mission design that places the satellite
       Vector Magnetograph                        into a geosynchronous orbit. This allows for
                                                   continuous contact at high data rate with a
A Total Solar Irradiance (TSI) Monitor             dedicated ground station. A geosynchronous
should also be included if redundant obser-        orbit can also be used to minimize the data
vations are not available concurrently with        interruptions caused by the satellite passing
SDO. The detailed instrument capabilities          through the Earth‟s and the Moon‟s sha-
are traced back to the scientific questions in     dows. The SDO Mission Concept (Section
the following sections. However, a brief de-       6) calls for a launch in late 2006 or early
scription of the instruments should provide        2007 on a Delta booster. The mission would
some initial guidance.                             be for a nominal 5 years with an additional
                                                   5-year extended mission. The SDO mission

will help us make great strides toward un-        weather represent one broad area. There are
derstanding the solar variability that affects    three significant types of solar influences:
life and society. The instruments will pro-       photons (irradiance), energetic particles (as-
vide the observations that lead to a more         sociated with flares and CMEs), and plasma
complete understanding of the solar dynam-        (coronal structure and solar wind variabili-
ics that drive variability in the Earth‟s envi-   ty). The mechanisms of solar variability
ronment.                                          represent the other broad area. While these
                                                  mechanisms are all magnetic in nature, they
3 Current Scientific Under-                       can be neatly separated according to the
  standing and Outstanding                        timescales associated with each: long-term
                                                  (the solar cycle), mid-term (active region
                                                  evolution), and short-term (small-scale
                                                  magnetic element interactions). SDO must
We have learned much about the Sun and
                                                  address all these areas to advance our under-
solar variability in the last few years. The
                                                  standing of the nature and source of the solar
scientific discoveries from missions such as
                                                  variations that affect life and society.
SOHO and TRACE make headlines in the
popular press and are occasionally featured
on the nightly news. The very nature of           3.1 Solar Influences on Global
scientific inquiry, however, teaches us that          Change and Space Weather
the more we learn the more we find that we
don‟t understand. We have learned much            Understanding the solar influences on global
about the solar influences on global change       change and space weather is increasingly
and space weather, but we have also found         important as society becomes more reliant
that we do not fully understand all sources       on technology. Global warming and ozone
of the irradiance variations and we can‟t re-     depletion can be influenced by solar irra-
liably predict energetic particle eruptions or    diance variations. The health and safety of
solar wind variations. Likewise, we have          satellites and astronauts can be influenced
learned much about the structure and dy-          by energetic particles from solar flares and
namics of the solar interior and the evolution    CMEs. Electrical power distribution to our
of active region magnetic fields, but we still    cities can be interrupted by geomagnetic
don‟t understand the solar dynamo and can‟t       storms induced by variations in the solar
reliably predict the size of the next solar       wind. While our understanding of these ef-
cycle or the emergence of the next active         fects and their solar origins has improved
region. The understanding of the mechan-          greatly over the past decade, many outstand-
isms of solar variability that we have gained     ing questions remain.
from previous missions, ground-based ob-
servations, and theoretical studies leads us to   3.1.1 Irradiance Variations
additional questions that require new obser-
vations. In the following subsections we de-      The radiation that the Sun provides to the
scribe our current scientific understanding       Earth varies at all wavelengths and on all
and list some of the outstanding questions        timescales. While the Sun's fundamental
that must be addressed by SDO.                    energy source is the nuclear conversion of
                                                  hydrogen to helium in the core, the imme-
We have divided the science into two broad        diate radiative energy source for the Earth‟s
areas with three subdivisions each. The solar     surface and lower atmosphere is the solar
influences on global change and space             photosphere. The chromosphere and corona

produce the UV, EUV and X-ray radiations
that are the primary source of energy for the
Earth‟s upper atmosphere and ionosphere.
With million-year photon diffusion times in
the core, the emergent luminosity at its outer
boundary should be effectively constant on
any total solar irradiance measurement time-              Figure 3.2. Total solar irradiance (W/m2) as a func-
scale. Given the observed variability, it fol-            tion of time. Total solar irradiance varies by about
lows that the mechanisms that deliver ener-               0.1% over a solar cycle and by several times that on
gy to the photosphere must be variable.                   shorter (solar rotation) timescales.

                                                          The spectrally integrated „total‟ irradiance,
                                                          of relevance to climate on Earth, varies by a
                                                          few parts per million in association with p-
                                                          mode oscillations, a few tenths percent dur-
                                                          ing solar rotation, about 0.1% in recent solar
                                                          cycles (Fig. 3.2), and possibly by a few
                                                          tenths percent on longer timescales. The UV
                                                          radiation from 160 to 300 nm that produces
                                                          stratospheric ozone and modulates the mid-
                                                          dle atmosphere, varies by 1 to 20% during a
                                                          solar rotation, about twice that during the
Figure 3.1. Solar spectral irradiance and its solar
cycle variability as functions of wavelength. The so-     solar cycle, and possibly more on longer
lar cycle variability at wavelengths shorter than about   timescales. Although the radiative output at
100 nm is typically greater than 100%.                    ultraviolet wavelengths less than 400 nm
                                                          constitutes only 8% of the total radiative
The magnitude of the irradiance variability               output, the stronger variability at these
depends on the timescale and the wave-                    shorter wavelengths makes a significant
length of the radiation (Fig. 3.1). Most vari-            contribution (30%) to the total solar irra-
able is EUV radiation at the shortest wave-               diance variations. The EUV radiation from 1
lengths. Emitted from outer layers of the so-             to 120 nm that produces the ionosphere and
lar atmosphere, this radiation exhibits rapid             is the primary heat source for the thermos-
short-term order-of-magnitude fluctuations                phere varies by 50% to factors of two or
during solar flares and more gradual varia-               more during solar rotations and the solar
tions of a few orders of magnitude during                 cycle. Significant variations are expected
the solar cycle. Least variable is the near               during solar flares, but are presently un-
infrared spectrum, emitted from the deepest               known.
layers of the photosphere, where flare-
related variability is negligible and solar
cycle changes are thought to be a few tenths     Irradiance Variations –
of a percent, at most.                                           Current Understanding

                                                          Magnetized features in the solar atmosphere
                                                          are associated with solar irradiance varia-
                                                          tions across the entire electromagnetic spec-
                                                          trum. Magnetic fields in sunspots redirect

the upward flow of energy from the convec-
tion zone and affect the emergent radiation     Ground-based measurements of the solar
locally in the photosphere. In bright magnet-   radius exist over the last 300 years, but these
ic regions, specifically active region plage,   results are controversial and inconsistent.
faculae and dispersed network, irradiance is    Historical measurements show that the Sun's
locally enhanced. The interplay of locally      radius may even have been larger during the
“dark and bright” features causes variations    Maunder Minimum - during extremely cold
in photospheric radiation (wavelengths >        periods in Europe and the Atlantic regions.
200 nm) on timescales ranging from days to      The French CERGA radius measurements
a solar cycle. Bright, magnetized features      detected a larger solar radius during solar
largely control the variability of chromos-     minimum. Others have found the opposite
pheric emission that includes some of the       positive correlation between apparent radius
strongest EUV emission lines. In the outer      changes and the solar activity cycle. There
solar atmosphere, magnetic fields in active     are also hints of periodic solar radius varia-
regions expand to form coronal loop struc-      tions over timescales of 1,000 days to 80
tures that at times cover a significant frac-   years, but taken as a whole these measure-
tion of the global solar atmosphere. Density    ments generally are neither consistent nor
and temperature enhancements alter the          conclusive. It is apparent that the results to
emission in coronal magnetic loops and their    date have been severely limited by the at-
presence causes significant increases in X-     mosphere.
ray and extreme ultraviolet lines, especially
during flares.                                  Because the nuclear energy generation rate
                                                is effectively constant on solar cycle time-
Fluctuations in total (spectrally integrated)   scales the Sun must be storing a fraction of
irradiance are traceable to both sunspot and    this energy flux in thermal, gravitational, or
facular sources on short (solar rotation)       magnetic forms in order for its net luminosi-
timescales. The movement across the visible     ty to vary. Each of these mechanisms leads
solar disk of a large sunspot group can cause   to distinct perturbations in the equilibrium
the total irradiance to decrease by a few       stellar structure and potentially detectable
tenths of a percent. These short-term varia-    changes in the solar diameter. It follows that
tions are superimposed on an overall total      a sensitive determination of the solar radius
solar irradiance increase from sunspot min-     fluctuations can help reveal the cause of the
imum to maximum. Empirical models of            solar cycle changes.
sunspot darkening and facular brightening
account for recent solar irradiance cycles,     Quantitative understanding of the magnetic
but the models require a factor of two more     sources of EUV irradiance variations is far
facular brightening than sunspot darkening      less developed. Until recently, the only
during maximum. It is not clear that all the    models of EUV irradiance variability were
variation can be found in the faculae. Mod-     simply parameterizations of the 10.7 cm ra-
els for the flow of heat through the convec-    dio flux. In reality the database of reliable
tion zone suggest that heat flow blockage by    EUV irradiance observations is simply too
sunspots should restructure the convection      limited to realistically constrain even the
zone and produce bright rings around active     most simple of empirical models. This has
regions. Recent observations of bright rings    lead to a new approach of using differential
around active regions and changes in the        emission measures (combined with atomic
solar radius add to the controversy.            parameters of the emitting species) of spe-

cific magnetic features (active regions, co-      – rather than merely proportional to – the
ronal holes) and the quiet background Sun,        total irradiance variations attributed to them.
to construct full-disk EUV irradiance. Solar      An accurate (~10%) equality would be an
images (e.g., Ca K, SOHO/EIT, Yohkoh/             important test of the models. Equality of the
SXT) provide estimates of the fractional vo-      year-to-year photometric and radiometric
lume covered by the respective features at        contributions probably offers the cleanest
any time during the solar cycle. This ap-         discriminator between the enhanced net-
proach depends on having reliable emission        work, and other explanations of the 11-year
measure distributions. Unfortunately, we          component of irradiance variation. Merely
have only a small number of suitable, cali-       showing proportionality leaves open the
brated, high spectral and spatial resolution      possibility that more global changes also
observations covering the needed wide             proportional to activity level might be play-
range of emission temperatures. Neverthe-         ing a significant role.
less, the new approach to quantifying mag-
netic sources of EUV irradiance variability       [Text highlighted in gray provides the     tra-
from a more physical understanding of the         ceability between the scientific goals,     the
temperature, density and atomic parameters        measurement characteristics, and the        re-
is promising. The major challenge here is to      quirements on potential instruments         for
obtain better data in the EUV lines of coron-     SDO.]
al and transition region origin. Many of
these lines arise in structures whose areas       The total solar irradiance must continue to
are poorly estimated by traditional chromos-      be monitored to address these questions. Ef-
pheric indices such as Ca K and Mg II.            forts toward identifying the sources of these
                                                  variations are of little use without know-
                                                  ledge of the variations themselves. Irradiance Variations –
       Outstanding Questions                      Photometric images, photometrically accu-
                                                  rate narrowband images and broadband bo-
                                                  lometric images, are needed to determine the
Where do the observed variations in the
                                                  actual sources of the irradiance variations.
Sun’s total and spectral irradiance arise,
                                                  These images must have the spatial resolu-
and how do they relate to the magnetic
activity cycles? This is the overarching          tion to resolve sunspots and faculae and the
question for the solar irradiance variations.     accuracy and precision to determine their
It leads to a series of more focused questions    contribution to the total irradiance.
in specific areas.
                                                  Magnetograms with comparable spatial
                                                  resolution are also needed in order to dis-
What are the sources of the total irra-
diance variations? The projected areas of         criminate between the magnetic and the non-
sunspots and faculae are well correlated with     magnetic irradiance sources.
the total solar irradiance variations. Howev-
er, alternative sources such as luminosity        Helioseismic images will aid in measuring
changes due to radius variations associated       variations in the size of the Sun that should
with sunspots and faculae can also give this      be related to luminosity variations.
correlation. Present photometry of spots and
faculae is not accurate enough to demon-          Heliometry – precise measurements of the
strate that the irradiance effects are equal to   size and shape of the Sun – will also aid in

measuring variations in the size of the Sun       pendent, spectrally-compatible, EUV irra-
that should be related to luminosity varia-       diance observations (to specify the actual
tions.                                            variations and to verify the understanding
                                                  developed from analysis of the images).
How does the EUV spectral irradiance
change on timescales of seconds to dec-           Atmospheric images like those obtained
ades? There is a severe lack of knowledge         with SOHO/EIT and Yohkoh/SXT will aid
of solar spectral irradiance variability at wa-   in the identification and quantification of the
velengths shorter than 120 nm, where the          spectral irradiance variations. Full-disk, nar-
database is characterized by intermittent ob-     rowband EUV images at a series of EUV
servations made with poorly calibrated in-        wavelengths should be taken simultaneously
struments. Solar cycle EUV irradiance va-         with the EUV spectral irradiance measure-
riability amplitudes are uncertain by 50% or      ments. These images should have the spatial
more, and variations associated with flares       resolution to identify the nature of the fea-
are essentially unknown. Only for the total       tures and the temporal resolution to follow
irradiance and for the spectral irradiance        the rapid variations in flares. The measure-
from 120 to 400 nm do continuous, well-           ments must be made in a sound radiometric
calibrated databases exist. NASA‟s Office of      way that eliminates spurious instrumental or
Earth Science (OES) has made these latter         spacecraft effects. The spectral bands should
observations during the past decade at long-      be chosen such that emission measures
er wavelengths for understanding natural          spanning a range of temperatures from 104K
variations in climate and ozone.                  to a few 106K can be constructed for indi-
                                                  vidual or classes of magnetic elements.
The EUV spectral irradiance shortward of
120 nm must be monitored. The observa-            UV/EUV spectra are needed to provide de-
tions should cover the neglected wavelength       tailed information about the density and
range from 1 nm to 120 nm with enough             temperature of the plasma in individual
spectral resolution to be useful to solar, up-    magnetic regions on the Sun's disk. It is evi-
per atmospheric and ionospheric studies.          dent that a major contributor to irradiance
The temporal resolution should be short           changes are the variations in UV/EUV emis-
enough to follow flares, and the temporal         sion lines associated with regions of strong
coverage should extend through a solar            magnetic fields in the photosphere. Without
cycle. These measurements should have suf-        sufficient spectral and spatial resolution it is
ficient short-term precision and long-term        not possible to understand the physical me-
stability to monitor the EUV variations on        chanisms responsible for the variations.
timescales of flares to the solar cycle.          High spectral resolution would provide sig-
                                                  nificantly more reliable emission measure
What are the sources of the spectral irra-        determinations, and the coordination with
diance variations? Although magnetic re-          the full-disk images would provide global
gions are understood to be the primary cause      context for these spatially resolved observa-
of EUV and X-ray irradiance variations, the       tions.
details of this association have yet to be de-
termined. This is because of the complete         Were solar irradiance variations larger in
absence of simultaneous well-calibrated full-     the historical past? Present specifications
disk images (that resolve the spatial features    of solar total and spectral irradiance variabil-
with adequate spectral resolution) and inde-      ity for terrestrial research are based largely

on multi-component empirical models. Al-
though these models can be quite complex,
they represent a very simplified view of the
structure of the solar atmosphere.

The ability to understand solar variations
outside of the current era requires the devel-
opment of physical models based on obser-
vations of the solar sources of the total and
spectral irradiance variations at the neces-
sary spectral and spatial resolution.
                                                 Figure 3.3. LASCO images of the solar corona be-
                                                 fore (left) and just after (right) the large flare of July
                                                 14, 2000 (the Bastille Day event). Energetic particles
3.1.2 Flares and CMEs as the                     from the flare eventually overwhelmed the detector
                                                 and made observations impossible for some time.
      Source of Energetic Par-
      ticles                            Flares and CMEs – Cur-
                                                        rent Understanding
Flares and CMEs are the primary sources of
energetic particles from the Sun. Solar flares
and coronal mass ejections are the largest       Solar flares have long been recognized as a
explosions in the solar system, and are the      key component of solar variability on short
root cause of much of the Earth‟s geomag-        timescales. The advent of space-based ob-
netic and ionospheric disturbances. The          servations has also added CMEs and shocks
energetic particles produced in these explo-     in the solar wind as significant components.
sions can be harmful to humans and elec-         Much has been learned about these pheno-
tronic instruments in space. Figure 3.3          mena and about how they accelerate par-
shows LASCO images of the solar corona           ticles to cosmic ray energies. Our under-
before and after the flare of July 14, 2000.     standing and predictive abilities have im-
The energetic particles produced in this         proved but not to the point where we can
event showered the LASCO detector and            reliably predict the occurrence and geoeffec-
made observations impossible for some            tiveness of these events.
time. The fundamental physics of these
events remains elusive and predictions must      Solar flares can be detected at virtually all
become more reliable. These extremely            wavelengths. The NOAA GOES satellites
complex phenomena require the explosive          include solar X-ray monitors that are used to
conversion of magnetic energy into accele-       detect and classify solar flares. The occur-
rated particles, bulk mass motions, light, and   rence of flares is well correlated with the
heat. SDO must examine the evolving mag-         magnetic complexity of active regions. Delta
netic configurations associated with these       spots, sunspots with opposite magnetic po-
events and identify their precursors.            larities included within a common penum-
                                                 bra, often produce flares. Detailed mea-
                                                 surements of the magnetic field also reveal
                                                 stressed magnetic field configurations in
                                                 flaring active regions. Figure 3.4 shows a
                                                 flaring active region along with the observed
                                                 magnetic field and the stress-free field de-

rived from the observed line-of-sight field
component. The magnetic field vectors are                 Traditionally CMEs have been observed
twisted or sheared in the region where this               with white light coronagraphs using polariz-
flare occurred. Other indicators of stressed              ers and broadband filters. With the launch of
field configurations include sigmoid, or S-               SOHO, considerable progress toward a de-
shaped, coronal loops observed in images                  tailed description of CMEs is being made
obtained in the UV/EUV. While these mag-                  through a combination of observations with
netic structures indicate the likelihood of a             the EIT, LASCO and UVCS instruments.
flare, they are not highly reliable and cannot            SOHO observations with the LASCO coro-
indicate when a flare will occur or how                   nagraphs show that CMEs have a wide
strong it will be. Detailed determinations of             range of speeds from 100 km/s to greater
the magnetic configurations in many flaring               than 1000 km/s. EIT observations show that
and non-flaring active regions are needed for             a large CME can empty a substantial frac-
systematic studies of these and other flare               tion of the corona. We know typical masses
indicators.                                               (1016 g) and total energies (1032 ergs) of
                                                          CMEs, but not what causes the variations of
                                                          these parameters from one CME to another.
                                                          We need to know how the magnetic field
                                                          energizes and accelerates CME material.

                                                          There is evidence that initially slower-speed
                                                          CMEs that tend to have higher accelerations
                                                          are related to prominence eruptions while
                                                          the faster CMEs are associated with flare
                                                          events but the correlation is not exact. SMM
                                                          observations show that the range of CME
                                                          speeds does not appear to be related to the
                                                          amount of thermal input into the CME, so
                                                          the variation must be related to the nonther-
                                                          mal energy from the magnetic field. UVCS
                                                          observations indicate ionization states
                                                          formed at temperatures over a broad range
                                                          (from 104 to 107 K) and heat input compara-
                                                          ble to the kinetic energy in the ejected ma-

                                                          By their explosive nature, CMEs also reflect
                                                          short-term changes in the configuration of
                                                          the coronal magnetic field. On short time-
                                                          scales, magnetic reconnection of field lines
                                                          is often seen just before and sometimes after
                                                          a CME has erupted, but whether reconnec-
Figure 3.4. Stressed magnetic field configuration in a    tion is a cause or an effect of the CME is
flaring active region (off-band H - top panel). The      still debated. There has been some success
observed magnetic field vectors (center panel) are        in using the presence of sigmoid magnetic
twisted relative to the stress-free field vectors (bot-   fields in the low corona as a precursor for
tom panel) in the region of the flare.
                                                          CME eruptions but it is not yet possible to

predict which sigmoid structures will erupt      licity) may be about the same as the amount
or when.                                         of free energy removed convectively by
                                                 CMEs during a solar cycle. While there is
The geoeffectiveness of CMEs that strike         good evidence for the expected flux and he-
the Earth is known to depend, in part, on the    licity transport in the past three decades of
shock strength and on the direction of the       solar wind data, quantitative inferences
magnetic field. Those CMEs that have             about magnetic helicity from solar observa-
southward directed interplanetary magnetic       tions have been difficult to obtain.
fields are known to produce the largest
geomagnetic storms. Studies of the relation-
ship between the direction of the magnetic
field in CMEs and their geoeffectiveness
conclude that accurate storm forecasting re-
quires a means of predicting the magnetic
field configuration in CMEs.

The process of magnetic reconnection is
thought to be the principle mechanism by
which magnetic energy is converted into
thermal energy in flares and CMEs. A pri-
mary result of magnetic reconnection is the
                                                 Figure 3.5. Helical structure in a CME observed with
creation of new field line connectivities. On    LASCO on June 2, 1998.
large spatial scales, the new plasma loops
are often seen to connect pre-existing pho-
tospheric flux elements with newly emerged Flares and CMEs – Out
photospheric flux elements. The rate at
which these new loops appear is on the order
                                                        standing Questions
of 0.1% to 10% of the Alfvén time, indicat-
ing that this occurs in the regime of fast re-   What magnetic field configurations lead
connection. The high electrical conductivity     to the CMEs, filament eruptions, and
of the coronal plasma implies that small spa-    flares that produce energetic particles?
tial scales are often needed in order for        Our current understanding of flares and
magnetic reconnection to occur at a reason-      CMEs suggests that the magnetic field con-
able rate. These spatial scales are below the    figuration and its evolution determine the
resolution limit of current instrumentation.     development of these explosive events. The
                                                 photospheric magnetic fields often indicate
Besides providing clues about the storage        the existence of stressed magnetic field con-
and release of magnetic energy, CMEs pro-        figurations. However, the lack of significant
vide a major avenue (in addition to the fast     changes in the photospheric fields after an
and slow speed wind) for coupling the Sun's      eruption suggests that the actual restructur-
atmosphere to the more extended helios-          ing of the field occurs high in the solar at-
phere. CMEs may be the fundamental means         mosphere where the magnetic forces domi-
by which the Sun sheds both magnetic flux        nate. Significant restructuring of the corona
and helicity (Fig. 3.5) over the 11-year solar   is often observed before, during, and after
cycle. The rate of free energy built up over     these events.
the solar cycle (in the form of increasing he-

Vector magnetic flux measurements are            ciated with the magnetic field pattern evolu-
required to determine the configuration of       tion.
the surface magnetic fields. While longitu-
dinal magnetograms provide some indica-          Is the magnetic helicity removed from the
tion of the field's complexity, vector magne-    Sun by CMEs consistent with the rate of
tograms in the photosphere (and ideally the      helicity generation under the surface and
chromosphere where the fields are closer to      with the rate of helicity transport in the
a force-free state), are required to determine   solar wind? The Sun has a weak preference
the presence and magnitude of the magnetic       for left-helical fields in the north and right-
stresses associated with these eruptions.        helical fields in the south but the cause of
Useful observations require adequate spatial     this hemispheric pattern is not yet unders-
resolution (~1 arcsec) to resolve the small-     tood. Measurements of the patterns and the
scale elements that may trigger the erup-        net amount of helicity generation would
tions, adequate temporal resolution to follow    place important constraints on solar dynamo
the evolution (minutes) and the spatial and      models. From a global point of view, CMEs
temporal coverage to capture all events and      are an inevitable response to helicity genera-
to determine long-range interconnections         tion, but we do not know whether the helici-
between magnetic regions.                        ty is primarily generated by the dynamo, by
                                                 convection, or by shearing of coronal fields.
Atmospheric images are required to deter-        A better understanding of helicity generation
mine the configuration of the magnetic           and removal is crucial to understanding
fields in the lower corona associated with       CMEs.
flares and CMEs. These images provide di-
rect evidence of the occurrence of these         Can the magnetic field direction and twist
events and provide critical information on       of CMEs be related to their geoeffective-
their spatial, temporal, and physical charac-    ness? The direction of the interplanetary
teristics.                                       magnetic field in the solar wind is known to
                                                 be a good indicator of the geoeffectiveness
Coronagraphic images are required to de-         of the disturbance. To what extent can this
termine the configuration of the magnetic        field component be determined from obser-
fields in the outer corona associated with       vations of CMEs close to the Sun?
flares and CMEs. These images also provide
direct evidence of the occurrence of these       Vector magnetic flux measurements are
events and provide critical information on       required to determine the helicity of the sur-
their spatial and temporal characteristics.      face magnetic fields.

UV/EUV spectra are required to determine         Atmospheric imaging and UV/EUV spec-
the physical conditions at the site of the       troscopy can provide information on the
eruption – both before and after the events      sign of the magnetic helicity, and the field
themselves. These measurements may help          geometry for the twisted magnetic fields in
to identify the triggering events for these      CMEs.
                                                 Where do CMEs form shock waves, and
Helioseismic images are required to meas-        how are coronal shocks related to the
ure the surface and sub-surface flows asso-      fluxes of energetic particles at the Earth?
                                                 Energetic particles are accelerated in shocks

associated with CMEs. The locations and            of shocks that accelerate particles which
development of these shocks must be stu-           contribute to the space weather environment.
died in more detail.

EUV coronal spectroscopy would provide
important information on the physical condi-
tions inside of CMEs as they evolve during
their transit through the outer corona. Veloc-
ity distributions, line of sight velocities, and
the detection of high temperature spectral         Figure 3.6. Geomagnetic fluctuations during the de-
                                                   clining phase of Cycle 22. Disturbances recurring at
lines can be used to determine shocks and
                                                   27-day intervals are due to high-speed streams in the
current sheets within CME structures.              solar wind.

What is the role of magnetic reconnection Coronal Structure and
in the heating and acceleration of CMEs?                  Solar Wind Variations –
Some models of CMEs give a prominent
role to a reconnection current sheet connect-             Current Understanding
ing the CME flux rope to a post-CME flare
arcade.                                            In the high-temperature, low-density ex-
                                                   tended corona the plasma beta (ratio of gas
UV/EUV spectra can provide evidence of             pressure to magnetic field energy density) is
extremely hot structures at the reconnection       low enough that motions of the coronal
sites with observations of high temperature        plasma are usually dominated by the mag-
spectral lines and measurements of the             netic field configuration. We know that the
plasma temperature, density, and heating           high-speed wind originates in the open mag-
rate. The presence of predicted MHD waves          netic field regions in coronal holes and that
created at reconnection sites can be detected      the slow-speed wind emanates from above
by their effects on observed spectral line         streamers.
profiles for comparison with models of the
reconnection process.                              Magnetic field extrapolation models, using
                                                   photospheric magnetic fields as input, can
3.1.3 Coronal Structure and So-                    now fairly reliably reproduce the large-scale
      lar Wind Variations                          structure of the inner corona (Fig. 3.7).
                                                   These models have become more sophisti-
                                                   cated by including non-potential fields and
Solar wind variations also produce geomag-         MHD effects. Maps of regions where the
netic disturbances. This is particularly evi-      magnetic field is opened to the heliosphere
dent late in each solar cycle when flares and      correspond well to coronal holes and the
CMEs are less likely and the Sun‟s global          source of high-speed wind streams. Yet, the
field geometry is more stable. As the Sun‟s        physics involved in the heating of the corona
rotation brings around the sources of high-        and the acceleration of the solar wind is still
speed streams we see recurring geomagnetic         hotly debated.
disturbances at ~27-day intervals (Fig. 3.6).
Corotating Interaction Regions (CIRs) form         While much has been learned in recent years
within the heliosphere when such streams           about coronal heating and solar wind varia-
interact with ambient slow solar wind. These       bility, no unifying theory about the physical
interactions eventually include the formation      mechanisms responsible has emerged. Theo-

ries of coronal heating and solar wind acce-
leration generally fall into two broad catego-          While Alfvén waves with frequencies high
ries: (1) heating and acceleration by Alfvén            enough to probe cyclotron resonances (10 to
or other MHD waves and (2) heating and                  10,000 Hz) have not yet been observed di-
acceleration by the impulsive release of                rectly in the solar wind or corona, UVCS
energy at sites of magnetic reconnection.               coronal hole observations have given rise to
Observations from current solar missions                renewed interest in the theoretical investiga-
(e.g. SOHO, Yohkoh, and TRACE) have                     tion of resonant wave dissipation. Spectral
provided evidence for both types of mechan-             line widths indicating large anisotropic ion
isms.                                                   velocities have been observed. It is not
                                                        known if the fluctuations originate primarily
                                                        at the coronal base or are generated conti-
                                                        nuously in the acceleration region of the
                                                        wind (via turbulent cascade or plasma insta-

                                                        Figure 3.8. Coronal structures from 1.0 to > 5 R
                                                        (August 11, 1999 LASCO C2 image and total eclipse
                                                        photo from S. Koutchmy). Solar wind speed can be
                                                        measured using moving features in the outer corona
                                                        but much of the restructuring takes place much closer
                                                        to the surface.

                                                        SOHO/MDI magnetogram observations
                                                        suggest that a “magnetic carpet” of twisted
                                                        magnetic fields that reconfigure on time-
                                                        scales of minutes may be responsible for
Figure 3.7. Coronal structures derived from an MHD      heating the quiet corona. Thin current sheets
field extrapolation model (top and middle panel,        are generated between the twisted and
SAIC) for the August 11, 1999 eclipse (bottom pan-
el). There are good agreements with many structures
                                                        braided field lines, and magnetic reconnec-
in spite of the lack of complete (back side) magnetic   tion occurs stochastically to relax the system
field information.                                      to a lower-energy state (with net particle

heating). “Microflares” from the reconnec-        Can the structure and dynamics of the
tion sites on small loops seen in Yoh-            solar wind be determined from the mag-
koh/SXT images and vector magnetograms            netic field configuration and atmospheric
may provide enough energy to heat the co-         structure near the solar surface? The tem-
rona at least locally in active regions.          peratures and outflow speeds of the large
Whether these local heating processes can         polar coronal holes at solar minimum have
heat the extended corona above a few tenths       been shown to differ from those of the co-
of a solar radius is still an open question.      ronal holes that appear primarily at low lati-
                                                  tudes at solar maximum. For the low-speed
Recent progress has also been made on the         wind, the large equatorial streamer belt at
slow speed wind and its relation to coronal       solar minimum seems to be hotter than the
streamers. UVCS observations of high tem-         more sporadic streamers that form at all lati-
perature ions high in streamers suggest pre-      tudes at solar maximum. Precisely how
ferential heating of ions with high mass-to-      these thermal changes depend on the coronal
charge ratios. These results, while not con-      field morphology is not well understood.
clusive, are consistent with ion cyclotron        The solar corona as a whole is brighter in its
resonance absorption that could lead to a         EUV emission at solar maximum than it is at
steady expansion of material into the slow        solar minimum. Whether this arises because
speed wind. Other streamer observations           of an overall density increase, a change in
made with LASCO show moving “blobs” or            the radial and latitudinal distribution of co-
density enhancements that appear to emerge        ronal heating, or is related to photospheric
from the tips of streamers. This suggests that    brightness changes, is not yet known.
some of the mass in the slow speed wind is
released by intermittent reconnections of the     Magnetograms are required to determine
open and closed magnetic field lines in           the configuration of the surface magnetic
streamers. This view is consistent with the       fields associated with the coronal and solar
results of earlier investigations that link the   wind structures. Magnetic measurements at
origin of the slow speed wind to the strea-       the boundaries between streamers and co-
mer belt. However coronal abundance mea-          ronal holes will elucidate the role of open
surements show that most of the slow speed        and closed fields in producing the slow
wind probably originates from the streamer        speed wind. Longitudinal magnetograms are
edges, since these regions have similar dep-      the minimum required observation, and to
letions of high First Ionization Potential        fully support the science outlined above vec-
(FIP) elements that are found in the slow         tor photospheric magnetic flux measure-
speed wind. More detailed measurements            ments are required. While longitudinal mag-
are needed to establish, with confidence,         netograms provide some indication of the
what fraction of the slow speed wind comes        field configuration, vector magnetograms in
from streamer edges versus the closed-field       the photosphere, and ideally in the chromos-
regions of streamers.                             phere where the fields are closer to a force-
                                                  free state, are required to provide a more
                                                  accurate determination. Coronal Structure and
       Solar Wind Variations –                    Atmospheric images are required to reveal
       Outstanding Questions                      the location, morphology, and thermody-
                                                  namic characteristics of atmospheric fea-
                                                  tures associated with the outer coronal struc-

tures. The locations of polar plumes and the     the magnetic field. Observations are needed
edges of streamers can be identified in such     to differentiate between the alternative me-
images. They also assist in the determination    chanisms of MHD wave absorption and im-
of the magnetic field configuration.             pulsive Joule heating. Different proposed
                                                 mechanisms of wave generation (i.e., turbu-
Coronagraphic images are required to de-         lent cascade or local micro-instabilities) de-
termine the positions of streamers and co-       pend in different ways on the evolving co-
ronal holes, the solar wind speeds associated    ronal flux tube area, photospheric field
with them, and to follow their evolution.        strength, and plasma beta.

UV/EUV spectra are required to determine         UV/EUV Spectra are needed to character-
the physical conditions associated with the      ize the density, bulk velocity, energy state,
solar wind structures. Temperatures, densi-      ionization, and elemental abundance of solar
ties, flow speeds, and the presence of waves     wind source regions. A key means of identi-
and turbulence need to be determined within      fying the dominant physical processes will
these structures. Spectroscopic measure-         be to observe how the measured plasma pa-
ments of bulk coronal outflow velocities at      rameters and inferred wave properties
the boundaries between streamers and co-         evolve on timescales from days to solar
ronal holes will elucidate the role of open      cycles. Reconnection sites on the boundaries
and closed fields in producing the slow          of streamers can be identified by measuring
speed wind.                                      spectral line widths in these regions. Mag-
                                                 netic reconnection sites should also produce
EUV coronal spectroscopy would also              ions in highly excited charge states and their
provide important information on solar wind      characteristic spectra.
structures. Velocity distributions of ions and
electrons could be measured to determine         3.2 Mechanisms of Solar Varia-
the spectrum of waves and turbulence in co-          bility
ronal structures. Doppler shifts and ion ab-
undances could be measured to characterize       Magnetic structure is produced in the Sun
the source regions of the high-speed and         through a broad range of spatial and tempor-
low-speed solar wind structures.                 al scales. The global field reverses on a solar
                                                 cycle timescale, while magnetic elements at
Helioseismic images are required to meas-        the limit of resolution evolve on timescales
ure the surface and sub-surface flows. These     of minutes or seconds. Ultimately, the
flows control, to some extent, the magnetic      source of this magnetic activity can be
field pattern evolution and the subsequent       traced to the heat released by nuclear reac-
restructuring of the corona and solar wind.      tions in the solar core. However, the me-
                                                 chanisms by which the activity is produced
What are the fundamental processes re-           involve the fluid flows driven by this heat-
sponsible for the heating and acceleration       ing and the magnetic field itself. Our ability
of the high-speed and low-speed solar            to understand and ultimately predict solar
wind? Solar wind variations have a direct        activity hinges on our understanding of how
impact on space weather. We must better          these magnetic structures are produced. Pre-
understand the processes involved in pro-        dictions of the size and timing of the next
ducing these variations. Heating and accele-     solar cycle will not be reliable until we un-
ration of the corona is ultimately related to    derstand how the cycle is produced. Predic-

tions of flares and CMEs will not be reliable    tohydrodynamic dynamo seated within the
until we understand how active regions form      solar interior. Magnetic fields are effectively
and evolve. Predictions of solar wind varia-     “frozen” into the highly conducting ionized
tions will not be reliable until we understand   plasma within the Sun. Shearing motions
how the wind is accelerated. SDO will pro-       within the plasma can amplify the embedded
vide crucial observations needed to improve      magnetic field. Regions with strong magnet-
our understanding of the mechanisms that         ic field become buoyant in the convection
produce solar magnetism.                         zone and rise rapidly to the surface. Magnet-
                                                 ic loops emerge at the surface to form bi-
                                                 polar active regions with a characteristic tilt
3.2.1 The Solar Cycle                            imposed by the solar rotation that leaves the
                                                 following polarity closer to the poles than
The solar cycle has proven notoriously diffi-    the leading polarity. At the surface a meri-
cult to predict. Once a cycle is well under-     dional flow slowly transports magnetic ele-
way its smoothed behavior can be predicted       ments from the active latitudes to the poles
with some reliability using statistical models   while cellular flows like supergranules dis-
for the shape of the cycle. Predictions prior    perse them randomly across the surface.
to the start of a cycle are, however, much
less reliable and longer range predictions are   The cycle is completed through magnetic
virtually useless. Currently, all methods of     reconnection between elements of opposing
cycle prediction are empirical in nature.        polarities. These ingredients (rotation, shear
While we understand many of the processes        flow, meridional flow, cellular flows, rising
involved in producing the solar cycle we do      tilted loops, and reconnection) are thought to
not have a physical model that will take ini-    be critical components of the dynamo. Ob-
tial conditions and predict future behavior.     servations and theoretical models exist for
Long-range predictions of the solar cycle (as    each of these but conflicts and questions re-
well as extensions backward in time) are         main and a comprehensive model containing
important for understanding the climate          all these ingredients is yet to be formulated.
connection, for predicting satellite drag, and
for predicting the frequency of eruptive
events such as flares and CMEs. SDO will
improve our understanding of the solar cycle
by providing continuous observations and
extended spatial coverage of the two main
components of the solar cycle – the magnet-
ic field and the fluid flows within the Sun. The Solar Cycle –
                                                 Figure 3.9. Longitudinally averaged magnetic field
       Current Understanding                     as a function of latitude and time. Active region
                                                 magnetic flux is evident in the “butterfly” pattern
The solar cycle is a magnetic cycle in which     within about 50 of the equator. The reversal of the
                                                 magnetic polarities from cycle-to-cycle is evident at
the polarity of sunspots and the Sun‟s global    the poles and in the equatorward edges of the butterf-
magnetic field reverse approximately every       ly patterns.
11 years (Fig. 3.9). The source of this mag-
netic cycle is widely believed to be a magne-

The well-known differential rotation seen on           The torsional oscillation pattern represents a
the surface of the Sun is one example of a             shear flow on a smaller scale. Bands of en-
shear flow – latitudinal shear. Radial shear           hanced rotation rate appear near the poles
has long been thought to be the principal              and migrate toward the equator by the mid-
driver for the dynamo. Early kinematic dy-             dle of each cycle. This torsional pattern pro-
namos for the Sun required a radial shear              duces enhanced latitudinal shear in the ac-
from a rotation rate that increased towards            tive latitudes. The detection of the torsional
the interior while hydrodynamic models of              pattern in the outer 5 percent of the Sun's
the convection zone gave a rotation rate that          radius by helioseismic probing makes it evi-
decreased. Both of these models were shown             dent that the structure is more than a super-
to have problems when helioseismic obser-              ficial process. In layers below this outer 5
vations revealed the true nature of the inter-         percent, the nature of the velocity structures
nal rotation profile. The rotation rate is near-       becomes more obscure.
ly constant across the bulk of the convection
zone with the shear layers at the top and bot-
                                                       A meridional flow is observed both on the
tom. The region of high shear found at the
                                                       surface and in the interior. The strength of
base of the convection zone (Fig. 3.10) is
                                                       this flow, its latitudinal structure, and its
thought to be the source of the solar cycle.
                                                       variations in time are still somewhat uncer-
The rate of shear across this “tachocline”
                                                       tain. Tracking features on the surface gives a
can wrap a magnetic field line once around
                                                       flow speed about half that given by the di-
the Sun‟s equator every 16 months. Recent
                                                       rectly observed Doppler velocities. Heliose-
observations suggest that there is a periodic
                                                       ismic studies also give higher flow speeds
variation in the rotation rate at the tachocline
                                                       and show that the poleward flow extends
with a timescale of 1.3 to 1.8 years. Others
                                                       well into the convection zone. “Flux trans-
analyzing the MDI and GONG data only
                                                       port” models for the large-scale surface
find evidence for chaotic variations. Should
                                                       magnetic field have been developed over the
there be additional time periods associated
                                                       last 40 years. These models do a remarkable
with the solar cycle, the constraints on mod-
                                                       job of reproducing the magnetic flux distri-
els will tighten.
                                                       bution using only surface flows and the
                                                       eruption of magnetic flux in active regions.
                                                       However, the meridional flows employed in
                                                       these models are quite different from the
                                                       observed flows in both strength and struc-
                                                       ture. There are serious concerns about why
                                                       the models of the polar cap size and field
                                                       strength work as well as they do given that
                                                       the poleward flow systems vary as much as
                                                       is observed.

                                                       The hierarchy of velocity structures in the
                                                       Sun begins with differential rotation and me-
Figure 3.10. Internal rotation rate as a function of
                                                       ridional circulation at the largest scale. At
latitude and depth. The tachocline (dashed line -
where the rotation velocity changes rapidly) is        smaller scales is the torsional oscillation pat-
thought to be the location of the magnetic dynamo      tern. Superposed on the torsional oscillations
associated with the 11-year cycle.                     is a persistent cell-like or wave-like pattern

with a geometric scale of roughly 10 percent       flow itself has been exceedingly difficult to
the solar diameter. At still smaller scale are     measure. Its form, in both latitude and ra-
supergranules. The supergranule pattern            dius, is still quite uncertain and its time de-
dominates the smaller scale distribution of        pendence is unknown. The poleward surface
the magnetic field and clearly plays a role in     flow extends well into the interior but the
moving magnetic flux across the surface.           equatorward return flow has not been de-
                                                   tected. Better characterizations of the meri-
                                                   dional circulation will aid our understanding The Solar Cycle –                          of the solar cycle.
       Outstanding Questions
                                                   What role does the torsional pattern play
                                                   in the solar dynamo? The extension of the
What mechanisms drive the quasi-
                                                   torsional oscillation pattern well below the
periodic 11-year cycle of solar activity?
This key question leads to others directly         surface suggests that this phenomenon is
concerning the relevant processes them-            more than a small perturbation in rotation
selves – differential rotation, meridional cir-    due to the presence of active region magnet-
culation, torsional oscillations, and cellular     ic fields at the surface. While it is probably
flows.                                             not the driving force behind the dynamo, it
                                                   may very well play a significant role in that
                                                   process and may signal variations in the be-
How are variations in the solar cycle re-
                                                   havior of the solar cycle.
lated to the internal flows and surface
magnetic field? The amplitude of the solar
cycle can vary dramatically from one cycle         Are the small latitudinal surface bright-
to the next. Can we identify an early indica-      ness variations related to the torsional
tor of solar cycle magnitude? We expect            pattern or the meridional flow? Latitudin-
these variations to be related to differences      al bands of enhanced surface brightness are
in the internal flows (e. g., differential rota-   seen in MDI data and the extreme polar re-
tion and meridional flow) and the magnetic         gions appear darker. These features may be
field itself (e. g. polar field strength).         a thermal signature related to the meridional
                                                   circulation or the torsional pattern. Changes
                                                   in their structure and position may hold im-
How is the differential rotation pro-
duced? The processes that produce the dif-         portant clues to understanding the solar
ferential rotation are still uncertain. Hydro-     cycle.
dynamical models for the solar convection
zone reproduce the latitudinal differential        Magnetic Images are needed to follow the
rotation but not its variation with depth or       evolution of the surface magnetic field. The
time. Within these models the differential         magnetograms themselves should cover the
rotation is maintained by angular momen-           visible disk to allow mapping the field to all
tum fluxes driven by large-scale convective        latitudes and longitudes. The sensitivity
flows. The detection of these cellular flows       should be sufficient to follow changes in
and confirmation of the presence of the            quiet areas as well as in active regions and
momentum flux would help our understand-           to accurately measure the polar flux.
ing of the solar cycle.
                                                   Helioseismic Images are required for stu-
What is the structure of the meridional            dies of the structures and flows in the solar
flow and how does it vary? The meridional          interior. Helioseismic probing of the critical

near surface layers (a region that all acoustic Active Region Evolution
waves travel through and, because of the                 – Current Understanding
lower sound speed, spend much of their time
in) requires high spatial resolution and rapid
                                                  The probability of the appearance of an ac-
cadence observations. Probing of the three-
                                                  tive region can be given as a function of the
dimensional structures deep in the convec-
                                                  size of the region for a given latitude and the
tion zone requires good spatial resolution
                                                  phase and amplitude of the solar cycle. The
near the limbs to allow for widely separated
                                                  number distribution of active region flux has
measurement areas.
                                                  been well measured through the solar cycle.
                                                  It is an exponential function of the flux that
Atmospheric Images are needed in con-
                                                  does not change slope over the solar cycle.
junction with the magnetograms to deter-
                                                  The amplitude of the function, and hence the
mine the large-scale magnetic field configu-
                                                  total active region flux, changes by about a
ration associated with the solar cycle.
                                                  factor of eight from solar minimum to max-
                                                  imum depending upon the amplitude of the
Coronagraphic Images are also needed in
conjunction with the magnetograms to de-
termine the large-scale magnetic field confi-     Once an active region emerges, there is a
guration associated with the solar cycle.         high probability that additional eruptions of
                                                  flux will occur in the neighborhood, or even
Photometric Images are needed to deter-           in the same region (activity nests, active
mine the actual source of the solar cycle var-    longitudes). There is some weak evidence
iations in irradiance. These images may also      that activity tends to concentrate around the
reveal thermal structures associated with the     edges of giant cellular patterns. Once an ac-
solar dynamo process.                             tive region starts to become visible, its ulti-
                                                  mate size and complexity can be estimated
                                                  by the rate of flux growth and the complexi-
                                                  ty of its early appearance. However, none of
3.2.2 Active Region Evolution                     these predictions are based on physical
                                                  models for the emergence of active region
Active regions are the most visible evidence      flux. There is a clear need for a better under-
of solar variability. The intense magnetic        standing of active region formation.
fields in active regions are the source of
flares and most CMEs. The chromospheric           The source of the active region flux is
and coronal structures that accompany ac-         thought to be in the tachocline, the shear
tive regions are the source of irradiance var-    layer just below the convection zone. Diffe-
iations in the UV, EUV, and X-rays. Under-        rential rotation should amplify the field until
standing and predicting the structure and         the magnetic energy density is so large that
evolution of active regions is key to our un-     it becomes buoyant and rises to the surface.
derstanding and predictions of explosive          The magnetic fields must be amplified to
events and spectral irradiance variations.        about 105 Gauss to become sufficiently
Detection of emerging magnetic structures         buoyant to begin their rise. Typically, active
in the interior and understanding their physi-    region magnetic flux emerges through the
cal properties can make important contribu-       surface in the form of sunspot groups and
tions to space weather forecasting.               plage. The spots grow in size over the
                                                  course of days while the preceding and fol-

lowing spots drift apart in longitude. Active
regions have characteristic tilts with respect   As to the prediction of specific active re-
to lines of constant latitude and asymmetries    gions at specific times and places, very little
in the morphology of the preceding and fol-      can be done at present. Searches for thermal
lowing spots.                                    shadow precursor signals in advance of the
                                                 eruption of an active region have been in-
Models of buoyant flux tubes have been           conclusive using currently available obser-
constructed to follow their rise from the bot-   vations. Helioseismic tomography can be
tom of the convection zone to within about       used to probe the flow field beneath active
10,000 km of the solar surface. Above that       regions (Fig. 3.11). These techniques might
height the rate of change of pressure is so      detect emerging active structures in the deep
great that current computers are not suffi-      interior before they are visible. However, a
cient for the calculations required. (Six of     clear case of a precursor signal has not been
the fifteen scale heights between the bottom     observed. Initial results using the MDI high-
and top of the convection zone occur in the      resolution data indicate that the flux propa-
top 10,000 km.) These models show how the        gates rapidly, in less than 8 hours, through
effects of solar rotation can produce active     the top 10,000 km of the convection zone.
regions with both tilt and morphological         One could speculate that this rise time is suf-
asymmetry between preceding and follow-          ficiently rapid and the cadence of existing
ing spots. However, these models neglect         observations is too slow to detect a precur-
the turbulent motions within the convection      sor signal. Alternatively, the precursor sig-
zone. Even neglecting turbulence it is diffi-    nals may simply be too weak to detect. Sur-
cult to create a stable rising flux bundle.      face Doppler measurements do, however,
                                                 show persistent convergence at the sites of
Observations of the interior rotation (Fig.      active regions prior to their emergence.
3.10) show that there is a sharp decline in      These observational techniques provide
the angular rotation rate over the top 10,000    great promise for furthering our understand-
km of the convection zone. Given the high        ing of active regions.
shear between the surface and the deeper
convection zone it is hard to understand how
the active regions that result from flux
emergence can long remain connected to
their origin in the tachocline.

MDI high-resolution measurements have
demonstrated that it is possible, using time-
distance helioseismology and acoustic to-
mography, to form images of the critical top
10,000 km. Time sequences of such images         Figure 3.11. The material flows at the depth of 0-3
should do much to increase our understand-       Mm (a) and at 6-9 Mm (b). The outline of the sunspot
ing of active region flux emergence and the      umbra and penumbra is shown. The colors are vertic-
subsequent detachment of this flux from the      al motion with positive values (red) corresponding to
deep convection zone. The behavior of the        downflow. The arrows are horizontal motion. Mea-
                                                 surements like these may reveal active regions before
flux in the top 10,000 km of the Sun may be      they emerge.
key to the understanding of the solar dyna-
mo process that generates the active region      Another recent success is the new ability to
flux.                                            make crude images of sound speed anoma-

lies near the surface on the far side of the           Can active region magnetic flux be ob-
Sun (Fig. 3.12). Thus, it has become possi-            served before it erupts through the sur-
ble, in principle, to predict that a large active      face? Helioseismic studies can see changes
region will rotate onto the visible disk days          in the sound speed below active regions but
to weeks before this happens.                          have not seen these variations before they
                                                       erupt through the surface.

                                                       To what extent are the appearances of
                                                       active regions predictable? An important
                                                       goal for SDO is to develop an understanding
                                                       of the locations and times of magnetic re-
                                                       gion eruption that goes beyond simple statis-
                                                       tics. Can we go beyond empirically identify-
                                                       ing the locations of activity?

                                                       What roles do local flows play in active
                                                       region evolution? Some aspects of active
                                                       region evolution are obviously dependent
                                                       upon the characteristics of the emerging flux
                                                       itself. Other aspects depend upon the sur-
                                                       rounding flows that are themselves influ-
                                                       enced by the presence of the active region.

                                                       Magnetic Images are required to determine
                                                       the configuration of the surface magnetic
                                                       fields associated with active regions. Vector
                                                       magnetograms are required to provide a rea-
                                                       listic assessment and to indicate the presence
                                                       of electrical current systems and non-
                                                       potential magnetic field systems. Vector
                                                       magnetograms will also give evidence as to
Figure 3.12. Helioseismic maps of the Sun. This set    whether flux leaves the surface predomi-
of three maps constructed 13.5 days apart shows that   nantly as bubbles, or whether it is principal-
the “big spot” (AR9393) was seen on the far side
before it was seen on the Earth side.                  ly the outcome of local annihilation of fields
                                                       of opposing polarity. High cadence MDI
                                                       observations have shown that the magnetic
                                                       configuration changes in intervals as short as Active Region Evolution                        minutes. Therefore the fields must be meas-
                                                       ured on a comparable cadence in order to
       – Outstanding Questions                         separate temporal and spatial variations.

How is magnetic flux synthesized, concen-              Helioseismic Images are needed to con-
trated, and transported to the solar sur-              struct subsurface maps with sensitivity and
face where it emerges in the form of                   turnaround time superior to present capabili-
evolving active regions?                               ties. We also need helioseismic mapping of
                                                       the far side of Sun with fast turnaround time.

Helioseismic probing of the critical near sur-   of the energy that heats the corona. In order
face layers (a region that all acoustic waves    to understand the distribution of fields on
travel through and, because of the lower         the Sun we must identify the types of
sound speed, spend much of their time in)        sources of magnetic flux, the mechanisms
requires high spatial resolution and rapid       for the spreading the flux, and the manner in
cadence observations. Probing of the three-      which flux disappears from the surface. The
dimensional structures deep in the convec-       various mechanisms of flux disappearance
tion zone requires good spatial resolution       are key to understanding the local heating of
near the limbs to allow for widely separated     the outer atmosphere, the acceleration of the
measurement areas.                               solar wind, and the contribution these ele-
                                                 ments make to the solar irradiance varia-
Atmospheric Images are needed in con-            tions.
junction with the magnetograms to deter-
mine the magnetic field configuration asso-
ciated with active regions and their evolu-
tion. Simultaneous observations in several
temperature regimes are needed to follow
the evolution of the overlying loop systems.
The rapid evolution (seconds) of these loops
requires rapid cadence.

Coronagraphic Images are also needed in
conjunction with the magnetograms to de-
termine the magnetic field configuration as-
sociated with active regions and their evolu-    Figure 3.13. Small-scale magnetic elements (red and
tion.                                            blue contours) and the coronal network bright points
                                                 (SOHO/EIT image). The bright coronal features are
Photometric Images may reveal thermal            well correlated with magnetic elements in general
                                                 and magnetic dipoles in particular.
structures associated with the active regions
before they emerge.
                                        Small-Scale Magnetic
UV/EUV Spectra are needed to provide in-                Structure – Current Un-
formation on the physical characteristics of            derstanding
atmospheric structures within active regions.
These measurements may provide important         Away from active regions, most of the pho-
information on reconnection events asso-         tospheric magnetic flux appears to exist in
ciated with the evolution of active region       the form of flux tubes. These structures have
magnetic fields.                                 complex dynamics. They form when a suffi-
                                                 cient amount of flux is aggregated by local
3.2.3 Small-Scale Magnetic                       velocity flows to affect the transport of
      Structure                                  thermal energy. A cooling and subsequent
                                                 collapse of the mass in the tube leads to a
While the large-scale distribution of magnet-    compression of the field into a kiloGauss-
ic flux on the Sun determines the coupling       strength configuration. This structure pers-
from the Sun to the heliosphere, the small-      ists until torn apart by convection.
scale mixed polarity flux may provide most

An unknown amount of flux exists outside          Outside plage, in the quiet Sun, the average
of active regions and the strong flux tubes.      field flux is about 5 Gauss. Because about
Some estimates indicate that the total flux       90% of the flux in the quiet Sun also has
may be as much as or more than that con-          field strength of a kilogauss, the quiet Sun
tained in active regions over the solar cycle.    filling factor is about 0.5%. Subsurface im-
This pattern of mixed polarity has been cha-      aging should collect data that will allow in-
racterized as a “salt and pepper” pattern or      sights into the structure of the flow and
the “magnetic carpet”. How it is created,         magnetic fields below the plage that give
how it evolves, and how it is destroyed are       raise to the remarkable bimodal character of
all uncertain. Presumably it is a manifesta-      plage and quiet Sun.
tion of a local dynamo at work in the upper
photosphere, but its relation to other scales
of magnetic activity is not known.

Observations during the last few years have
established that a significant fraction of the
observed flux on the solar surface emerges
in ephemeral regions – bipolar regions with
lifetimes of hours. The ephemeral region
number distribution, like the active region
number distribution, is also exponential. Al-
though it has not been as well measured as
the active region flux over the cycle, there is
strong indication that the amplitude of the
cyclic variation is a factor of two or less.
Consequentially, the ratio of total active re-
gion to ephemeral region flux varies consi-
derably during the cycle. Another compo-          Figure 3.14. Histogram for the number of magnetic
nent of the flux is the inter-network field.      regions as a function of size (area). Ephemeral re-
This component has not yet been well cha-         gions (open circles) appear as an extension of the
                                                  distribution function for active regions (closed cir-
racterized. There is evidence that the field      cles).
strength of the inter-network field is 500
Gauss or less, while the active region and        Once field elements leave the plage, they are
ephemeral region magnetic structures, ex-         dispersed over the surface by the diffusive
cluding sunspots, have field strengths of         action of convection, by the differential rota-
1100 100 Gauss.                                  tion, and by meridional flows. Linear mod-
                                                  els of flux distribution by diffusion, rotation,
When active region flux emerges it forms          and flows have been developed over the past
plage as well as sunspots. Plage are regions      40 years and have done an excellent job of
in which the average magnetic flux is 125        predicting the mean characteristics of the
25 Gauss. Since the typical magnetic ele-         surface flux distribution. In particular, given
ments in plage have field strengths of about      the observed plage strength, the mean field
a kilogauss, the magnetic filling factor for      with a spatial resolution of a few arc-
plage is about 12%. Quite remarkably, plage       minutes (see Fig. 3.9) and the mean field at
maintain their average field strength as the      the pole can be predicted. However, these
active region grows and finally disappears.       models do not properly reproduce the num-

ber distribution of flux, the total flux with    disappear in days without continuous re-
resolutions of a few arcseconds, or the spa-     placement. The most recent measurements
tial distribution of flux over the surface.      of the ephemeral flux emergence rate yields
                                                 about 5 1023 Mx/day, which is equivalent to
By proper choice of the value of the diffu-      10 to 20 large active regions. This rate is
sion coefficient, the speed of the meridional    sufficient to replace the quiet Sun network
flow, and the latitude that the meridional       in less than a day and the flux in an active
flow terminates, these linear models produce     region, if the emergence rate is the same as
remarkable agreement with low-resolution         in quiet Sun, in a few weeks. Because the
observations of the flux distribution over the   rate of emergence of ephemeral flux is so
surface. Unfortunately, the value of the dif-    high, 10 to 30% of the quiet Sun flux may
fusion coefficient that is required by the       reside in the cell centers just because the
models is higher by a factor two than the        travel time to the boundary is only a small
measured values reported in the literature.      fraction of the total flux lifetime on the sur-
This is not very serious because the majority    face.
of the measurement techniques for determin-
ing the diffusion constant strongly weight       Ephemeral region flux emerges inside su-
high concentrations of flux. Recent MDI          pergranulation cells and is transported to the
observations have shown that the dispersal       cell boundary where is cancels with existing
of flux is a function of the magnitude of the    flux in the magnetic network. This means
size of the flux concentration and that larger   that ephemeral flux is not just the result of
flux concentrations disperse more slowly         magnetic loops emerging through and then
than small ones. (Of course, these results       sinking back below the solar surface. Rather
mean that the uses of a fixed diffusion con-     ephemeral regions must disappear by some
stant and consequently linear model of the       type of reconnection process. There are a
surface spreading of magnetic field are not      number of methods for estimating the ener-
sufficient for an accurate description of flux   gy released in the reconnection. But using
dispersal.) Much more serious is that both       the initial field strength and the foot point
the strength and the latitudinal structure of    separation of the emerging loop to estimate
the meridional flow used in the models are       a magnetic volume energy, the result is 3x
not consistent with the observations. The        107 ergs/cm2/sec. This energy emergence
poleward flow speed and the latitude of the      rate is two orders of magnitude more than
upper cutoff control both the diameter and       necessary to heat the corona, but most of the
the magnetic flux of the polar cap field.        energy maybe dissipated in the chromos-
There are strong indications from both           phere and transition region.
ground and MDI measurements that these
poleward current systems may also vary           There is clear evidence from very high-
both in longitude and time.                      resolution filter magnetograms that magnetic
                                                 field emerges on the scale of granulation.
Kinematic modeling that includes the super-      Spectropolarimetry indicates that there is a
granulation flow shows that there must be a      component of the field with a strength that is
constant eruption of new flux to maintain        probably less than 500 Gauss, and that there
the measured total flux observed with a          is a significant amount of weak horizontal
resolution of a few arcseconds. Constant         field in quiet Sun. Taken together these ob-
eruption of flux is also required to maintain    servations constitute a strong suggestion that
the observed quiet Sun network. Simula-          there are surface magnetic fields associated
tions show that the magnetic network would       with every form of convection. Numerical

simulations indicated that a weak seed field     of undulating flux ropes with the photos-
is amplified sufficiently by convective flows    phere.
to generate a mixed polarity field that con-
tains 25% of the kinetic energy originally in    Magnetic Images are required to identify
the convection.                                  and to follow the evolution of these ele-
                                                 ments. Full-disk observations with enough
Local dynamo action is extremely interest-       spatial resolution (~1″) to identify them will
ing as a process that has the capacity to con-   be needed to determine their characteristics.
tinuously generate energy that can then be       The temporal cadence should be fast enough
used to heat the chromosphere, transition        to follow their evolution and interactions (~
region and corona. In addition models sug-       5 min is probably the best that can be
gest that local and global dynamos can           achieved). Longitudinal magnetograms
couple and thus produce variations in the        should be sufficient for most studies involv-
length and amplitude of the solar cycle.         ing the small-scale magnetic elements but
                                                 vector magnetograms will be required to un-
                                                 derstand the flux ropes forming the small- Small-Scale Magnetic                     scale elements.
        Structure – Outstanding
        Questions                                Helioseismic Images are needed to deter-
                                                 mine the nature of the fluid flows that sur-
How does magnetic reconnection of solar          round these elements and to examine their
magnetic fields on small spatial scales re-      sub-surface structure. Since flows close to
late to coronal heating, solar wind accele-      the surface will be the most significant, the
ration and the transformation of the             oscillation images will need the spatial (~1″)
large-scale field topology? There are indi-      and temporal (<50 sec) resolution to resolve
cations that the interactions between oppo-      them and their temporal evolution.
site polarity elements contribute to coronal     Atmospheric Images are needed to deter-
heating. These interactions may also provide     mine the effects of these elements on coron-
the triggers for eruptive events that restruc-   al and chromospheric heating. These images
ture the large-scale field.                      should also have the spatial resolution (~1″)
                                                 to identify the elements and the temporal
How are the small-scale magnetic features
produced? There is now good evidence for         resolution (~10 sec) to follow their evolu-
magnetic dynamo activity in the thin layer       tion. These images need to cover a wide
just below the photosphere. There is strong      range of temperatures to determine the ef-
shear across this layer and the rate at which    fects of these elements on atmospheric heat-
magnetic flux appears and disappears sug-        ing and dynamics.
gests local generation.                          UV/EUV Spectra are needed to provide in-
                                                 formation on the physical characteristics of
What are the dynamics of magnetic struc-
tures on very small spatial scales? Several      atmospheric structures above these small-
mechanisms may be involved in the appear-        scale magnetic elements. These measure-
ance, disappearance, and interactions be-        ments should provide important information
tween these small magnetic structures. High-     on events associated with coronal heating by
resolution vector magnetograms show that         these magnetic elements.
many magnetic elements seen in longitudin-
al magnetograms may just be intersections

                                                  return from SDO but should not be included
4 Required Observations                           at the expense of higher priority observa-
An array of observations is needed to ad-
dress the scientific questions posed for SDO.     The highest priority observations include
Each of these observations and their charac-      (from the interior outward):
teristics can be traced back to one or more of         Helioseismic Images
the outstanding questions stated in the pre-           Longitudinal Magnetograms
vious section. (Note that all six of the scien-        Atmospheric Images
tific areas discussed in the previous Section          EUV Spectral Irradiance
are deemed critical to the SDO Mission. The            Total Solar Irradiance (should con-
scientific links among the different areas                current measurements not be availa-
suggest that all would suffer from the omis-              ble from other platforms)
sion of any one.) Since single instruments        The high priority observations include:
might be designed to provide more than one             Photometric Images
measurement (e.g., helioseismic images and             Vector Magnetograms
magnetograms), we present the requirements             UV/EUV Spectra
for the measurements themselves in the fol-            Coronagraphic Images
lowing sub-sections. Potential instruments        The important observations include:
and the possible allocation of resources are
                                                       Coronal Spectroscopy
given in Section 5.
                                                       Heliometry
                                                  Further refinement of priorities within these
All of these observations are important for
                                                  groups is unproductive at this phase of the
one or more of the scientific areas to be ad-
                                                  mission definition. The actual resources re-
dressed by SDO. However, some are
                                                  quired to obtain the measurements, the ac-
deemed to be of higher priority than others
                                                  tual instruments to be flown on other mis-
for a variety of reasons. Some of these ob-
                                                  sions, and other programmatic considera-
servations are either unique to SDO or have
                                                  tions could easily change those priorities.
requirements that can be uniquely met by
SDO (e.g. full-disk spatial coverage, conti-
nuous time coverage over several years, rap-      4.1 Helioseismic Images
id cadence, high bandwidth). Other observa-
tions may be provided by other programs,          Helioseismic images are required for studies
may be more speculative in nature, or may         of the Sun‟s internal flows and structures.
be more difficult to obtain with current          This impacts all aspects of SDO science.
technology and resources. We recognize the        Helioseismic images provide information on
need for setting priorities for SDO and have      solar radius changes for irradiance studies
done so by grouping the observations and          (Section and on sub-surface flows
their respective instruments into three priori-   that drive magnetic field evolution asso-
ty categories: highest priority – SDO must        ciated with flares and CMEs (Section
make these observations; high priority – and with coronal structures and so-
due effort should be made to include these        lar wind variations (Section He-
observations but for one reason or another        lioseismic images provide critical measure-
(as discussed in the following sub-sections)      ments of the internal flows that drive the so-
the mission could go on without them; im-         lar cycle (Section, active region
portant – these would enhance the scientific      evolution (Section, and the dynam-
                                                  ics of the small-scale magnetic elements

(Section One of the highest priori-    Field Of View. In order to detect fields and
ties for SDO is to obtain the helioseismic       flows nearer the base of the convection
observations required to provide a detailed      zone, we need simultaneous observations
view of the upper 10-20 Mm of the convec-        across the full disk (again 75° from disk
tion zone. These measurements are of high-       center is a reasonable goal, this is 0.97 in
est priority for SDO. They will not be avail-    units of the radius).
able with the required spatial resolution and
full-disk coverage from any other source.        Completeness. Gaps in the data stream
                                                 compromise the helioseismic objectives.
Accuracy. Signals near the base of the con-      Experience with MDI and GONG indicate
vection zone likely generate wave travel         that nearly complete coverage (more than
time variations as small as 0.01 second. The     95% of the time) is needed.
total travel time approaches 2 hours so the
timing accuracy must be on the order of a        Table 4.1: Measurement Characteristics
part in 106 over intervals of hours to days.     for the Helioseismic Imager

Dynamic Range. Previous experience               Observable              Osc. Time Series
shows that the best signal-to-noise can be       Accuracy (Clock)               10-6
obtained with Doppler observations as com-       Dynamic Range                13 km/s
pared to brightness observations in the 1 to 5   Time Cadence                < 50 sec
mHz region. Therefore dynamic range is           Spatial Resolution          1 arcsec
discussed here in terms of velocity. The in-     Field of View               Full Disk
strument must have a velocity dynamic            Duration                    10 years
range large enough to accommodate both the       Completeness            99.99% coverage
spacecraft orbit and the Sun‟s rotation. The                               95% of time
orbit will have a range of about 6 km/s and
the Sun adds about 7 km/s from East to
                                                 4.2 Longitudinal Magneto-
West limbs giving a dynamic range of about
13 km/s.                                             grams

Time Cadence. There is useful wave power         Magnetic images are critical for all of the
up to about 8 mHz and measurable power to        scientific areas to be addressed by SDO.
at least 12 mHz. A sample with a 10 mHz          Longitudinal magnetograms are needed: to
Nyquist frequency leaves the spectrum clean      distinguish between magnetic and non-
up to 8 mHz. This gives a time cadence of        magnetic sources of the irradiance variations
about 45 seconds per measurement.                (Section; to determine the configu-
                                                 ration of the magnetic fields associated with
Spatial Resolution. A reasonable goal is to      flares and CMEs (Section and solar
sample the wave-speed profile and flows          wind variations (Section; to follow
with about 1 Mm depth resolution in the up-      the evolution of the magnetic field over the
per 10-20 Mm of the convection zone. This        solar cycle (Section and as active
requires sample points 3 Mm apart. At 75°        regions evolve (Section; and to
from disk center, this corresponds to about 1    identify and follow the small magnetic ele-
arcsec resolution. This can be provided with     ments (Section Longitudinal mag-
0.5 arcsec detector pixels.                      netograms are of highest priority to SDO.
                                                 Full-disk, high-resolution magnetograms
                                                 will not be obtained with the required tem-

poral resolution and coverage by any other
means.                                          4.3 Atmospheric Images
Precision. A pixel-by-pixel precision be-       Atmospheric images are required for all as-
tween 5 and 50 G over a 5 minute integra-       pects of the SDO mission. They indicate the
tion is needed to follow the evolution of the   sources of the EUV irradiance variations
small-scale magnetic elements.                  (Section, the occurrence and nature
                                                of flares and CMEs (Section, and
Accuracy. The disk integrated magnetic          the structure and dynamics of the low coro-
flux should be measured with an accuracy of     na (Section They reveal the mag-
about 0.1 G. This level of accuracy allows      netic structures associated with the solar
for studies of active region flux imbalances    cycle (Section, active region evolu-
and determinations of the global field confi-   tion (Section, and small-scale mag-
guration.                                       netic elements (Section These ob-
                                                servations are of highest priority to SDO.
Dynamic Range. Sunspot umbrae harbor            Full-disk images at the required spatial and
the strongest magnetic fields with peak val-    temporal resolution will not be obtained by
ues of about 3000 G. A dynamic range of 6-      any other means.
7 kG would capture even the strongest flux
concentrations.                                 Precision/Accuracy. A photometric accura-
                                                cy of about 10% is needed for these images
Time Cadence. Individual magnetograms
                                                to be useful as a tool for the determining the
should be obtained at a cadence of at least 1
                                                sources of the spectral irradiance. Similar
minute to account for the Doppler shifts due
                                                accuracy is needed in making useful ratios
to the 5-min oscillations.
                                                of the images to determine physical charac-
Spatial Resolution. High spatial resolution     teristics of the atmospheric features. These
(~1 arcsec) is required to identify the small   imagers must be properly calibrated to in-
intranetwork magnetic elements.                 sure the stability and repeatability of the
                                                measurements at the 10% level.
Field Of View. Full-disk measurements are
required to provide the global field configu-   Dynamic Range. A high dynamic range
ration, to monitor all active regions, and to   (103 or greater) is needed to provide obser-
provide complete coverage for studies of        vations with sufficient signal to noise and to
irradiance and small magnetic features.         span the range of emission features from
                                                intra-network loops to bright active regions.
Table 4.2: Measurement Characteristics
                                                Time Cadence. An important diagnostic of
for the Longitudinal Magnetograph
                                                coronal activity may lie with transient flow
                                                patterns that are perhaps created by short-
Observable                 Longitudinal B
                                                term heating events. The flow velocities on
Precision                  5-50 G / 5 min
                                                the order of 100 km/s, near the sound speed,
Accuracy                        0.1 G
                                                are often observed. A cadence of 10
Dynamic Range                Several kG
                                                seconds would be sufficient to track these
Time Cadence                   ~ 1 min          flows from resolution element to resolution
Spatial Resolution            1 arcsec          element between consecutive exposures.
Field of View                 Full Disk
Duration                      10 years

Spatial Resolution. We know from mea-             needed to effectively span this range is un-
surements with HRTS and SUMER that the            certain. The minimum is probably four but
atmospheric structures are highly filamenta-      seven would be far more useful.
ry. TRACE observations show that with 1
arcsec spatial resolution, solar structures are   Table 4.3: Measurement Characteristics
recorded with sufficient clarity that much of     for the Atmospheric Imagers
the confusion produced by the fine structure
is overcome. CCDs with a 4096x4096 for-           Observable                     Intensity
mat will soon become available and would          Precision/Accuracy               10 %
provide a 1.2 arcsec resolution over the 40       Dynamic Range                  103 – 105
arcmin field of view.                             Time Cadence                    10 sec
                                                  Spatial Resolution            1.2 arcsec
Field Of View. Full-disk images are re-           Field of View               40 x 40 arcsec2
quired to observe flares and CMEs and to          Spectral Resolution           / ~ 20
identify sources of EUV irradiance variabili-     Temperature Range            0.02 – 4 MK
ty. CMEs are often large-scale events that
can involve a significant portion of the solar
disk. In addition, it would be useful to ob-
                                                  4.4 EUV Spectral Irradiance
serve the initial acceleration of CMEs near
                                                  The EUV irradiance and its variations influ-
the solar limb with some overlap with the
                                                  ence ionospheric and thermospheric compo-
coronagraph. Consequently, a field of view
                                                  sition, density, and temperature. The need
(FOV) of 40 arc-min (1.25 RSun) is speci-
                                                  for continuous observations of these varia-
fied.                                             tions (Section drives the SDO re-
                                                  quirements for an EUV Spectral Irradiance
Spectral Resolution. Relatively high spec-        Monitor. These observations are of highest
tral purity (resolving power of 20 or so) is      priority to SDO. While EUV spectral irra-
needed so that the individual images refer to     diance observations will be obtained by the
only a small temperature range.                   SEE instrument on the TIMED mission,
                                                  these observations will not be concurrent
Temperature Range. Solar activity often           with the SDO mission and are of lower spec-
involves the nearly simultaneous rapid evo-       tral and temporal resolution. The SDO EUV
lution of plasmas over a wide range of tem-       observations play a critical role in helping us
peratures, such as hot flare plasmas, the         to understand how solar variability influ-
eruption of cool prominence material and          ences the ionosphere and thermosphere.
the reconfiguration of coronal loops during a     Tracking the solar cycle variations, and the
flare or CME event. To understand the dy-         possibility of longer-term secular variations
namics of the coronal field, we must be able      is desirable. The observations should be ob-
to trace the loops as they evolve in tempera-     tained continuously to maintain a calibrated
ture. That requires narrow pass bands be-         time series, and to provide continuous inputs
cause only those allow the clear identifica-      for geophysical studies undertaken else-
tion of loops while minimizing the line of        where within the LWS program.
sight confusion. The ability of the Atmos-
pheric Imagers to observe plasmas over a          Precision/Accuracy. The absolute uncer-
wide range of temperature (0.02 – 4 MK)           tainty (accuracy) translates into uncertainties
will help untangle the relevant physical          in geophysical parameters (densities, tem-
processes. The number of image bandpasses         peratures) calculated by geophysical models

that use these inputs, so the goal is maxi-       photoelectron production and heating, re-
mum accuracy. Accuracies of 10% reflect           quires input spectra with 0.1 nm resolution.
the state of the art irradiance calibrations
using synchrotron irradiance standards. De-       Spectral Range. Understanding ionospheric
gradation of the optics and detectors forces a    and neutral density responses to solar varia-
requirement for some on-board calibration         bility requires knowledge of the solar energy
as well. The measurements should have             inputs over a spectral range from 1-120 nm
long-term stability on the order of 5% for        in both lines and continua.
useful solar cycle studies.
                                                  Table 4.4: Measurement Characteristics
Dynamic Range. The solar cycle variability        for the EUV Spectral Irradiance Monitor
ranges from 50 percent to two orders of
magnitude over this spectral range, with          Observable              Spectral Irradiance
even larger variations (a factor of up to 1000    Precision/Accuracy            10 %
in some lines) occurring during flares. A dy-     Dynamic Range                  103
namic range of about 1000 can be obtained         Time Cadence                  10 sec
with a combination of detector dynamic            Spatial Resolution            None
range and different exposures.                    Field of View                   1
                                                  Spectral Resolution         ~ 0.1 nm
Time Cadence. Synchronization with the            Spectral Range             1 – 120 nm
SDO Atmospheric Imagers is desirable for
tracing the flux to the magnetic (and possi-
bly other) sources of its variability and for     4.5 Photometric Images
formulating physical mechanisms. This in-
dicates a similar time cadence of about 10        Photometric image data is essential to find-
sec for the EUV irradiance measurements.          ing the origins of solar irradiance and lumi-
                                                  nosity variability (Section These
Field Of View. A slightly larger FOV than         images may also reveal thermal structures
the solar disk will minimize pointing error       associated with the solar cycle (Section
problems. The FOV should be sufficient to and active region evolution (Section
capture off-limb radiation that may contri- These observations are of high
bute to the flux of some hot coronal lines.       priority for SDO, as they address three im-
                                                  portant areas. Data of high photometric sen-
Spectral Resolution. Solar emission lines         sitivity or extended bandwidth will only be
that are close in wavelength can nevertheless     obtained from space and would be unique to
have quite different variabilities if they have   SDO. However, empirical modeling of the
different emission temperatures i.e., sources     sources of the irradiance variations suggests
in different solar atmospheric regions. At        that the sources are well understood and he-
wavelengths longer than 10 nm, line and           lioseismic determinations of solar radius
continuum energy inputs to the upper at-          variations indicate that these variations
mosphere and ionosphere must be separately        probably play only a minor role in produc-
measured and distinguished to achieve the         ing the irradiance variations. The ability to
required geophysical understanding. AURIC         image thermal perturbations associated with
(Atmospheric Ultraviolet Radiation and Io-        active regions before they emerge is some-
nization Code), which is used to calculate        what speculative but may prove to be revo-

It is likely that the photometric mapper (PM)    Table 4.5 Photometric Mapper Characte-
will operate with two channels. In its first     ristics
channel it will achieve the highest possible
photometric accuracy and spatial resolution.     Channel          Photometric    Bolometric
The second channel captures the widest           Observable         Surface      Bolometric
possible spectral coverage (approaching to-                        Intensity      Intensity
tal solar irradiance bolometric spectral cov-    Precision           0.1 %           3%
erage) with sufficient spatial resolution to     Dynamic             >103            30
identify photospheric magnetic contributions     Range
as small as faculae.                             Time Cadence        1 min         1 min
                                                 Spatial            1 arcsec      10 arcsec
Precision. In its first channel the PM must      Resolution
have photometric stability and sensitivity       Field of View     Full Disk      Full Disk
sufficient to detect faint diffuse brightness    Spectral           Narrow         Broad
changes associated with surface magnetic         Resolution          band           band
features like sunspot bright rings, facular      Completeness        95%            95%
shadows and the network irradiance pertur-
bations. Detecting these subtle features will
require a precision of about 0.1%. In its
                                                 4.6 Vector Magnetograms
second channel the PM must have the ability
                                                 Vector magnetograms are needed to deter-
to tally the contributions to the irradiance
                                                 mine: the magnetic stresses and current sys-
variations from the various photospheric
                                                 tems associated with flares and CMEs (Sec-
features. A precision of 3% at each pixel
                                                 tion; the nonpotentiality of coronal
should be sufficient for the number of pixels
                                                 magnetic fields (Section; and the
given by the spatial resolution requirement.
                                                 large-scale magnetic field pattern evolution
                                                 associated with the solar cycle (Section
Dynamic Range. The dynamic range at
                                        and active region evolution (Section
each pixel should be sufficient to capture the
                                        These are high priority observa-
range of intensities from the darkest sunspot
                                                 tions for SDO. They address critical needs in
umbrae to the brightest plage and faculae.
                                                 several areas. However, full-disk vector
                                                 magnetograms with the required spatial res-
Time Cadence. The images should be ob-
                                                 olution may be difficult to obtain with cur-
tained at about 1 minute intervals to resolve
                                                 rent technology and mission resources. Li-
the p-mode oscillation component of the ir-
                                                 mited field of view observations at higher
radiance variations.
                                                 resolution will be acquired with instruments
                                                 on Solar-B (Section 7.2). Full-disk mea-
Spatial Resolution. Photospheric faculae
                                                 surements from the ground will be acquired
should be resolved in these images. A spatial
                                                 with the SOLIS instruments (Section 7.6).
resolution between 1 arcsec and 10 arcsec
                                                 These alternative observations do not, how-
should be sufficient.
                                                 ever, fully satisfy the SDO requirements.
                                                 Solar-B has a very limited field of view and
Field Of View. The full photospheric disk
                                                 SOLIS will have poorer spatial resolution
must be imaged for the mapper to provide
                                                 and temporal coverage.
the observations needed to determine the
sources of the irradiance variations.
                                                 Precision. The precision of the transverse
                                                 field direction measurements should be a

few degrees. This translates into a polariza-
tion precision of ~10-4. The precision of the    4.7 UV/EUV Spectra
field strength measurements should be better
than 50 G for a 10-minute integration time.
                                                 The UV/EUV Imaging Spectrometer will
                                                 provide spectral images and measurements
Accuracy. The vector field measurements
                                                 that reveal quantitatively the dynamics of
should yield the correct vector field direc-
                                                 the solar atmosphere, from the photosphere,
tion in each resolution element to within five
                                                 through the transition region to the corona.
degrees. The longitudinal component accu-
                                                 The UV/EUV Imaging Spectrometer will
racy should be consistent with a disk inte-
                                                 provide quantitative constraints on the phys-
grated flux accuracy of about 0.1 G.
                                                 ical mechanisms associated with: the
                                                 sources of the spectral irradiance variations
Dynamic Range. Sunspot umbrae harbor
                                                 (Section the impulsive release of
the strongest magnetic fields with peak val-
                                                 energy that results in flares and CMEs (Sec-
ues of about 3000 G. A dynamic range of 6-
                                                 tion; and the sources of high-speed
7 kG would capture even the strongest flux
                                                 and low-speed solar wind (Section
                                                 UV/EUV spectra are also needed to provide
                                                 information on the physical characteristics
Time Cadence. Polarimetric scans should
                                                 of atmospheric structures in active regions
be obtained at a cadence that allows the
                                                 (Section and above small-scale
Doppler shifts due to the 5-minute oscilla-
                                                 magnetic elements (Section These
tions to be removed or compensated for.
                                                 observations are of high priority to SDO.
Full-disk vector magnetograms should be
                                                 They are required to address several out-
obtained at a cadence of about 6 per hour.
                                                 standing questions. Although there is an
                                                 EUV Spectrometer planned for the upcom-
Spatial Resolution. High spatial resolution
                                                 ing Solar-B mission (Section 7.2), it is not
(~1 arcsec) is required to identify the small
                                                 ideally suited to the SDO scientific objec-
magnetic elements.
                                                 tives for two reasons. First, the Solar-B EIS
                                                 is primarily a coronal spectrometer, with
Field Of View. Full-disk measurements are
                                                 only one strong emission line below 106 K.
required to provide the global field configu-
                                                 Second, the estimated count rates of the So-
ration, to monitor all active regions, and to
                                                 lar-B EIS are too low to match the temporal
provide complete coverage for studies of
                                                 cadence of the Atmospheric Imager Array.
irradiance and small magnetic features.
                                                 The SDO UV/EUV Imaging Spectrometer
                                                 nicely complements the Atmospheric Imag-
Table 4.6: Measurement Characteristics
                                                 ing because it provides quantitative observa-
for the Vector Magnetograph
                                                 tions with the spatial and temporal resolu-
                                                 tion necessary to resolve ambiguities seen in
Observable                    Vector B
                                                 the imager observations.
Transverse Precision         50 G (~3°)
Polarimetric Precision          ~10-4            Precision/Accuracy. Spectral intensity
Dynamic Range                Several kG          should be measured to within 10% to contri-
Time Cadence                  ~ 10 min           bute to the spectral irradiance studies. Line
Spatial Resolution            1 arcsec           widths should be measured to within about
Field of View                 Full Disk          10% to provide useful information on non-
Duration                      10 years           thermal broadening associated with physical

mechanisms in the relevant features. Dopp-
ler velocities should be measured to within      Table 4.7: Measurement Characteristics
1-5 km/s to determine the nature of the dy-      for the UV/EUV Imaging Spectrometer
namic events.
                                                 Observable                   Line Profiles
Dynamic Range. Spectral intensity varies         Precision/Accuracy         Intensity 10 %
widely from one spectral feature to another                                    Width 10%
and from quiet sun to active plage. A dy-                                  Velocity 1-5 km/s
namic range of 103 to 105 should capture all     Dynamic Range                  103 – 105
these features.                                  Time Cadence                    10 sec
                                                 Spatial Resolution            ~1 arcsec
                                                 Field of View              16 to 34 arcmin
Time Cadence. Ideally, the time cadence of
the UV/EUV Imaging Spectrometer should           Spectral Resolution         / ~ 30,000
match the 10 sec cadence of the Atmospher-       Temperature Range            0.02 – 4 MK
ic Imaging Array. This can be accomplished
with limited FOV raster motions about a          4.8 Coronagraphic Images
given location (e.g. central meridian) or tar-
get (e.g. active region), repeated throughout    Coronagraphic images are important for
the majority of the observing period. Synop-     many aspects of the SDO Mission. These
tic observing programs providing raster im-      images indicate the occurrence of CMEs
ages of the entire disk may also be per-         (Section and reveal the structure of
formed periodically.                             the corona and presence of solar wind varia-
                                                 tions (Section They also provide
Spatial resolution. A spatial resolution of      important information on changes in the
1.3 arcsec should be sufficient to resolve the   magnetic configuration of the corona asso-
facular elements associated with the spectral    ciated with the solar cycle (Section
irradiance variations.                           and active region evolution (Section
                                        The coronagraphic images are of
                                                 high priority to SDO. They provide critical
Field Of View. The routine raster FOV will       information on events and processes asso-
be limited by the necessity to match the time    ciated with several outstanding questions.
cadence of the Atmospheric Imaging Array.        However, the STEREO mission (Section
However, it will also be necessary to ob-        7.1) is designed to address many of these
serve the full solar disk and inner corona by    questions. Nonetheless, coronagraphic im-
slit rasters or other techniques.                ages from SDO will be needed to span the
                                                 duration of the mission and provide informa-
Spectral Resolution. A spectral resolution       tion on the occurrence of CMEs and solar
(resolving power) of 30000 should be suffi-      wind variations.
cient to measure the required line profile
information.                                     The SDO coronagraphic images will proba-
                                                 bly require two separate channels in order to
Temperature Range. The measurements              map the structure of the corona from 1.1
should include spectral features that cover      RSun to 15 RSun. The stray light suppression
the temperature range of the Atmospheric         requirements are such that it is not practical
Imaging Array – 0.02 to 4 MK.                    for a single coronagraph channel to be used
                                                 for the entire range of heights. Further, a de-

sired spatial resolution of 12 arcsec or better   nels respectively, are required and sufficient
at the inner edge of the FOV (1.1 RSun) re-       for accurate determinations of liftoff times,
quires an internally occulted coronagraph         helical motions, and speed profiles for dif-
system. The channel for the outer corona          ferent parts of coronal mass ejections.
should provide overlapping coverage with
the other coronagraph channel. That implies       Spatial Resolution. Observations of CMEs
that this channel has to be an externally oc-     should have a high enough spatial resolution
culted system. The requirement for halo           to discern their small-scale structures.
CME detection is a high priority for the          Plumes and polar rays are on the order of a
LWS mission, in general, and SDO, in par-         few arc-minutes in width but they have fine-
ticular since these are CMEs that affect          scale structure on the sub arc-minute level
Earth and have their origins from regions         that is probably related to their magnetic
directly observed on the disk with the At-        field configurations. A 2048 x 2048 CCD
mospheric Imaging instruments.                    detector would give a spatial resolution of
                                                  6” for the inner coronagraph and 30 arcsec
Precision. The polarization brightness            for the outer coronagraph. This would give
should be measured with a precision of            SDO twice the resolution of LASCO C1 and
about 10% to provide useful information on        a comparable resolution to C2; it is more
coronal structures and their variations.          than adequate to meet the SDO science re-
Dynamic Range. A dynamic range of about
103 captures most of the observed variations      Table 4.8: Measurement Characteristics
in the inner corona. A somewhat wider dy-         for the Coronagraph
namic range of about 104 captures most of
the observed variations in the outer corona.      Channel                Inner         Outer
                                                  Observable           Polarized     Polarized
Field Of View. An inner height of 1.1 RSun                             Intensity     Intensity
is required to overlap the FOV of the At-         Precision              10 %          10 %
mospheric Imaging instruments and thus            Dynamic Range           103           104
allow for continuous tracking of events like      Time Cadence           1 min         5 min
CMEs from their initiation in disk imagers        Spatial Resolution   6 arcsec      30 arcsec
through their development in the extended         Field of View        1.1-3. RSun   2.5-15 RSun
corona. An outer height of about 15 RSun is       Spectral Range        400-700       400-700
needed in order to allow detection of halo                                nm            nm
CMEs that are directed toward the Earth.
Experience from LASCO observations
shows that beyond 15 RSun the propagation
properties of CMEs don‟t change much and          4.9 Total Irradiance
so this outer limit would be acceptable.
                                                  The total solar irradiance must be accurately
Time Cadence. High time cadence will be           and precisely monitored to determine the
required to study the dynamics of coronal         nature and source of the irradiance varia-
disturbances (CMEs, streamer blowouts,            tions (Section These observations
eruptive prominences, etc.). SOHO/LASCO           are of highest priority to SDO but will likely
observations have shown that 1-min and 5-         be obtained from both SORCE (Section 7.4)
min cadences, for the inner and outer chan-       and GOES/NPOESS (Section 7.5) during

the SDO mission and are therefore not in-         Table 4.9: Measurement Characteristics
cluded as part of SDO. If, however, it ap-        for a Total Solar Irradiance Monitor
pears that these observations will not be
provided by these alternative sources then a      Observable                 Total Irradiance
Total Solar Irradiance Monitor should be          Precision/Accuracy              0.01%
placed on SDO.                                    Repeatability              0.001% per year
                                                  Time Cadence                    1 min
Precision/Accuracy. The absolute uncer-           Duration                     Solar Cycle
tainty (accuracy) translates into uncertainties   Completeness                  continuous
in the energy input to the terrestrial climate    Field of View                     2°
system. The goal is maximum accuracy to
minimize uncertainties in climate models,
and also to ensure that the instrument is
                                                  4.10 Coronal Spectroscopy
properly characterized to achieve the needed
high repeatability. The actual solar energy
                                                  Spectroscopic measurements in the extended
input to the radiometer depends on the en-
                                                  corona are needed to for characterizing the
trance aperture, but is typically of order 100
                                                  mechanisms that accelerate CMEs (Section
milliWatt. The ability to measure the change
                                         and the solar wind (Section
in this signal due to changes in total solar
                                                  These observations are important for SDO
irradiance depends, in part, on the noise
                                                  but are limited in scope and would require
floor of the radiometer electronics. This ra-
                                                  significant resources. Spectroscopic mea-
tio sets the dynamic range and it must be
                                                  surements in the corona up to about 2 RSun
sufficient to enable the required repeatability
                                                  can be accomplished with either an internal-
and uncertainty, at a cadence of 1 observa-
                                                  ly occulted coronagraph design (see, e.g.
tion per minute.
                                                  LASCO-C1) or a very sensitive “wide an-
                                                  gle” EUV spectrograph. However spectros-
Time Cadence. The 5-minute oscillations
                                                  copic measurements in the extended corona
affect total solar irradiance, and the instru-
                                                  (beyond 1.5 RSun), require an externally oc-
ment must be capable of resolving these in
                                                  culted telescope (see, e.g. SOHO/UVCS)
                                                  due to the rapid decrease with height of co-
                                                  ronal emission line intensities. A large aper-
Duration. Tracking the solar cycle varia-
                                                  ture, externally occulted coronagraph-
tions, and the possibility of longer term se-
                                                  spectrometer system would have the proper
cular variations, is desirable.
                                                  stray light suppression and sensitivity to al-
                                                  low for the measurement of dozens of faint
Completeness. The instrument should be
                                                  coronal lines out to heliocentric distances of
operated continuously to maintain a cali-
                                                  10 RSun. Such an instrument would be capa-
brated time series, and to provide continuous
                                                  ble of characterizing sites of magnetic re-
inputs for geophysical studies undertaken
                                                  connection and shock formation in CMEs by
elsewhere within LWS.
                                                  observing high charge state ions and non-
                                                  thermal line broadening. It will also measure
Field Of View. A FOV slightly larger than
                                                  velocity distributions of H, He, electrons,
the solar disk will minimize pointing error
                                                  and heavy ions to determine the power spec-
                                                  trum of resonant MHD waves that may be
                                                  responsible for heating and acceleration.
                                                  Helical 3D velocities can be determined

from Doppler shifts and Doppler dimming.           “acoustic” radius fluctuations. The Photo-
Coronal source regions for CME and solar           metric Images may be obtained in a manner
wind plasma can be determined from abun-           that also allows for heliometry. The magni-
dance determinations.                              tude of the radius fluctuation, compared to
                                                   the irradiance change during a solar cycle
While advanced coronagraph-spectrometers           contains important information on where
would be desirable, we felt that such instru-      and how energy is stored. If W is the ratio of
ments could not be accommodated with the           relative radius and irradiance changes, then
present SDO spacecraft resources. Howev-           physical models predict a wide range of val-
er, if an opportunity does arise for flying this   ues for W. Depending on the mechanism and
type of instruments it would be a valuable         depth of the interior perturbation estimates
complement to the SDO Mission.                     of W range from 210-4 to 7.510-2. Given
                                                   a solar cycle irradiance change of about 10-3,
 Table 4.10: Measurement Characteristics           a desirable goal for SDO is to achieve a ra-
for a UV Coronagraph Spectrometer                  dius sensitivity of at least 10 milliarcsec on
                                                   solar rotation time-scales. Such measure-
Observable                  Line Profiles          ments will allow us to clearly discriminate
Precision/Accuracy         Intensity 15 %          between competing solar cycle luminosity
                             Width 10%             variation models.
                           Velocity 5 km/s
Dynamic Range                    105
Time Cadence                   10 min
Spatial Resolution               4”                5 Potential Instruments and
Field of View              4~30”  2000”             Allocation of Resources
Range of View                1.1-10 RSun
Spectral Resolution        / ~ 10,000           We have examined several generic instru-
Spectral Range               28-140 nm             ments for inclusion in the SDO payload to
                                                   estimate the total mass, data rate, power and
4.11 Heliometry                                    volume required to accommodate them.
                                                   These estimates are included in Table 5.
Measurements of small changes in the solar         Several assumptions have been made in ar-
radius and limb shape (heliometry) are             riving at these estimates. The masses do not
needed to determine how and where the              include electronics boxes, mounting, radia-
emergent solar luminosity is gated and             tors, etc. The data rates assume the use of
stored (Section These measure-           image compression. The range of masses,
ments are important for SDO but should not         data rate and volume for the Atmospheric
require the resources of an additional in-         Imaging Array is due to the number of poss-
strument. The Helioseismic Images from             ible telescope tubes.
SDO will be important for observing solar

Table 5. Allocation of Resources

Instrument             Mass            Data Rate       Power            Volume
                        (kg)            (Mbps)          (W)              (cm3)
HMI                      40               25             60            90x40x25
Atmospheric            40-70             20-50           45           100x15x30
Imaging Array                                                         100x45x60
EUV SIM                20-30              <1             45            44x48x21
Coronagraph             30                 1             35           135x17x17
UV/EUV                 40-60             15-30           40           160x60x30
Photometric              30                 5            50           100x30x30
Vector                  10+                 5           20+           90x40x40
Total                 210-270            62-117         295

6 Mission Concept                                  downlink. The large data rate, along with the
                                                   strict limitations on on-board storage capaci-
This section details the spacecraft, launch        ty, result in an effective requirement of con-
vehicle, ground system and data system for         tinuous contact. An inclined geosynchron-
SDO. These items have been discussed as            ous (GEO) orbit will allow nearly conti-
being sufficient to support the science as         nuous observation of the Sun, and can
defined in this Science Definition Team re-        downlink data to a single dedicated ground
port. Future adjustments and variations will       station.
undoubtedly occur, as the mission concept
matures into the design and development            The mission will launch into a geosynchron-
phase.                                             ous transfer orbit (GTO) and then use an
                                                   apogee kick motor (AKM) or other orbital
The science of the Solar Dynamics Observa-         transfer mechanism to boost the spacecraft
tory optimally will be performed on a space-       into geosynchronous orbit. The spacecraft
craft that allows nearly continuous observa-       will be three-axis stabilized and will main-
tions of the Sun and a scientific data rate        tain solar pointing with occasional maneuv-
well in excess of 100 Megabits per second.         ers to unload the momentum accumulated in
These two requirements drive the orbit and         the reaction wheels. Twice a year SDO will
spacecraft specification and the definition of     undergo “eclipse seasons,” which will last 2-
the SDO Mission. Nearly continuous obser-          3 weeks, with a maximum Earth eclipse pe-
vations can be obtained from other orbits,         riod of approximately 70 minutes. Space-
such as a low Earth orbit (LEO), but a LEO         craft maneuvers, eclipses, and occasional
orbit would require on-board storage of            ground system outages will interrupt the
large volumes of scientific data pending           scientific observations.

                                                 6.1 Orbit Selection
Missions of the Living With a Star program
are designed to perform investigations of the    The orbit of the Solar Dynamics Observato-
long-term variations of the Sun-Earth con-       ry will allow a high science data rate (160
nected system. The SDO mission will be           Mbps) and nearly continuous contact via a
designed for a 5-year baseline with expen-       single dedicated ground station. This ground
dables to last an additional five years of an    station can be built and operated at a fraction
extended mission.                                of the cost of a mission that would rely on
                                                 existing ground contact networks. The
Table 6.1 illustrates the mass breakdown         ground track of a geosynchronous orbit with
used in a preliminary definition study for       an inclination of 28.5 degrees orbit is shown
SDO. Compared are the mass estimates for a       in Figure 6.1, projected onto 102 degrees
custom-built spacecraft and a spacecraft         Earth longitude.
from the Rapid Spacecraft Development Of-
fice (RSDO) catalog. An instrument module
mass of 200 kg, an Apogee Kick Motor
(AKM) mass of 668 kg, and the RSDO cata-
log mass estimates were used in the study
calculations. Modifications of a custom-built
bus would include a lighter battery, a lighter
structural composition of the bus, and ad-
justments to the propulsion and communica-
tion systems.

Table 6.1: SDO Preliminary Study Bus
and Payload Mass Breakdown (kg)

                  Custom          RSDO
ACS                 54              42
Power               60              92
Harness             15              19
RF comm.            35         72 w/ C&DH        Figure 6.1: Ground track of geosynchronous orbit
C&DH                11                           with an inclination of 28.5 degrees.
Propulsion          10              61
Hydrazine           20              20           The disadvantages of this orbit include
AKM Stage           10               8           launch and orbit acquisition costs (relative to
Balance Wt.         10               7           LEO) and eclipse (Earth shadow) seasons
Thermal             60                           twice annually. During these 2-3 week ec-
Mechanical          34         114 w/ therm      lipse periods, SDO will experience a daily
Subtotal           319             434           interruption of solar observations. The max-
                                                 imum duration of these interruptions is 70
Instruments         200             200          minutes, during which solar observations
AKM                 668             668          will be interrupted. The spacecraft attitude
Total              1187            1302          control system (ACS) and power system
                                                 must recover from each of these eclipse pe-
                                                 riods, which may involve a longer duration

of the interruption of the scientific observa-         ous launch vehicles, using an AKM such as
tions. Three lunar shadow events also occur            the Star-30E and a spacecraft mass of 634
annually, with durations of approximately              kg. The Delta 2925 meets the launch mass
30 minutes. The total duration of these inter-         criteria, but other vehicles such as the Delta
ruptions is approximately 45 hours annually.           Lite and the Taurus would require use of a
Eclipse and shadow periods are shown in                much lighter spacecraft and AKM, or would
Figure 6.2.                                            require the consideration of an alternate or-
                                                       bit profile. During the design of the SDO
                                                       mission, contingency and mass margins as
                                                       well as mass and spin balance requirements
                                                       must be taken into consideration; these items
                                                       will constrain the total mass able to be ac-

Figure 6.2: Annual periods of Earth and lunar sha-
dow (minutes per day).

                                                       Figure 6.4: Mass margins for various launch ve-
                                                       hicles. The total mass of the spacecraft and AKM is
                                                       shown in gray. The launch vehicle capabilities (in
                                                       mass to GTO) are shown in blue. A Delta 2925 has a
                                                       margin of nearly 500 kg.
Figure 6.3: Illustration of a launch to geosynchron-
ous transfer orbit (GTO) and orbit circularization     6.2 Attitude Control System
with an Apogee Kick Motor.
                                                       Most of the requirements of the SDO mis-
SDO's orbit can be achieved with a launch              sion can be met by a geosynchronous space-
into a geosynchronous transfer orbit (GTO),            craft from the RSDO catalog, with some ne-
with an apogee kick motor (AKM) to circu-              cessary adjustments. The standard geosyn-
larize the orbit at geosynchronous altitude            chronous ACS would include a startracker,
(Figure 6.3. The total mass to be lifted to            course and fine sun sensors, gyroscopic sen-
GTO includes the spacecraft and instru-                sors and reaction wheels. Four reaction
ments as well as the AKM. Figure 6.4 com-              wheels in a pyramid configuration will allow
pares the mass capabilities to GTO of vari-            efficient unloading of accumulated momen-

tum and will provide redundancy for a long-        based on the field of view and the design of
term mission design. Attitude control system       the coronagraph, the Sun must be centered
considerations include the adaptation of a         behind the occulting disk to within a desira-
geosynchronous spacecraft to maintain a            ble tolerance. The accuracy and stability of
sensitive three-axis stabilized solar pointing.    the roll of the spacecraft is specified in rela-
These modifications would include a mea-           tive terms; the HMI requires a stable view-
surement from a guidescope in the instru-          ing angle, though the angle does not neces-
ment package which will link to the ACS to         sarily have to be aligned at solar North.
meet the stability requirements required by
the instrument package, and the use of low-        Attitude Knowledge: Because of its re-
jitter reaction wheels (such as the SMEX           stricted field of view, the spectrometer
Lite IRWA) to restrict the jitter introduced       would require a knowledge of a fraction of
to the system.                                     the instrument resolution. Therefore, the
                                                   knowledge in pitch and yaw would be set at
The spacecraft ACS requirements which              a fraction of an arc second (which can be
were discussed were as follows:                    provided by the ACS or by the instrument
                                                   package). The HMI requires a roll know-
Jitter: The instruments with higher resolu-        ledge of 30-60 arcsec (3 sigma), correspond-
tion place restrictions on the amount the          ing to a fraction of a pixel at the solar limb.
pointing can vary during the collection of
data. The HMI and AIA require that the jit-        SDO will have several modes of operation
ter in both pitch, yaw and roll not vary by        including: Science mode - 3-axis zero-
more than a fraction of a pixel over the in-       momentum control, pointing roll axis toward
terval of time required to collect the data.       the Sun, run wheels in bias speed, using
Therefore, the image must be stabilized to         measurement from guide telescope for pitch
.25 arcsec (3 sigma) over a few seconds (de-       and yaw and star tracker measurement for
termined by the image collection time) in          roll; Calibration mode - includes possible
pitch and yaw. It is likely that the spacecraft    offsets and maneuvers for instrument cali-
ACS will not be able to satisfy this require-      bration; Safehold mode - uses CSS and
ment, and will have a pitch/yaw jitter near 5      wheel to point solar array normal to the Sun,
arcsec over 45 seconds. The instruments            similar to Sun acquisition; Delta V mode -
would incorporate image stabilization using        for orbit maintenance; and Delta H mode -
data from the guidescope. Because the im-          to unload momentum using propulsion
age stabilization system cannot correct for
jitter in the roll axis, the roll jitter must be   These operational modes may need to in-
held to within 50 arcsec (3 sigma) over a          clude periods of recovery from Earth sha-
period of 45 seconds.                              dow periods, and maneuvers to assist in in-
                                                   strument calibration.
Accuracy/Stability: It is estimated that an
image stabilization system cannot function         6.3 Data and Communication
properly at angles greater than 10-15 arcsec;          System
therefore, if the spacecraft introduces 5 arc-
sec jitter, the greatest deviation from the        Commands to the science instruments will
overall pointing must be within 5-10 arcsec        be scheduled for 1 contact per day. The high
(3 sigma) in pitch and yaw. A coronagraph          rate science data downlink, at 160 Mbits/sec,
requires a similar accuracy in pitch and yaw;      and the low rate housekeeping data will be

continuously maintained between the ground      6.5 Instrument Module
and the spacecraft.
                                                An instrument module must accommodate
                                                the SDO instruments and provide an accept-
                                                able launch environment. The instrument
                                                module consists of a mounting structure,
                                                instrumentation, cables, heaters and radia-
                                                tors. The mounting structure shall be de-
                                                signed to serve as an optical bench to allow
                                                for mounting and alignment of the sensitive
                                                SDO instruments. Figure 6.6 shows an illu-
                                                stration of the launch configuration of SDO,
                                                including the instrument module, AKM, and
                                                solar panels in a 9.5-foot Delta fairing.

Figure 6.5: SDO CD&H system.

6.4 Spacecraft Power
The instruments were assumed to require a
combined power of 250 Watts, with 50
Watts for survival. (The estimates were
based on 24% efficient Triple Junction
GaAs cells, with 2 spacecraft wings, each
with solar array area of 3.2 square meters,
including losses from UV and energetic par-     Figure 6.6: SDO instrument module launch configu-
ticle radiation, thermal cycling, assembly      ration
losses, and losses from SA to battery, bat-
tery to load, & SA to load.) The estimates of   6.6 Ground System
the spacecraft power requirements are listed
in Table 6.2                                    The SDO mission utilizes an inclined geo-
                                                synchronous orbit to take advantage of a
Table 6.2. Spacecraft bus eclipse power         single dedicated ground station. This ground
(W)                                             station would ideally be located in a location
                                                with minimal effects of rain attenuation. X-
               Custom          RSDO             band frequencies are becoming less availa-
ACS            84              53               ble to missions in the space sciences; it is
Power          21              52               likely that the primary downlink of SDO
RF comm.       100             41               data will use Ka- or Ku-band frequencies.
C&DH           35              12               For space research 37 GHz is currently allo-
Propulsion     5               12               cated, with a possibility of allocation at 21
Thermal        50              91               or 27 GHz. The Ku and Ka frequencies suf-
Harness        23                               fer greater rain attenuation than X-band, but
Total          295             284              the antennae are much smaller and can be
                                                built inexpensively, and backup stations can

be considered to meet data completeness

Data latency (including delivery to final us-
ers) requirements may drive ground system
considerations; several of the LWS partners
may require latency significantly less than
an hour. Additionally, the high science data
rate indicates that only the health and safety
data would be able to be stored on board in
the event of a contact interruption. The data
gathering architecture will be selected to
maximize long continuous streams of valid
                                                 Figure 6.7: SDO Mission Operations and Ground
data. The high data completeness require-        System Network
ment will also be a design driver for the
ground system and the downlink margins.          The Mission Operations Center, Science
                                                 Operations Center and Ground System could
During the nominal mode of spacecraft op-        be distributed or centralized. Centralization
eration, the ground system can operate as a      requires less transport of data over the Inter-
semi-autonomous system, allowing standard        net and greater communication between the
operations to proceed on a 40 hour per week      mission operations and science teams. How-
schedule. The geosynchronous orbit and on-       ever, the cost of data transport at the time of
board safing systems make the use of a           the SDO mission is unknown, and the cost
semi-autonomous ground system a low risk         advantage gained from less transport re-
and low cost way of meeting the mission          quires further study. Moreover, the cost of
requirements.                                    temporary storage of SDO data at the
                                                 ground system site (to compensate for tem-
6.7 Mission and Science Opera-                   porary network and distribution failures) is
    tions                                        dependent on the future price of bulk sto-
Most of the SDO planned instruments are
full-disk instruments and the observations of    The ground station will be able to strip the
the mission are not “event-driven,” (e.g. res-   science data packets from the downlink
ponses to flares or CMEs). There is consi-       stream and send them to the storage sites,
derable value in collecting observations as      either at PI institutions or at a centralized
routinely, and under as stable observing         data service. The packets will be assembled
conditions and repeatable operation scena-       into Level Zero data sets and may receive
rios, as possible. Thus, the nominal science     additional processing prior to being made
mode of operation can be accomplished with       available for distribution.
daily command loads similar to those used
by the TRACE mission. Instead of near-
real-time commanding, daily command
loads compiled by the PIs will be sent by the
Mission Operations Team.

                                                 Table 7: Concurrent Observations

                                                 Facility        Measurements           Date
                                                 STEREO       Atmospheric Images        2006-
                                                                Coronal Images          2011
                                                 Solar-B      Atmospheric Images        2005-
                                                                UV/EUV Spectra
                                                 Solar        Atmospheric Images        2010
                                                 Probe          Coronal Images          2015
Figure 6.8: SDO Distributed Data System                          Magnetograms
                                                 SORCE          Total Irradiance        2002-
Data generated by LWS missions are to be                       Spectral Irradiance      2007
free and publicly available for analysis.        GOES/          Total Irradiance        2010-
Proper support of an open data policy re-        NPOESS        Spectral Irradiance
quires the provision of the development of       SOLIS           Magnetograms           2001-
software and analysis tools and calibration                         Spectra             2025
algorithms by the instrument team. These         ATST            Magnetograms            N/A
services will be provided through the Prin-      Solar        Atmospheric Images         N/A
cipal Investigations.                            Sentinels    Helioseismic Images
                                                 Solar        Atmospheric Images        2009-
                                                 Orbiter        Coronal Images
7 Concurrent Observations                                        Magnetograms
                                                                UV/EUV Spectra
Several space-based and ground-based in-         FASR               Coronal              N/A
struments will be operational at times during                    Magnetograms
the SDO mission. Observations from some
of these instruments [particularly those on      7.1 STEREO
other LWS missions] will, at the very least,
complement those from the SDO instru-            STEREO (Solar-TErrestrial RElations Ob-
ments. In some cases (e.g. total solar irra-     servatory) will provide new perspectives on
diance) these observations may replace           the structure of the solar corona and CMEs
those that otherwise would need to be ob-        by moving away from our customary Earth-
tained with SDO. The following sub-              bound vantage point and by using two
sections describe the instruments as they        spacecraft to provide information on three-
were specified at the time of this report. Fu-   dimensional structure. Both spacecraft carry
ture changes in these specifications may in-     a suite of instruments including: two coro-
fluence the final choices for SDO instru-        nagraphs (covering the range 1.25 - 4 RSun,
ments. Table 7 lists the facilities along with   and 2-15 RSun), an extreme ultraviolet im-
their anticipated dates of operation and key     ager (full-disk, 1 arcsec pixels), a helios-
measurements.                                    pheric imager (an externally occulted coro-
                                                 nagraph that can image the heliosphere from
                                                 12 RSun to beyond Earth‟s orbit), an inter-
                                                 planetary radio burst tracker, in situ particle

and field detectors, and a plasma composi-      netic variability and how this variability
tion experiment.                                modulates the total solar output and creates
                                                the driving force behind space weather.
This mission is designed: to further our un-
derstanding of the origins and consequences     The spacecraft will accommodate three ma-
of CMEs, to determine the processes that        jor instruments: a large solar optical tele-
control CME evolution in the heliosphere, to    scope (SOT), an X-ray telescope (XRT), and
discover the mechanisms and sites of solar      an EUV Imaging Spectrograph (EIS). All
energetic particle acceleration, to determine   three major instruments will give extremely
the 3D structure and dynamics of coronal        high resolution observations of a field of
and interplanetary plasmas and magnetic         view on the Sun that is restricted to an active
fields, and to probe the solar dynamo           region spatial scale. The SOT includes focal
through its effects on the corona and the he-   plane instruments that will provide longitu-
liosphere.                                      dinal magnetograms with 1-5G sensitivity
                                                and vector magnetograms with 30-50G sen-
The STEREO Mission spacecraft are ex-           sitivity to transverse fields. The XRT pro-
pected to be launched in December 2005.         vides atmospheric images at wavelengths
The two spacecraft will slowly drift apart in   from 2 to 6 nm. The EIS provides UV/EUV
ecliptic longitude with a total separation of   spectra with a resolving power of about
45° after the first year and 90° after the      10,000 over wavelength ranges from 17-21
second year. The mission is expected to last    nm and 25-29 nm (covering temperatures
two years at minimum and, more likely, five     from 105 to 107 K but with only one strong
years.                                          emission line below 106 K).

STEREO will overlap with SDO and pro-           The Solar-B spacecraft is scheduled for
vide coronagraphic images like those needed     launch in the fall of 2005. It will be placed
for SDO. STEREO‟s lifetime, however, is         in a polar, sun-synchronous orbit about the
shorter than SDO‟s and during parts of the      Earth. This will keep the instruments in con-
STEREO mission the STEREO instruments           tinuous sunlight, with no day/night cycling
will be viewing solar regions far removed       for nine months each year. Solar-B will ad-
from the SDO observations. The STEREO           dress some of the same problems that SDO
coronagraphs will complement the mea-           will address but with higher spatial resolu-
surements made from SDO but cannot be           tion and a smaller field of view. The combi-
used to replace them.                           nation of vector magnetograms, atmospheric
                                                images, and UV/EUV spectra should pro-
7.2 Solar-B                                     vide the measurements needed to answer
                                                many of the outstanding questions concern-
Solar-B is a Japanese mission proposed as a     ing the initiation of flares and CMEs.
follow-on to the highly successful Ja-
pan/US/UK Yohkoh (Solar-A) collaboration.       7.3 Solar Probe
The mission consists of a coordinated set of
optical, EUV and X-ray instruments that         The Solar Probe mission is an unprecedent-
will investigate the interaction between the    ed exploration of the inner heliosphere,
Sun's magnetic field and its corona. The re-    which will achieve unique science by flying
sult will be an improved understanding of       over the pole of the Sun and as close to the
the mechanisms that give rise to solar mag-     Sun's surface, through the solar corona, as is

technologically feasible today. It will first       polar sub-surface flow patterns are essential
travel to Jupiter for a gravity assist, leave the   to answering fundamental questions related
ecliptic plane, fly over the Sun's poles to         to the solar dynamo and the origins of the
within 8 solar radii, and reach perihelion          solar cycle, which are central to the SDO
over the equator at 4 solar radii. A unique         primary scientific objectives, but which can-
aspect of the Solar probe orbit is that the tra-    not be obtained with the SDO spacecraft.
jectory is orthogonal to the Sun-Earth line
during perihelion passage so that there is
continuous radio contact throughout the fly-        7.4 SORCE
by. Two perihelion passes are planned, the
first near the 2010 solar maximum and the           SORCE (SOlar Radiation and Climate Ex-
second near the 2015 solar minimum. This            periment) is a program within NASA‟s Of-
orbit ensures that the mission will probe           fice of Earth Science (OES) for measuring
both the high speed solar wind streams and          solar irradiance. The solar-pointed SORCE
the equatorial low-speed streams.                   spacecraft carries five instruments to meas-
                                                    ure both total and spectral solar irradiances.
The results from SOHO and Ulysses have              These are the Total Irradiance Monitor
focused our understanding of the solar coro-        (TIM), Spectral Irradiance Monitor (SIM),
na to the point where in situ measurements          two identical Solar Stellar Irradiance Com-
are now necessary for further progress. Both        parison Experiments (SOLSTICE) and the
imaging and in situ measurements will pro-          X-ray Photometer System (XPS).
vide the first three-dimensional view of the
corona, high spatial and temporal resolution        The TIM measures total solar irradiance us-
measurements of the plasma and magnetic             ing electrical substitution radiometers. The
fields, as well as helioseisology and magnet-       observations have an uncertainty goal of
ic field observations of the solar pole.            0.01% and a long-term repeatability of
                                                    0.001% per year.
The Solar Probe Nadir-Viewing Imagers
will provide the only full view of the Sun's        The SIM measures solar UV, visible and IR
Poles, imaging both the North and South             spectral irradiance from 200 nm to 2000 nm
Pole within one day. The Solar Probe Mag-           with spectral resolution ranging from a few
netograph/Helioseismograph will provide             nm at UV wavelengths to tens of nm at IR
out-of-the-ecliptic observations of the polar       wavelengths. It uses a prism for wavelength
magnetic field and polar sub-surface flow           dispersion and a bolometer and diodes for
patterns essential to answering fundamental         signal detection. The SIM spectral irra-
questions related to the solar dynamo and           diances have uncertainties of 0.03% and
the origins of the solar cycle.                     long-term repeatabilities of 0.01% per year.

The observations of the solar corona and            The two SOLSTICE instruments measure
solar wind that Solar Probe will provide are        the solar UV spectral irradiance from 120 to
critical to understanding fundamental               300 nm with spectral resolution of about 0.1
processes that can be obtained in no other          nm, using grating spectrometers that have
way, and will provide a set of measurements         the capability also to observe bright blue
that complement, but are distinct from, the         stars for calibration tracking. The SOLS-
SDO observations. Finally, the Solar Probe          TICE UV spectral irradiance uncertainties
observations of the polar magnetic field and

are in the range 3% to 6% and repeatabilities    by NPOESS, expected to commence around
are 0.5%.                                        2010, depending on existing resources.

The XPS is a bank of broadband X-ray pho-        Measurements of solar EUV radiation in
tometers in the range 1 – 31 nm with spec-       five broad bands in the range 10 to 130 nm
tral bands 5 to 10 nm, uncertainty of 12%        are planned to be made from future GOES
and repeatability of 3% per year.                platforms commencing in late 2002, depend-
                                                 ing on the health and status of existing
SORCE will be launched in mid 2002, with         GOES spacecraft. The EUV broadband
expected mission duration of 5 years. It will    fluxes are recorded every 10 seconds, with
overlap with ACRIMSAT, which has pro-            an uncertainty of 10% and a repeatability of
vided total solar irradiance data since 2000,    5% over 7 years. The instrument uses dif-
thereby extending the continuous record of       fraction gratings to disperse the light, thin-
total solar irradiance that commenced in late    film filters to further remove unwanted wa-
1978. NASA OES plans to continue the so-         velengths, silicon diode detectors to collect
lar irradiance measurements with a follow-       the light and tantalum shielding to eliminate
on solar irradiance mission in the time frame    effects from radiation. Two GOES space-
of 2006-2011 that measures total solar irra-     craft are planned to be operational at any
diance (e.g., TIM) and spectral irradiance       one time providing redundancy and cross-
from 200 to 2000 nm (e.g., SIM).                 calibration for the EUV sensor. With the
                                                 launch of each new GOES EUV Sensor
7.5 GOES/NPOESS                                  every (2-5 years), a new calibration will be
                                                 applied to the data set.
GOES (Geosynchronous Operational Envi-
ronmental Satellites) and NPOESS (National       In addition to these major resources, at least
Polar-orbiting Operational Environmental         two European programs are planning to
Satellite System) are satellite systems dep-     measure solar irradiance during the next
loyed by NOAA to monitor the environ-            decade. Three solar instruments (ACES,
ment.                                            SOVIM and SOLSPEC) will measure the
                                                 total solar irradiance and the solar spectrum
Solar irradiance will be monitored by            from the EUV to the IR. The solar package
NPOESS. Measurements of total solar irra-        is presently scheduled as part of the Colum-
diance and of the solar spectral irradiance in   bus module of the International Space Sta-
the wavelength range from 200 to 2000 nm         tion, to be launched during 2004-2005 with
with accuracies and precisions similar to        a priority of solar minimum observations.
those of the TIM and SIM instruments on          PICARD, a small spacecraft carrying in-
SORCE are specified. The solar irradiance        struments to measure the total solar irra-
measurements are designated for flight on        diance and solar diameter, is under devel-
one of the three NPOESS spacecraft. Since        opment in France, for launch by CNES
the priority for the solar measurements is       around 2006 with a 2-3 year mission dura-
relatively low among other NPOESS mea-           tion.
surements, failure of the spacecraft would
not trigger an immediate replacement of the      These measurements may facilitate the cali-
solar instruments. This means that gaps may      bration of the SDO EUV Spectral Irradiance
occur in the total solar irradiance measured     measurements but will not have the spectral
                                                 resolution or coverage to replace them.

                                                  lar spectrum lines using one arcsecond pix-
7.6 SOLIS                                         els. Quick-look SOLIS data will be availa-
                                                  ble on the Web within 30 minutes and more
SOLIS (Synoptic Optical Long-term Inves-          accurately reduced data within 24 hours.
tigations of the Sun) is a ground-based           Special campaigns and user-proposed pro-
project of NSO to provide regular observa-        grams can be interleaved with the regular
tions of the Sun for at least 25 years. It will   synoptic observations. The synoptic data
replace many of NSO's current synoptic fa-        will be openly available.
cilities. The primary science objectives are
to increase understanding of solar activity       SOLIS and SDO will complement each oth-
and its effects on earth by means of observa-     er in a number of ways. Lightweight space
tions of the Sun's full vector magnetic field     magnetographs will be filter based and it
and the dynamics and evolution of solar           will be very useful to compare results from
changes related to the magnetic field. Aside      such an instrument with the higher spectral
from basic research on the solar activity         resolution SOLIS measurements to seek out
cycle, the observations will also be used as      systematic errors in both types of observa-
inputs to test models that are alleged to be      tions. Similarly, the higher spatial resolution
able to forecast activity.                        space observations will permit a study of
                                                  how ground-based observations are de-
The first SOLIS facility is nearing comple-       graded by terrestrial seeing. In case a vector
tion and the recent NAS/NRC report “As-           magnetograph is not flown on SDO, the
tronomy and Astrophysics in the New Mil-          SOLIS vector observations will provide an
lennium” recommends building two addi-            obvious enhancement. The same holds true
tional systems to be located at longitudes        for the chromospheric magnetograms. SO-
different from the US. This would increase        LIS will provide regular monochromatic
the average 24-hour duty cycle from about         measurements of intensity and dynamics in
30% to about 80%. The US system includes          the cool solar atmosphere that will be valua-
three instruments: a vector spectromagneto-       ble for use with the SDO high temperature
graph (VSM), a full-disk monochromatic            measurements. Perhaps the best complemen-
imager, and an integrated sunlight spectro-       tarity would be one that we cannot predict,
meter for sun-as-a-star spectroscopy.             namely, some SDO discovery that stimulates
                                                  follow-up observations with SOLIS, or vice
The VSM is the most unique instrument and         versa.
would be the common network instrument.
The VSM is a 50-cm telescope and a high-          7.7 ATST
resolution spectrograph. It can provide full-
disk vector magnetograms in about 15 mi-          ATST (Advanced Technology Solar Tele-
nutes with a polarimetric noise level of          scope) is a proposed ground-based telescope
about 3 x10-4 using one arcsecond pixels. It      facility designed for high-resolution studies
will also provide high-sensitivity photos-        of the Sun. The proposal is for a 4-meter
pheric and chromospheric line-of-sight            telescope operating in the visible and infra-
component magnetograms as well as He I            red (0.3 to 35 microns) with very high reso-
dynamics images.                                  lution (0.1arcsec or better) and a large pho-
                                                  ton flux for sensitive polarimetry. The facili-
The monochromatic imager provides inten-          ty will have the spatial resolution and sensi-
sity and Doppler images in a number of so-        tivity to study the ubiquitous weak magnetic

field elements in the photosphere and meas-       to follow the development of structures
ure magnetic fields in the corona. It will        within the Sun.
have the capability to examine waves in
magnetic flux tubes to test models of chro-       The fleet will consist of four Inner Helios-
mospheric and coronal heating. It will have       pheric Sentinels in heliocentric orbits rang-
a 5 arcmin field of view that will allow stu-     ing between 0.5 and 0.95 AU, a FarSide
dies of active region evolution and the initia-   Sentinel in a 1 AU orbit opposite Earth, on
tion of flares and CMEs.                          the far side of the Sun, and a single L1 Sen-
                                                  tinel to provide solar wind input information
ATST will address some of the same prob-          to the geospace components. These elements
lems that SDO will address (e.g. small-scale      will work together to track solar distur-
magnetic elements) but with higher spatial        bances as they evolve and transit the inner
resolution, smaller field of view, and less       heliosphere. The inner heliospheric sentinels
complete coverage. The proposal suggests          are spinning satellites. The FarSide Sentinel
that operations begin in about 2008.              is three-axis stabilized.

                                                  Solar Sentinels will supplement SDO obser-
7.8 Solar Sentinels                               vations to provide continuous and whole
                                                  surface imaging of the photosphere and solar
The Solar Sentinels will consist of a fleet of    corona, hence allow the study of evolution
spacecraft distributed throughout the helios-     of solar active regions. The observations
phere. They will help to improve the accura-      complement those from SDO but with little
cy of models of CMEs and other solar wind         or no redundancy.
transients. They will resolve geoeffective
solar wind structures and map them back to
solar features. They will search for the loca-    7.9 Solar Orbiter
tions and mechanisms of energetic particle
acceleration, and provide tomographic im-         The Solar Orbiter was selected by ESA at
ages of the Sun. When fully deployed, the         the end of 2000 as a “flexi”- mission, for
Sentinels will increase the lead-time and ac-     launch in the 2008-2013 time frame. The
curacy of geospace forecasts.                     key mission objectives of the Solar Orbiter
                                                  are: (a) to study the Sun from close-up (45
The Inner Heliospheric Sentinels will make        solar radii, or 0.21 AU), and (b) to provide
in situ observations of the heliospheric vec-     images of the Sun's polar regions from he-
tor magnetic field, the solar wind plasma         liographic latitudes as high as 22 degrees
properties, and the spectrum of high-energy       during the nominal mission, and over 30 de-
particles. They will make remote measure-         grees during the extended mission.
ments of the propagation of interplanetary
shocks by tracking radio bursts. A Farside        Solar Orbiter‟s unique heliosynchronous,
Sentinel will also provide EUV images of          near-Sun trajectory will allow, in conjunc-
the solar corona along with photospheric          tion with concurrent high-resolution remote
magnetograms. Radio occultations will be          sensing observations, in situ investigations
employed along with these images and those        of the energetic particle environment in
from SDO to identify the birthplace of tran-      close proximity to different source regions,
sients. Helioseismic measurements will be         such as active regions, flare locations, CMEs
made in conjunction with those from SDO           and associated shocks. Solar Orbiter‟s

unique high-latitude trajectory will allow it    and chromospheric fields, and in this regard
to determine the longitudinal extent of          promises to be an important supporting in-
CMEs and provide, in conjunction with            strument for SDO studies of the magnetic
SDO and ground-based observatories, full         field in the corona. FASR has been rated
coverage of the entire Sun over 360º in lon-     highly by the NRC panel on the future of
gitude.                                          ground-based solar astronomy, and recom-
                                                 mended as a moderate-sized initiative by the
The potential payload includes two instru-       decadal NRC Astronomy and Astrophysics
ment packages: the Heliospheric in situ in-      Survey Committee. The telescope will be a
strument package and the Solar Remote            solar-dedicated instrument providing excel-
sensing instrument package. The Helios-          lent images of the full Sun with arcsecond
pheric in situ instruments include: a solar      spatial resolution at a wide range of fre-
wind analyzer, radio and plasma wave ana-        quencies nearly simultaneously, with both
lyzers, a magnetometer, energetic particle       targeted research and synoptic capabilities.
detectors, an interplanetary dust detector, a
neutral particle detector, and a solar neutron   Radio observations are able to measure
detector. The Solar remote sensing instru-       magnetic fields due to the gyroresonance
ments include: an EUV full-Sun and high          effect: electrons spiraling in the coronal
resolution imager, a high-resolution EUV         magnetic fields provide opacity at radio wa-
spectrometer, a high-resolution visible-light    velengths at low harmonics of the electron
telescope and magnetograph, an EUV and           gyrofrequency, 2.8  10-3 B GHz where B is
visible-light coronagraph, and a radiometer.     measured in Gauss. A given observing fre-
                                                 quency and sense of circular polarization is
We hope that the interested parties within       sensitive to a single value of magnetic field
ESA will do everything possible to ensure a      strength; by changing frequencies FASR
launch in a timely manner, i.e. in 2009 or       will be sensitive to magnetic fields in the
soon after. From recent solar and heliospher-    range 100 - 2000 G. Surfaces of constant
ic physics missions, e.g. SOHO, Yohkoh,          magnetic field strength show up in radio
TRACE, and Ulysses, we have learned that         maps at the appropriate frequency as bright
by far the best scientific return from mis-      regions: they have coronal brightness tem-
sions is through efficient coordination. A       peratures because gyroresonance opacity
launch of Solar Orbiter in 2009, or soon af-     makes them optically thick, whereas the sur-
ter, would provide a significant overlap with    rounding atmosphere with lower magnetic
SDO. The combination of Earth-orbit high-        field strength is optically thin and has much
resolution observations from SDO with the        lower brightness temperature. The depen-
close-encounter and polar observations from      dence of opacity on viewing angle introduc-
Solar Orbiter would allow an invaluable,         es some complications but the theory is well
thorough analysis of many aspects of solar       understood, and radio data have proven to be
activity and its influence on the Earth.         excellent diagnostics for testing extrapola-
                                                 tions of photospheric fields into the corona.
7.10 FASR

FASR (Frequency Agile Solar Radiotele-           8 Acknowledgements
scope) will provide radio observations of
coronal magnetic fields that complement          The Science Definition Team would like to
optical/IR measurements of photospheric          acknowledge the following people for their

assistance in the production of this report:
John Leon, Paul Caruso, Sahag Dardarian
and Ron Miller from the GSFC Project team
for their support during the discussions of
mission requirements; Art Poland, Dick
Fisher, Karel Schrijver and the LWS
Science Architecture Team for LWS pro-
gram science support; Members of the SDO
Mission Science Preformulation Team: Leon
Golub, Russ Howard, Steve Kahler, Dana
Longcope and Vic Pizzo; Military Space
Weather advisers during preformulation: Lt.
Col. Michael Bonnadonna, Maj. Peter En-
gelmann, Capt. Riley D. Jay, Maj. Phyllis
Kampmeyer and Lt. Col. Erwin Williams;
Joe Gurman and Terry Kucera at GSFC for
mission development; Jennifer Rumburg for
information systems; Scientists who assisted
in the instrument study and discussions:
Rock Bush, Darrell Judge, Don McMullin,
Jesper Schou, Jake Wolfson and Tom


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