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Solar Dynamics
Observatory
“…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
UNITED KINGDOM
Barbara Thompson
Project Scientist
Code 682
NASA/GSFC
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
cycles?
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
disturbances.
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
2
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
3
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
4
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
5
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
6
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
7
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
Questions
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
8
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 3.1.1.1 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
9
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-
10
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.
3.1.1.2 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
11
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
12
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 3.1.2.1 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-
13
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-
terial.
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
14
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 3.1.2.2 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-
15
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.
eruptions.
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
16
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 3.1.3.1 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-
17
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-
bilities).
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
18
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.
3.1.3.2 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-
19
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-
20
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.
3.2.1.1 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-
21
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
22
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
3.2.1.2 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
23
near surface layers (a region that all acoustic 3.2.2.1 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
cycle.
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-
24
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-
25
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
3.2.2.2 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.
26
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.
3.2.3.1 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
27
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-
28
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
29
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-
3.2.3.2 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
30
return from SDO but should not be included
4 Required Observations at the expense of higher priority observa-
tions.
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 3.1.1.2) 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 – 3.1.2.2) and with coronal structures and so-
due effort should be made to include these lar wind variations (Section 3.1.3.2). 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 3.2.1.2), active region
portant – these would enhance the scientific evolution (Section 3.2.2.2), and the dynam-
ics of the small-scale magnetic elements
31
(Section 3.2.3.2). 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 3.1.1.2); to determine the configu-
ration of the magnetic fields associated with
Spatial Resolution. A reasonable goal is to flares and CMEs (Section 3.1.2.2) and solar
sample the wave-speed profile and flows wind variations (Section 3.1.3.2); 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 3.2.1.2) and as active
requires sample points 3 Mm apart. At 75° regions evolve (Section 3.2.2.2); 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 3.2.3.2). 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-
32
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 3.1.1.2), the occurrence and nature
of flares and CMEs (Section 3.1.2.2), and
Accuracy. The disk integrated magnetic the structure and dynamics of the low coro-
flux should be measured with an accuracy of na (Section 3.2.3.2). 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 3.2.1.2), active region evolu-
and determinations of the global field confi- tion (Section 3.2.2.2), and small-scale mag-
guration. netic elements (Section 3.2.3.2). 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
33
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 3.1.1.2) 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
34
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 3.1.1.2). 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 3.2.1.2) and active region evolution (Section
capture off-limb radiation that may contri- 3.2.2.2). 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-
lutionary.
35
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 3.1.2.2); the nonpotentiality of coronal
should be sufficient for the number of pixels
magnetic fields (Section 3.1.3.2); 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
3.2.1.2) and active region evolution (Section
each pixel should be sufficient to capture the
3.2.2.2). 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
36
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 3.1.1.2): 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 3.1.2.2); and the sources of high-speed
7 kG would capture even the strongest flux
and low-speed solar wind (Section 3.1.3.2).
concentrations.
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 3.2.2.2) and above small-scale
Doppler shifts due to the 5-minute oscilla-
magnetic elements (Section 3.2.3.2). 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
37
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 3.1.2.2) and reveal the structure of
formed periodically. the corona and presence of solar wind varia-
tions (Section 3.1.3.2). 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 3.2.1.2)
irradiance variations. and active region evolution (Section
3.2.2.2). 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-
38
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-
quirements.
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 3.1.1.2). 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
39
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
3.1.2.2) and the solar wind (Section 3.1.3.2).
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)
time.
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-
problems.
trum of resonant MHD waves that may be
responsible for heating and acceleration.
Helical 3D velocities can be determined
40
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 210-4 to 7.510-2. 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 3.1.1.2). 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
41
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
Spectrometer
Photometric 30 5 50 100x30x30
Mapper
Vector 10+ 5 20+ 90x40x40
Magnetograph
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.
42
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
43
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-
commodated.
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-
44
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
45
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
46
be considered to meet data completeness
requirements.
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-
rage.
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.
47
Table 7: Concurrent Observations
Facility Measurements Date
STEREO Atmospheric Images 2006-
Coronal Images 2011
Solar-B Atmospheric Images 2005-
Magnetograms
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
48
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
49
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
50
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.
51
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
52
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
53
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
54
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
Woods.
55
