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					Tne Corona
  0610335
    宋颖
At the center of our solar system there is a
magnetic variable star, our Sun, which drives every
cubic centimeter of interplanetary space. The upper
atmosphere of the Sun, the solar corona extends
from the visible disk of the Sun outward,
eventually enveloping the earth. The earth, our
home planet, is located at a distance of about 200
solar radii from the visible surface of the Sun.
The dimension of a solar radius is roughly 700,000
km, approximately twice the distance from the earth
to the Moon, and the solar radius is a convenient
scale for discussing the solar corona, and the
heliosphere, the extension of the solar atmosphere
into interplanetary and interstellar space.
Astronomers feel comfortable using the solar radius
as a measure of length when discussing the corona,
the interplanetary medium, and the sizes of other
stars.
Total solar eclipse images of 1980 February (above)
and 1988 March (below) taken from sites located in
India (1980) and the Philippines (1988) by
expeditions from the High Altitude Observatory of
Boulder, Colorado. Note that the 1980 image, taken
near the maximum of the solar activity cycle shows
many streamers located at all azimuths around the
occulted disk of the Sun. Taken later in the cycle,
about a year past the minimum, the 1988 image shows
several large (bottle-shaped) helmet streamers which
are restricted to latitudes between N45 and S45. The
helmet streamers, which are large scale, dense
structures, have measured lifetimes from less than
one to more than several solar rotations. A special
telescope, known as the White Light Coronal Camera,
was used for both of these observations. Half of the
diameter of the dark central image of the moon is
equal to a distance of one solar radius.
Through most of history, coronal research has
been dominated by the simple fact that
observation was possible only during the special
astronomical circumstance of a total solar
eclipse. There are between two and five solar
eclipses each year, but many occur over the
oceans and are not easily documented. Some are
not total, being only partial or annular, and a
good opportunity for eclipse observation comes
along every only two or three years. Solar
eclipses are also brief, the average duration of
totality being only two to three minutes,
limiting efforts to study evolution of the
corona to following changes in the corona from
one eclipse observation to another.
 There are a number of natural timescales
operating on the Sun. The Sun rotates on its own
axis once every 27 days (as viewed from the
earth), and the period of the magnetic variation
most often detected using sunspots is an 11-year
fluctuation. Other types of changes in the
structure of the corona take place on a variety
of time scales ranging from minutes to a
fraction of a day. Thus progress in
investigating the solar corona was paced by the
availability to investigate the changes of the
solar corona by following ground-based
observations of a series of total solar eclipses.
The white light corona seen at the time of a
total eclipse is the result of scattering of
sunlight by electrons in the corona.
A combination of two coronal images, one taken from the
ground and one from space. The central image was made
in soft X-rays by an instrument on the Yohkoh
("Sunbeam") satellite (Japan); it shows the very hot
plasma in primarily closed magnetic structures in the
magnetically dominated lower corona. The blue-white
image was made at the same time with a white light
(electron scattering) coronagraph based at Mauna Loa,
Hawaii, and operated by the High Altitude Observatory
of Boulder, Colorado. In this case the large scale,
relatively weak magnetic field structures of the solar
corona are seen extending upward for roughly a solar
radius in altitude. In the 1930's, a French astronomer,
Bernard Lyot, solved the technical problem of creating
an artificial eclipse of the Sun within a telescope
system , and since that time it has been possible to
view the solar corona on regular basis. Even with this
development, there are practical limitations to ground-
based observing of the solar corona imposed by the
scattering of light by both dust and molecules in the
earth's atmosphere, as the brightness of the white
light corona ranges from one millionth to one billionth
of the central solar disk brightness.
The coronagraph flown on the 1973 Skylab mission
solved this problem by observing from a location on
the Apollo Telescope Mount, a cluster of instruments
used to view the solar atmosphere from this early
version of a space station. By using a coronagraph in
space, it became possible to made eclipse like
observations as often as one wished for an extended
period of time. In the case of Skylab, the mission
lasted almost nine months, about nine solar rotations,
but only one fifteenth of the duration of the solar
magnetic variability cycle. The Solar Maximum Mission
spacecraft, launched in 1980 and operated until 1989,
represented a further refinement of the use of a
coronagraph on a satellite observatory platform for
the investigation of the nature of the solar corona
since it was possible to accumulate thousands of
images of the solar corona over this nine-year period.
The lower solar corona as seen in soft X-rays on 1993
February 25. The bright regions of this image indicate
the magnetic complexity found in the corona above
sunspots and active regions. The base of a helmet
streamer structure is seen in the lower right, and the
dark lane at the lower, central portion of the disk is
a coronal hole structure. Coronal holes are large scale
features of reduced density (and are therefore dark in
soft X-ray images, since the soft X-ray intensity is
proportional to the square of the electron density in
the emitting region) and are identified as being open
magnetic field regions which are sources for high speed
streams of solar particles (electrons, protons, and
ions). By using a combination of eclipse and
coronagraph observations, a picture of the solar corona
has emerged which suggests that the solar corona is a
place where unique physical conditions and processes
exist. Spectroscopy of the corona suggests that, by
some not fully understood mechanism, the Sun has the
ability to create very high temperature material in the
corona.
Radiation characteristic of one to two million degrees
are regularly observed with coronagraph instruments.
Images of the corona made from satellites in low earth
orbit in the soft X-ray region of the spectrum
demonstrate a highly structured corona where besides
the forces of pressure and gravity, magnetic fields
play a role in the determination of the Sun's outer
atmosphere.
Occasionally observations of flare regions in the corona
demonstrate radiation which is interpreted to originate
at very high temperatures between 10 to 40 million
degrees C. These situations arise in areas where
coronal magnetic fields are relatively strong and it is
believed that the Sun has an effective mechanism for
converting magnetic field energy into thermal energy.
Current research indicates that in regions of
relatively high magnetic field strength in the solar
corona, corresponding to structures of small scale size
(a few hundredths of a solar radius in length), some of
the most energetic radiative processes originate in
these small scale, high magnetic field regions of the
corona.
A coronal mass ejection (CME) event in
progress. These two images were made with
the coronagraph flown on the Solar Maximum
mission spacecraft and demonstrate the scale
and speed of a CME event. The occulting disk
image is about 1.8 solar radii in diameter and
the images are taken a few minutes apart. The
large loop-shaped CME structure is roughly the
size of the Sun in the second image, and the
velocities estimated for this type of event range
from several hundred to a thousand kilometers
per second (well over a million miles an hour), a
velocity that would take a space traveler from
the earth to the Moon in twenty minutes.
In contrast to solar flares, which occur in small scale
 structures with relatively high magnetic field strength, there
 is a second kind of energetic phenomenon detected in the solar
 corona. These are the huge mass ejection events which were
 discovered and first studied in detail in the early 1970's with
 data collected with the Skylab and OSO-7 coronagraphs; a much
 larger data set was amassed with the later P78-1 and Solar
 Maximum Mission instruments. Evidently, some of the largest
 scale structures of the corona, which are governed by large
 scale, weak magnetic fields, become unstable and huge amounts
 of mass are occasionally discharged from the solar atmosphere
 out into the heliosphere. Particle detectors carried on
 research satellites operating between Venus and Jupiter have
 confirmed that these ejections are detected far from the Sun,
 and must sometimes impact the earth. At the time of peak solar
 magnetic activity near the maximum in the sunspot cycle, there
 are two or three such events per day. Near the minimum of the
 magnetic activity cycle this rate falls to approximately one or
 two mass ejection events every ten days. The size scales of
 such events are typically seen to be a large fraction of a
 solar radius, and the speed of ejection averages to a value of
 about 400 km/s. The detection, analysis, physical mechanisms
 and consequences of coronal mass ejections remains a topic of
 concentrated scientific research at this time.
  Composite of a SPARTAN 201, ground-based coronagraph, and
  Yohkoh soft X-ray image obtained during the first flight of
  the SPARTAN 201 system. Images such as this have been used
  to:
• construct models of the distribution of temperature and
  density for the large scale structures of the white light
  solar corona,
• investigate the heating of the coronas and the base of the
  solar wind, and
• determine the size and physical conditions of coronal
  features found in the polar regions of the Sun's atmosphere.
    Three forces are active in the solar corona at the base of
  the heliosphere; these are gas pressure and gravity forces
  similar to those experienced by humans near the earth's
  surface, and a third force produced by solar magnetic fields.
  As a consequence of these forces, a continuous flux of
  material is ejected from the Sun and blows outward through
  the heliosphere: the solar wind of charged particles.
   Within a few solar radii of the Sun's visible surface,
  magnetic forces are thought to be the cause of the
  structuring seen at the times of total solar eclipse, such
  as helmet streamers and coronal holes.
Coronal holes are now known to be regions where the density
  of the corona is considerably reduced, causing a
  relatively dark region to appear in soft X-ray and EUV
  (extreme ultraviolet) images. During much of the magnetic
  activity cycle there are semi-permanent polar coronal
  holes, and it has been known since the Skylab era that
  coronal hole structures seen in the solar corona are
  associated with the detection of high speed solar wind
  streams which sweep past the earth. The physical
  mechanisms for the acceleration of the solar wind and the
  conditions of interplanetary space, which slowly evolve in
  step with the change of the Sun's periodic variation of
  magnetic field, are also the subject of intense interest
  to the international research community.



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