The Corona Approaching Solar Maximum
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|Quantitative changes in the solar corona as seen by UVCS/SOHO from
1.5 to 3.5 so lar radii and by EIT/SOHO at lower heights during the rising
phase of solar cycle 23. The five solar images from left to right are composites
of UVCS/SOHO O VI 1032 Å (outer) and EIT/SOHO 284 Å (inner)
synoptic images. The center of each solar image is positioned above the
date of observation. The blue histogram represents the National Solar Observatory/Sacramento
Peak sunspot numbers (monthly averages). The hatched blue region corresponds
to predictions of future sunspot numbers from the Marshall Space Flight
Center Solar Physics Branch. Both the UVCS and EIT instruments are calibrated,
so the color scale represents the measured brightness of their respective
sections of the composite image, but their color scales are much different
with the same color representing a factor of about 10,000 greater brightness
in the EIT images of the solar disk compared to the brightness of the UVCS
images in the outer portion of the composite image. In each case, brightnesses
increase from the black region through the red and yellow regions to the
Caption: The corona is the Sun's outer atmosphere. The corona
has bright emission at ultraviolet wavelengths. The O VI emission shown
above comes from coronal oxygen atoms that have five missing electrons.
Artificial eclipses produced by UVCS block out the disk of the Sun and
filter out everything except the O VI 1032 Å emission. UVCS scans
the entire corona in a ``synoptic'' sequence. For this standard observation,
the UVCS instrument observes a narrow rectangle in the solar corona that
is 40 arcminutes long and 84 arcseconds wide. The rectangle can be scanned
in height at each of eight position angles around t he Sun. The images
are made by interpolating between positions to provide complete coverage
between the inner and outer heights. The above composites have been generated
from five UVCS/SOHO and EIT/SOHO synoptic images taken in December 1996,
December 1997, December 1998, December 1999, and October 2000, covering
over four years of SOHO's operation in the current solar cycle. The histogram
(blue) shows the number of sunspots from mid-1995 through September 2000.
For dates after October 2000, a prediction from the MSFC Solar Physics
Branch is provided. The corona displays a variety of features including
coronal holes and streamers. In the above images, those features and the
overall shape and brightness of the corona can be seen changing with the
sunspot activity cycle. From solar minimum in 1996 to mid-1998, the large
scale configuration of the corona consists of dark coronal holes over the
north and south heliographic poles and quiescent streamers at the equator.
Toward solar maximum, there is a striking difference in the appearance
of the corona. The streamers fill more of the coronal volume, and the extended
corona and disk become considerably brighter.
One way of tracking the solar activity is by observing sunspots. Sunspots
are relatively cool areas that appear on the face of the Sun. During solar
minimum there are only a few sunspots, and during solar maximum there are
about 20 times more spots. Every 11 years the Sun undergoes a period of
activity called solar maximum followed by a quiet period called solar minimum.
The sunspot number is calculated by first counting the number of sunspot
groups and then the number of individual sunspots. The ``sunspot number''
is then given by the sum of the number of individual sunspots and ten times
the number of groups. Although sunspots themselves produce only minor effects
on solar emissions, the magnetic activity that accompanies the sunspots
can produce dramatic changes in the ultraviolet and soft X-ray emission
levels. These changes over the solar cycle have important consequences
for the Earth's magnetosphere and upper atmosphere.
Picture Credit: Smithsonian Astrophysical Observatory; ESA &
Image processing/comments: Mari Paz Miralles (firstname.lastname@example.org
ard.edu), Alexander Panasyuk, Raid Suleiman, Steven Cranmer and John
Instrument/observatories: UVCS/SOHO (Ultraviolet Coronagraph
Spectrometer ); EIT/SOHO (Extreme ultraviolet Imaging Telescope); NSO/SP
(National Solar Observatory/Sacr amento Peak); MSFC (Marshall Space Flight
Center) Solar Physics Branch.