The
sun
in x-rays taken by the Soft X-ray Telescope (SXT)
built by
Lockheed
and flown aboard the Japanese satellite Yohkoh.
For a nice introduction to Solar Astronomy/Physics suitable for grades
K-12 see
The Stanford Solar Center Recommended.
For a more technical resource on the Solar Corona see this book:
"The Solar Corona"
, by Leon Golub and Jay M. Pasachoff.
There are several interesting facets of this image:
- Coronal heating problem:
The corona is hot, the sun is not (relatively). The fact that the sun's
outer atmosphere (the corona)
emits so strongly in x-rays means that its temperature is on the order
of 10^6 Kelvin. The suns surface (photosphere) temperature, which is
only about
5800 Kelvin, cannot produce this coronal temperature.
The mechanism via which the solar corona is heated is not fully understood.
The problem of the 5800 Kelvin Photosphere and the 10^6 Kelvin
Corona has been know for over 50 years. Many very talented people have
worked on this mystery, including Nobel prize winners. The mystery
continues.
- Structure, Magnetic fields and Energetics: Magnetic fields
are involved in the structure of the corona.
There is a clear
structure to the bright areas. Many structures have a filamentary character
linking two regions or poles. An investigation with a simultaneous
magnetogram, revealing photospheric magnetic field strength and
polarity, shows a strong correlation between these coronal structures and
photospheric magnetic structures (sunspots, etc). The bipolar
filamentary structures are called coronal loops. The two dark areas over
the top and bottom of the sun's image (the poles) are the polar coronal
holes, and the dark peninsula extending upward towards the center of the
disk from the bottom polar coronal hole, is called a coronal hole.
Generally the loops are hotter and denser and therefore brighter, and
are associated with closed magnetic structures.
The coronal holes are less dense and have a slightly lower temperature
and are therfore darker, and are associated with open magnetic
strucures. Most interestingly, coronal holes have nearly the same
energy flux requirements as coronal loops due to the significant loss of
kinetic energy in the outward flowing streams of coronal material which make
up a significant part of the solar wind.
- Time Scales: This hot corona and its structure
is not some short lived temporary outburst.
Although the corona is very dynamic with events occuring on time
scales of minutes (flares), there are
long lived structures that have
time scales of months. There are also periodicities seen in the
coronal structure
with time scales on the order of 11 and 22 years. The general feature of
an X-ray emitting corona has never ceased in the many years we have been
observing it. This
"movie"
and
other movies and the
latest images from SXT reveal something
about the variation of the X-ray corona.
If you ever get a chance to see a high quality video movie
of the solar corona taken in X-rays, take it. It is awe inspiring.
Complementing the X-ray images is
NASA's collection of recent solar images in other wavelengths
including magnetograms,
spectroheliograms and images from a white light coronameter.
Other sources are
NOAA's Space Environment Laboratory
which has
recent images in H-alpha. and
New Jersey Institute of Technology's Big Bear Solar
Observatory which has
daily images
including full disk white light images as well as selected regions and
images in the various diagnostic spectral lines. They also have several
very cool movies
of selected phenomena as well.
For a more varied perspective on Solar Physics see
NASA's Marshall Space Flight Center Solar Physics page.
Here are some links to other
solar related web sites
Back to
Dr. Boynton's home page ,
or
UM Physics home page
