The Sun's Corona
The Sun, in X-ray wavelengths.
Of all the billions
and billions of stars in the Universe, only one small and fairly
unremarkable star really matters to us: the star without which life
would be very different on Earth -- our star, the Sun.
Because of its proximity, the Sun is one of the most studied stars, but
it has not yet revealed all its secrets.
The photosphere, chromosphere, and corona
- Courtesy of Univ of Herts, UK
One of the most puzzling features of the Sun is what has been dubbed
"the solar corona problem." There is a region around the Sun,
extending more than one million kilometers from its surface, where the
temperature can reach two million degrees. This region, called the
solar corona, is where the solar wind originates. The corona has been
found to emit X-ray radiation (the
corona is a plasma; at temperatures greater than
a million degrees a plasma will radiate a lot of X-rays). The corona can be
seen only during solar eclipses, when the main radiation from
the Sun's surface is blocked by the passage of the Moon or with
The solar corona,
as seen in a
The problem is, no one can really explain how this corona exists.
Even if the temperature in the core of the Sun does reach 15 million
degrees, it drops to a mere 5000 degrees at the surface. The temperature
even lower farther away from the Sun, but the temperature of the
corona is measured at more than a million degrees. This incredibly hot
temperature requires a permanent heating mechanism, or the plasma would cool
down in about an hour. There are many mechanisms which could heat some gas
above the surface of the Sun, but none of those mechanisms could account
for the large rate of heating necessary to heat the corona to these
phenomenon remained a mystery for more than 50 years.
The SOHO spacecraft
Things have changed. Although the details of the answer are not
completely known, it does seem that the solution is near.
Using data from instruments onboard the SOlar and Heliospheric
Observatory (SOHO) and from the more recent Transition Region And
Coronal Explorer (TRACE), solar physicists have identified small
patches of magnetic field covering the entire surface of the Sun.
Contrary to the bright, large magnetic field loops which are linked
to the "active regions" during periods of solar maxima, these patches
seem to appear and disappear randomly in time scales on the order of
40 hours. Scientists now think that this magnetic carpet is probably
a source of the corona's heat.
A close-up of sunspots
According to Joseph Gurman, an astrophysicist at the Laboratory for
Astronomy and Solar Physics at the Goddard Space Flight Center, people
suspected that magnetic fields were playing an active role in the Solar
problem. They thought the heat energy was coming from the active regions,
where the spectacular giant loops are seen in Ultra-Violet and X-rays.
But there was a problem with that theory: those giant loops disappeared
solar minima, while the corona does not.
Now most scientists believe that the heating of the corona is linked to
the interaction of the magnetic field lines radiating out of the small
patches mentioned above.
Because the laws of electromagnetism prohibit the intersection of two
magnetic field lines, every time magnetic field lines come close to
crossing they are "rearranged," and this magnetic
reconnection continuously heats the solar corona. It's a faily
inefficient source of energy, but the sheer number of these small magnetic
patches on the surface of the Sun makes the process a viable solution to
the 50 year old problem of what heats the solar corona.
All is not quite clear yet. No one has directly observed any
magnetic field reconnection. Experts do not even agree on the approximate
length of time these patches remain active. As a direct consequence of
this theory, the heating
process should occur much closer to the surface of the Sun than previously
thought, but no one really knows how close. "We need to find proofs that
(magnetic reconnection) is really the story" says Gurman.
One proof could be plasma jets, which scientists expect to be produced by the reconnection process. They've observed the jets before, but
there were unfortunately too few observations to get conclusive evidence.
In this image of a sunspot, the dark
regions are locations
of positive magnetic
polarity and the
light regions are
negative magnetic polarity.
Also, no one really knows what could produce these magnetic patches,
which vary on such short timescales. Professor Edward Spiegel from
Columbia University has suggested that the patches are produced by small
dynamos located just beneath the surface of the Sun. But once again, no
one has any definitive proof of the theory.
The future of solar physics now resides in a better understanding of the
magnetic reconnection phenomena and in future missions -- including a
"Reconnection Microscope" devoted to the magnetic reconnection problem.
The problems are very complex, but as Gurman pointedly notes,
"If the Sun didn't have a magnetic field, it would be as boring as
astronomers believe it is".