A corona is the very hot outermost layer of a star's atmosphere. Our Sun's corona can only be seen during a total solar eclipse. (Source: NASA's "Starchild" Glossary.)

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 during solar eclipses, when the main radiation from the Sun's surface is blocked by the passage of the Moon.

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 should be 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. This phenomenon remained a mystery for more than 50 years. 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.  (Source: "The Sun's Corona" - NASA's Imagine the Universe.)