photosphere lies the chromosphere (which means "color layer"). The chromosphere is a pink-glowing region of gas at a temperature of 10,000 Kelvin. Its light is mainly the red H2 emission line. The chromosphere can be seen by the naked eye during a total solar eclipse. When the Moon covers the rest of the solar disk, this thin outer layer is visible as a ring of small, intense red emissions.
Composite image of the Sun, the corona, and the solar wind, as seen by the SOHO satellite. Click here for original source URL
The Sun's hot corona. Click here for original source URL
Above the chromosphere is the outermost, tenuous atmosphere of the Sun, the corona. Gas in the corona reaches the amazing temperature of 2 million Kelvin, due to heating by violent convective motions in the photosphere and chromosphere. At this very high temperature even heavy elements like oxygen and iron have all their electrons stripped off and all atoms are highly ionized. The gas in the corona is very low density, and it seems surprising that it could possibly be at the very high temperature of 2 million Kelvin. One way for gas to reach such a high temperature is by compression due to gravity — the situation in the Sun’s core. Another way is for a much less dense gas to have energy dumped into it by some other process.
In the corona, the energy comes from magnetic fields and convection in the Sun’s atmosphere. A fluorescent gas tube is a familiar example of this phenomenon. As you know, a fluorescent gas tube is cool to the touch. Yet the gas inside has enough electrical energy forced into it to emit visible light that indicates a thermal temperature of several thousand Kelvin! The explanation is that a fluorescent tube is almost completely evacuated — the gas inside is at very low density. So the rate of collisions of gas atoms on the walls of the glass tube is very low and the tube stays cool. However, the individual gas atoms have kinetic energy appropriate for a very high temperature.
The corona is extremely hot, and by Wien’s law the peak wavelength of thermal radiation scales with temperature. Since energetic particles tend to produce short wavelength radiation, we would expect the corona to emit most of its energy as X-rays. Imaging of the Sun at X-ray wavelengths from above the atmosphere has permitted us to visualize the remarkable appearance of the X-ray Sun, which vividly shows flares and active areas emitting X-rays. The flares shoot material upward into the corona, disturbing the coronal structure. It's interesting that the Sun has such a stable and non-varying nuclear reactor at its core while having such violent and variable "weather quot; on and above its surface.
One of the stranger mysteries of the Sun involves the heating of these outer two layers. It is believed that either some form of wave (called magneto-acoustic waves) pumps heat into these layers or that energy is released when magnetically induced currents rapidly collapse. This mechanism, called magnetic reconnection, is also behind solar flares. While there is currently no evidence that magneto-acoustic waves exist, observations by NASA's TRACE (Transition Region an Coronal Explorer) and SOHO (Solar and Heliospheric Observatory) missions don't show enough solar flares, and thus enough magnetic reconnection, to produce the necessary heating. Continued observation by solar observing missions may identify new phenomena that help us understand this "hot problem." A high energy plasma laced with magnetic fields is one of the most complex situations in physics, so it may take some time to fully understand the solar corona.