The energy source that keeps the Sun hot and shining is at the center of the Sun. Therefore, the temperature is expected to fall as you move away from this central "heater". The temperature of the solar atmosphere, however, instead of dropping as you move away from the Sun, actually increases as you move outward from the photosphere to the corona! (The temperature eventually reaches a maximum and begins to drop in the outer corona.) How is this possible?
What we know is that the outer layer of the Sun is very turbulent, like a pot of boiling water. This layer of the Sun is called the convection zone. The magnetic field in the corona is anchored in the convection zone. The gas pressure in the convection zone dominates the magnetic field pressure. As a result, the magnetic field is dragged around and twisted by the turbulent motions of the gas in the convection zone. These motions can propagate up the magnetic field lines to the corona, where the magnetic field pressure is greater than the gas pressure. There the extra energy transferred to the magnetic field is presumably transferred to the coronal plasma.
We do not know how the energy in the magnetic field
is converted to heat in the corona. One possibility is that the energy is released in many
tiny flares, each too small to be observable. Another
possibility is that the energy is in the form of hydromagnetic
waves that are damped in the corona. The study of mechanisms for transporting and
depositing energy into the corona is an active area of solar research.
A collage of images of the corona obtained at different times during the solar cycle.
The corona in 3-D! (This requires a pair of red/green 3-D glasses, or simply view the image with a red filter over your left eye and a green filter over your right eye.)
Educator's Guide to Convection, including images and MPEG movies.
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