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The Sun is our closest star, and naturally we are fascinated by the puzzles it poses. Learning about the Sun can, figuratively, throw light on the mysteries of the stars. One such mystery or problem is the coronal heating problem.
What is the coronal heating problem?
While the sun’s photosphere, which is the deepest layer we can observe directly, is at a temperature of about 6000 degrees Celsius (It is 6200 degrees Celsius at the bottom and about 3700 degrees Celsius at the top). This is understandable as the temperature drops as you go away from it. The photosphere is surrounded by the chromosphere, which shows a temperature of 3700 degrees Celsius at the inner edge and about 7700 degrees Celsius at the outer surface. This is already counter intuitive as it gets hotter as you move away from the heat producing centre. Surrounding the chromosphere is a thin transition layer and then comes the Corona. We cannot see the corona directly, except during a total solar eclipse or by using a coronagraph. In the Corona, the temperature reaches 500,000 degrees Celsius or more, up to a few million degrees. Given that the nuclear fusion reactions that produce the sun’s energy happen in the inner layers, how is this outermost layer hundreds of thousand degrees hotter than the inner layers such as the photosphere, this is the coronal heating problem.
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How is the problem resolved?
To reach such high temperatures, heat has to be transferred from the inner layers to the corona by a non-thermal process (conduction, convection or radiation) because this would violate the second law of thermodynamics which in effect forbids heat flowing from a cooler to a hotter body. Two theories have been remained as contenders to explain this phenomenon: the wave heating theory and the magnetic reconnection theory.
The wave heating theory says that heat is transferred from the solar interior to the chromosphere and corona in the form of special waves that the hot plasma of the Sun supports.
The magnetic reconnection theory relies on the magnetic fields in the Sun to induce electric currents in the Corona. These currents then suddenly collapse, releasing their energy in the form of heat. Magnetic reconnection is said to be the mechanism behind solar flares.
These two theories still stand as contenders, while other explanations have been almost given up. There isn’t sufficiently strong observational evidence for either theory right now. Thus, the final explanation could be either of these or a combination of the two effects.
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