The Sun's corona is millions of degrees Kelvin hotter than the main body of the Sun (reaching temperatures of up to 10 MK). A number of possible mechanisms have been proposed to explain why. They include:
• slow and fast magneto-acoustic surface waves • current (or magnetic field) dissipation • microflares/transients • mass/particle flows • magnetic flux emergence • afven body waves.
As yet, none has been confirmed as the cause.
Further reading Stanford Solar Center
Note: In 1997 the Goddard Space Flight Center issued a press release regarding a new 'Magnetic Carpet' theory.
Note: More up-to-date detail needed on this
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