The Sun's outer corona - stretching millions of kilometers out into space from the surface - reaches temperatures of up to 4 million °C.. This contrasts sharply with the surface of the sun which is measured as around 6 thousand °C.
A number of possible mechanisms have been proposed to explain why it's so hot. They include:
As yet, none has been definitely confirmed as the cause. See Wikipedia for details.
Further reading Stanford Solar Center
Note: In 1997 the Goddard Space Flight Center issued a press release regarding a new 'Magnetic Carpet' theory.
Also see :
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