Unsolved: Why is the Sun so hot?

The corona of the Sun is a layer of plasma that extends millions of kilo­metres into space above the Sun’s surface.

The problem is that the tem­per­ature of the corona is between one and three million kelvin, whilst the tem­per­ature of the surface is much, much cooler, only about 6000K.

There doesn’t seem to be any good explan­a­tion as to why the region above the surface is hotter than the surface itself. The Second Law of Ther­mo­dy­namics says that thermal energy cannot flow from the cool surface to the hot corona so a dif­ferent explan­a­tion is needed.

There are two com­peting theories as to what is respons­ible: wave heating, and magnetic recon­nec­tion. The first, wave heating, says that energy is trans­ferred by waves in the plasma a bit like sound waves; the second, magnetic recon­nec­tion says that energy is released as elec­trical currents induced by the collapse of the Sun’s magnetic field. This is the same phe­nomena that creates coronal loops like the one shown in the pho­to­graph below.

Most sci­ent­ists think that a com­bin­a­tion of the two theories is respons­ible; it is hoped that NASA’s Solar Probe+ will solve the coronal heating problem for good.