The sun's surface is blisteringly hot at 10,340 degrees Fahrenheit — but its atmosphere is another 300 times hotter. This has led to an enduring mystery for those who study the sun: What heats the atmosphere to such extreme temperatures? Normally when you move away from a hot source the environment gets cooler, but some mechanism is clearly at work in the solar atmosphere, the corona, to bring the temperatures up so high.

Clear evidence now suggests that the heating mechanism depends on regular, but intermittent explosive bursts of heat, rather than on continuous gradual heating. This solution to the coronal heating mystery was presented in a media briefing on April 28, 2015, at the Triennial Earth-Sun Summit, or TESS, meeting in Indianapolis, Indiana.

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Jim Klimchuk, a solar scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, explained that the new evidence supports a theory that the sun's corona is heated by tiny explosions called nanoflares. These are impulsive heating bursts that individually reach incredibly hot temperatures of some 10 million Kelvins or 18 million degrees Fahrenheit – even greater than the average temperature of the corona – and provide heat to the atmosphere. The research evidence presented by the panel spotted this super hot solar material, called plasma, representative of a nanoflare.

"The explosions are called nanoflares because they have one-billionth the energy of a regular flare," said Klimchuk. "Despite being tiny by solar standards, each packs the wallop of a 10 megaton hydrogen bomb. Millions of them are going off every second across the sun, and collectively they heat the corona."

https://www.scientificamerican.com/article/why-is-the-suns-corona-th/