An explanation has been offered for the bewildering fact that the sun's atmosphere is much hotter than its surface. The culprits, it is claimed, are small heating bursts known as nanoflares.

For those learning about the sun for the first time the discovery that the solar corona is described as 1-3 million degrees, while the surface is a mere 6000°C prompts disbelief. The corona, a component of the sun's atmosphere, is heated by radiation from the sun itself – how then can it be not just hotter, but hundreds of times hotter?

The question hasn't stopped nagging professional astronomers. "That's a bit of a puzzle," said Jeff Brosius of NASA's Goddard Space Flight Center. "Things usually get cooler farther away from a hot source. When you're roasting a marshmallow you move it closer to the fire to cook it, not farther away." 

While there have been various theories, none of them have achieved a great deal of confidence in the scientific community. However, Brosius has now published a paper in The Astrophysical Journal supporting the nanoflare theory.

This idea holds that small bursts release huge amounts of heat into the corona. Because the corona is so thin, the heat released by these flares has a huge impact, in a way that would not occur if there were more molecules to dilute the effects.

While space agencies have plenty of satellites devoted to the sun, as well as time on huge telescopes, Brosius' evidence comes from a much cheaper project. The Extreme Ultraviolet Normal Incidence Spectrograph (EUNIS) was launched aboard a rocket into the upper atmosphere, reaching a height of 300km. EUNIS collected just 6 minutes of data before returning to Earth, but its spectrograph provided us with extensive information on the far ultraviolet light emitted by the sun, which the Earth's atmosphere blocks from surface observations.

Scans of a region of the sun known to be magnetically complex (or “active”) at the time revealed radiation from a type of iron that forms at 8.9 million degrees. A patch this hot would cool, spreading heat to the surrounds and creating the typical coronal temperatures.

"The fact that we were able to resolve this emission line so clearly from its neighbors is what makes spectroscopists like me stay awake at night with excitement," said Brosius. "This weak line observed over such a large fraction of an active region really gives us the strongest evidence yet for the presence of nanoflares." 

While nanoflares may have firmed as the mechanism, we still do not know what is causing them, although again several theories exist. Moreover, just last year an alternative suspect, Alfven Waves, gained support in the same journal. 

Future missions will seek to investigate the material that makes up the flares.