The flashes of white light accompanying some solar flares are caused by the sun's acceleration of electrons to speeds greater than half the speed of light.

The phenomenon's new explanation derives from data recorded from a 2006 solar flare. The presence of high-energy X-rays in the same spot that scientists saw visible light tipped them off that some kind of non-thermal process was generating the light.

"These explosions are particle accelerators," said Säm Krucker, of the Space Science Laboratories at the University of California, Berkeley. "The whole surprising thing with these flares' light is that it could simply be heat. But that's not the case."

Solar flares occur when the sun's magnetic field lines rearrange and reconnect, releasing tremendous amounts of energy. There are different types of flares, which can generate geomagnetic storms of Earth, and only some of them are accompanied by the white light flares. These were first observed in 1859 by astronomer Richard Carrington, but no one really knew how they were produced until the new observations by the Japanese satellite Hinode and the NASA SMEX mission RHESSI.

Now, it looks as if the extremely powerful electromagnetic fields somehow deliver enormous amounts of energy into particles in the sun's photosphere. It's not unlike what humans do at a much, much smaller scale in particle accelerators like the Large Hadron Collider.

"As opposed to the LHC where you accelerate a few particles, it would be like accelerating the whole building basically," said Hugh Hudson, also of Berkeley's Space Science Laboratories, who worked with Krucker.

Astronomers haven't figured out how exactly the sun works as a particle accelerator just yet. "It's being done by electromagnetic effects that are not really understood," Hudson admitted.

It's possible that as the sun eases into a more active state over the next year, scientists will have more opportunities to study the flares.

A paper on the new work, with Kyoko Watanabe of Japan Aerospace Exploration Agency as lead author, appeared May 20 in The Astrophysical Journal.

Images: NASA/JAXA

*Citation: "G-band and Hard X-ray Emissions of the 2006 December 14 Flare Observed by Hinode/SOT and RHESSI," by Kyoko Watanabe, Säm Krucker, Hugh Hudson, Toshifumi Shimizu, Satoshi Masuda, Kiyoshi Ichimoto. The Astrophysical Journal, No.715, pp. 651-655, 2010. *

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