News in Science

Flare fallout creates bright spots on Sun

Solar spectacular The rare appearance of bright spots on the Sun's surface is caused by material raining down from solar flares, reveal close-up images of a solar eruption.

Researchers led by Associate Professor Fabio Reale of the University of Palermo studied a spectacular solar flare that occurred on 7 June 2011.

Their findings, in the journal Science, open a new window into the little-understood processes that occur during these violent eruptions.

"I think this is the first time that these brightenings are explained as direct consequences of the impacts," says Reale.

"Certainly this event is infrequent. Material can fall back from eruptions, but it is rare that it brightens so much as it hits the surface.

The discovery was a serendipitous outcome of a wider investigation by Reale and colleagues into the accretion of material onto the surface of young stars.

They developed computer simulations to see what happens as blobs of high density, million-degree plasma falls back into the Sun at speeds of up to 450 kilometres per second.

Reale and colleagues examined the eruption in visible, ultra-violet, extreme ultra-violet and x-ray wavelengths using NASA's Solar Dynamics Observatory spacecraft. They noted the x-ray emissions were far lower than expected.

"The solar event tells us that this is probably due to absorption from material along the line of sight," says Reale.

"The thick chromosphere and the dense bouncing material stop a large part of the x-rays."

They also observed very faint evidence of a bow shock around the falling material.

Bow shocks

Professor Iver Cairns of the University of Sydney suggests these bow shocks may provide a mechanism for helping to heat the Sun's outer atmosphere, the corona.

"This is exciting because those temperatures are standard coronal temperatures of a million degrees."

"There have been real questions as to whether much of the corona is heated higher up, or whether some of it is heated ... low down, 10,000 kilometres or so above the photosphere," says Cairns.

He says future studies should look more closely at the bow shocks.

"I think this is an example of where we can identify unexpected phenomena like shock waves associated with this material raining back down where it came from.

"There could be interesting radio emissions and other signatures which have been observed ... but not understood or properly interpreted, so I think the study might well have quite a lot more implications for other areas," says Cairns.

Reale, however, isn't convinced bow shocks heat the solar corona.

"There might be some local heating surrounding the impacts, but that requires more investigation.