Over the Arctic, Auroras Collide

SAN FRANCISCO ? Two curtains of light known as the aurora borealis have been caught in a collision by NASA cameras deployed around the Arctic, creating a spectacular explosion of light.

These auroral collisions, which had never been seen before or known to exist, were described for the first time here today at the annual meeting of the American Geophysical Union.

The unexpected collisions were spotted by a network of all-sky imagers set up by NASA and the Canadian Space Agency for the THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission. Their aim was to find out why some auroras occasionally exploded in light, an event known as a substorm.

Auroras are created when particles in the solar wind rushing off the sun interact with Earth's magnetic fields at its poles.

Earlier this year, UCLA scientist Toshi Nishimura assembled continent-wide scale movies from the individual cameras. The first movie he made showed a pair of auroras crashing together in December 2007.

"Our jaws dropped when we saw the movies for the first time," said space scientist Lary Lyons of UCLA, a member of the team that made the discovery. "These outbursts are telling us something very fundamental about the nature of auroras."

The collisions occur on such a vast scale that someone looking up from a single vantage point on Earth's surface wouldn't notice them. But the cameras, which look over a much wider distance, can see the whole picture.

After the evidence from the first movie, the team looked for more such collisions and "our excitement mounted as we became convinced that the collisions were happening over and over," Lyons said.

The scientists think that the spectacular light explosions are a sign of dramatic goings-on in the space around the Earth, or its "plasma tail." This region is millions of kilometers long and points away from the sun. Plasma in the tail is made up of charged particles captured from the solar wind and is held together by Earth's magnetic field.

These magnetic fields also connect the tail to Earth's polar region, where the auroras, or northern lights, can be seen dancing across the polar skies.

A collision happens when a broad curtain of slow-moving auroras and a small knot of faster-moving auroras fall apart. The curtain hangs in place, while the knot rushes in from the north. The auroras then collide and erupt in light.

Lyons thinks that this sequence of events is connect to the tail because the fast-moving jet might be associated with a stream of relatively lightweight plasma jetting through the tail. The stream starts at the end of the tail and rush inward toward Earth, and the fast-moving knot moves in synch with it. The slow curtain, meanwhile, is connected to a stationary inner boundary of the plasma tail.

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