Saturday science subject: Northern lights

— 12:14 PM on July 26, 2008

What causes the magnetic substorms responsible for the short auroras that illuminate northern skies? As Nature reports, NASA has used its Themis satellites in an attempt to detect the process that leads to the phenomena.

Before the findings emerged, researchers generally adhered to two competing hypotheses. "One camp holds that the key step is a disruption, which occurs about 60,000 kilometres [37,000 miles] away from the Earth, in the electrical current that travels across the magnetotail. . . . The other camp contends that the first step is actually a realignment of the Earth's magnetic field some 120,000 kilometres away." According to findings from Themis, the second explanation seems to be the correct one.

Themis aimed to resolve the debate by timing each event in a substorm. NASA strung five identical satellites in a line so that they could observe the magnetic realignment, the current flow and the planet. By observing the same substorm from different vantage points, the scientists hoped to discover which event occurred first. "It's like if you try to watch a race, and you're at a bend, you can't really figure out who's won. You need someone at the finish line," says Nicola Fox, a space physicist who researches radiation belts at Johns Hopkins University Applied Physics Laboratory (APL) in Laurel, Maryland. The scientists also used ground cameras to watch the aurora borealis.

On 26 February this year, the instruments monitored a substorm. And the first thing they picked up was magnetosphere realignment, far from Earth. "I think it's pretty convincing," says Jim Drake, a plasma physicist who studies magnetic realignment at the University of Maryland in College Park. "I thought this question was never going to be solved."

To illustrate the discovery, the New Scientist has put up a 3D animation of the process on YouTube:

All that said, proponents of the other hypothesis say more data is needed before their peers can reach a conclusion. Nature quotes space physicist Tony Lui as saying, "I would not call that event a classic substorm . . . I am surprised they picked that kind of event to look at." Lui added, "One has to look at a month of data . . . And do some statistics."

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