The Themis mission is an attempt to settle a long-standing debate on the origins of the rippling lights. Scientists know that the aurora are caused by electrons streaming from space and delivering a kick to gas molecules in the atmosphere. As the molecules relax, they release the energy as light — blue from nitrogen and green and red from oxygen. But the trigger that unleashes those electrons has remained a mystery.

The original energy source for the aurora is the solar wind, a stream of charged plasma that billows from the Sun and deforms the Earth's magnetic field, producing a long 'magnetotail' on the far side of the Earth.

While coronal mass ejections can cause larger plasma storms that last for more than 24 hours, deformation of the magnetotail can create smaller substorms that last just a few hours.

But scientists disagree on the precise order of events between the solar wind and the substorm. One camp holds that the key step is a disruption, which occurs about 60,000 kilometres 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, roughly one-third the distance to the Moon. According the the Themis results, published today by the journal Science,¹ this latter explanation is the correct one.

Which came first?

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."

However, other scientists remain sceptical. "I would not call that event a classic substorm," says Tony Lui, a space physicist who studies substorms at the APL. "I am surprised they picked that kind of event to look at."

Perfect storm

But project leader Vassilis Angelopoulos, a space physicist at the University of California, Los Angeles, insists that the February substorm was a classic substorm worthy of study. “In my mind, the small events are as important as the large events, because the physics is similar,” he says.

Both sides agree that there is more work ahead for the Themis satellites. “One has to look at a month of data,” says Lui. “And do some statistics.” Others say a few more examples would convince them, and that is what Angelopoulos plans to do. “We will collect so many more events that the evidence will be irrefutable,” he promises.

There's still a chance that Themis may make a more even-handed judgement. “My suspicion is that it’s not going to be so much of an either-or, but a both-and,” says Brian Anderson, a space physicist who studies magnetosphere currents at the APL. “Nature has more tricks up her sleeve than we imagined.”