The aurora borealis draws tourists from across the globe to see the phenomenon once feared and revered but now understood to be connected to substorms in the Earth's magnetic field. (Bill Rockwell/Canadian Press)

Canadian and U.S. scientists have zeroed in on the cause of the aurora borealis, tracing the shimmering northern lights to the way the "tail" of the Earth's magnetic field snaps back into shape in response to the solar wind.

They knew the aurora borealis were related to solar substorms, the energy releases resulting from the complex relationship between the Earth's magnetic field and the stream of high-speed ionized particles coming from the sun, called the solar wind.

While solar storms are known to result in major events on the sun's surface, the origin of substorms has been a subject of some debate, prompting the Canadian Space Agency and NASA to launch five satellites last year at various positions in the magnetosphere in conjunction with ground stations across Canada and Alaska to track the process.

The first results of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission, taken from a Feb. 26, 2008, solar substorm and published Thursday in the journal Science, suggests that an energy release in the magnetosphere about 20 Earth radii away from the planet starts off the process.

The solar wind has an average speed of 400 kilometres per second, streaming around Earth like water around a rock. (Mikko Syrjsuo/University of Calgary)

The Earth's magnetic field buffers the planet from much of the cosmic radiation of the solar wind, but in doing so takes on the shape of a teardrop, with the side facing the sun compressed while the opposite side stretches out into a far-reaching tail.

One theory on solar substorms suggests that as energy builds up in this tail it stretches it out farther and farther out until, stretched to its limit, the magnetic field snaps back like an elastic band, converting the built-up energy into kinetic and heat energy that returns to the Earth.

Some of the charged particles hit close to the planet's upper atmosphere, where they interact with the gas molecules to create the spectacular displays that are viewable at high latitudes.

Astronomers know this snap-back action occurs during substorms, but they didn't know if this is the cause or just a side effect of the real cause. Another theory suggested that the magnetosphere closer to Earth becomes unstable in response to the solar wind, ballooning out like a bubble forming on an inflated bicycle tire, causing both the northern lights and the magnetosphere reconnection.

The timing of the Feb. 26 substorm, however, supports the theory that the magnetic field reconnection starts the process. The THEMIS satellites reported the magnetosphere reconnection occurred at least 90 seconds before the aurora lights intensified and at least two minutes before the substorm expansion.

University of Alberta professor Ian Mann, a principal investigator with the THEMIS mission, said the results clearly favour magnetosphere tail reconnection as the cause.

"We had absolutely ideal conditions, and the results are crystal clear as to the timing," Mann told CBC News. "It's a beautiful result."

Mann said further study should give the THEMIS team a better understanding of the process as it happens and give them a more accurate sense of the timing of events.

"As we piece together more on these substorms, it should help us with practical applications, such as being able to make accurate space weather forecasts," he said.