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NORTHERN DE-LIGHTS: These shimmering waves of energy are produced when charged particles from the sun get trapped in Earth's magnetic field and then strike gases in the planet's atmosphere.
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People have long marveled at the majestic and mysterious northern lights that light up the skies over the polar regions of countries like Canada and in Scandinavia. Scientists have known for years that these undulating auroras are caused by a storm of charged particles high above Earth. And although a sight to behold, the forces triggering these lights can endanger satellites and air travelers near the poles. But researchers were in the dark about just what forces acted on these so-called magnetic substorms to produce the shimmering lightshows that dazzle us—until now.

Scientists have debated for decades whether local electrical disruptions in Earth's magnetic field or far-flung happenings in the so-called magnetotail (the tapering region of the magnetic field that points away from the sun) lead to the flare-ups of these substorms and their associated auroras.

Researchers say they were able to pinpoint the source by using measurements of magnetic fields recorded by five satellites that were sent into space as part of NASA's THEMIS (Time History of Events and Macroscale Interactions during Substorms) project, which is designed to track "space weather" events called substorms as they unfold. The answer: these substorms form when Earth's magnetic field lines collapse on each other, showering the upper atmosphere with captured radiation from the sun where it sparks the auroras primarily over Earth's polar regions known as the northern and southern lights (aka the aurora borealis and aurora australis, respectively).

"Charged particles from the sun blow up Earth's magnetic tail like a balloon, and then for some reason the balloon leaks," says study co-author Stephen Mende, a physicist at the University of California, Berkeley.

For the new study, set to be published next month in Science, researchers at ground monitoring stations earlier this year observed substorms erupting with the aid of information sent back by the THEMIS satellites. The five washing machine–size craft were lined up in a row on the dark, or nighttime, side of Earth stretching away from the sun to detect where the substorms start and how they progress. An analysis revealed that pinched magnetic field lines explosively "reconnect," or reform into single lines, in the magnetotail before the substorm is unleashed.

"With these magnetic field lines, it's like pulling back the string on a bow for an arrow, and then the string snaps forward, sending all these particles flying at the Earth," Mende says. This reconnection takes place at a distance of about 80,000 miles out (129,000 kilometers)—approximately a third of the distance to the moon.

Local electrical disruption, the other theorized source of the substorms, also takes place, but not until several minutes after the magnetic field lines reconnect. The findings are the first of their kind, and if supported through further observation and testing, will help unravel one of chief mysteries of space physics, says Robert Rankin, a professor of physics at the the University of Alberta in Canada.

Substorms, as well as more powerful geomagnetic storms unleashed by massive solar flares (which occur about once a year and threaten power grids) get their energy from the outflow of gases from the sun called the solar wind. This stream of high-energy particles, consisting of ionized hydrogen and helium, emanates from the sun in all directions and flies past Earth at one million miles (1.6 million kilometers) per hour.