Mysterious Northern Lights Understood

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.
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