Until recently, researchers knew that electromagnetic disturbances in space called “substorms” caused the colorful streaks of light, but they didn’t know what triggered those storms. Now, with the help of a string of five satellites acting as giant stopwatches and a network of ground-based observatories, they have begun to find out. NewsHour has the details:

[Scientists] traced the storms’ beginnings to a spot about one-third of the distance between the earth and the moon’s orbit, where the earth’s magnetic field lines snap together like rubber bands, flinging charged particles back toward earth.

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The earth is surrounded by magnetic field lines that stretch from the north to the south poles, like a giant bar magnet. But charged particles from the sun — called the solar wind — distort that magnetic field, pulling it into a long tail called the “magnetotail.”

The magnetic field lines from the north and the south poles stretch out and out, until finally, at the point about one third of the way to the moon’s orbit, they snap back together and send charged particles flying back towards the earth.

However, scientists weren’t sure whether this “reconnection point” snap triggered the substorms, or whether it was another event, called current disruption, which is a kind of large-scale “short circuit” of currents of charged particles that flow closer to the earth — about one-sixth of the distance to the moon’s orbit.

To find out which of these theories was right, NASA launched five washing machine-size satellites that orbit the earth at different distances. The satellites lined up once every four days, and when scientists were lucky, a substorm occurred while the orbiters were lined up. Sensors on board the satellites recorded electric and magnetic field strength, as well as the charged particle flow.

Using those observations, along with data from ground-based observatories on earth, [Vassilis Angelopoulos, Principal Investigator of NASA's THEMIS (Time History of Events and Macroscale Interactions in Substorms) poject] and his colleagues figured out that the substorm disturbances begin farther out, at the point where the magnetic fields reconnected, and not at the current disruption location.

Ask the Expert: Dr. Vassilis Angelopoulos

On July 22, 2008, Vassilis Angelopoulos answered selected viewer questions about the THEMIS mission, for which he is Principal Investigator, as well as general questions about the aurora and magnetosphere.

Audio Slideshow: Listen to the Aurora Borealis

View a slideshow featuring an audio recording of an aurora. You cannot hear the sounds it makes in nature; the aurora’s electromagnetic (radio) signals must first be converted into sound frequencies.

For more information about space exploration and NASA, including the Phoenix Mars Lander, visit NOVA ScienceNOW online.