Yearly Archives: 2008

Researchers have discovered what causes brilliant outbursts of Northern Lights: Gigantic plasma bullets launched toward Earth by explosions 1/3rd of the way to the Moon.

From NASA’s Science web site:

Duck! Plasma bullets are zinging past Earth.

That’s the conclusion of researchers studying data from NASA’s five THEMIS spacecraft. The gigantic bullets, they say, are launched by explosions 1/3rd of the way to the Moon and when they hit Earth—wow. The impacts spark colorful outbursts of Northern Lights called “substorms.”

“We have discovered what makes the Northern Lights dance,” declares UCLA physicist Vassilis Angelopoulos, principal investigator of the THEMIS mission. The findings appear online in the July 24 issue of Science Express and in print August 14 in the journal Science.

The THEMIS fleet was launched in February 2007 to unravel the mystery of substorms, which have long puzzled observers with their unpredictable eruptions of light and color. The spacecraft wouldn’t merely observe substorms from afar; they would actually plunge into the tempest using onboard sensors to measure particles and fields. Mission scientists hoped this in situ approach would allow them to figure out what caused substorms–and they were right.

The discovery came on what began as a quiet day, Feb 26, 2008. Arctic skies were dark and Earth’s magnetic field was still. High above the planet, the five THEMIS satellites had just arranged themselves in a line down the middle of Earth’s magnetotail—a million kilometer long tail of magnetism pulled into space by the action of the solar wind.

That’s when the explosion occurred.

A little more than midway up the THEMIS line, magnetic fields erupted, “releasing about 1015 Joules of energy,” says Angelopoulos. “For comparison, that’s about as much energy as a magnitude 5 earthquake.”

The blast launched two “plasma bullets,” gigantic clouds of protons and electrons, one toward Earth and one away from Earth. The Earth-directed cloud crashed into the planet below, sparking vivid auroras observed by some 20 THEMIS ground stations in Canada and Alaska. The opposite cloud shot harmlessly into space, and may still be going for all researchers know.

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Images by Jeff Hapeman and Karl Johnson via Space Weather’s Aurora Gallery