Colliding Auroras Produce Explosions of Light

A network of cameras deployed around the Arctic has made a startling discovery about the Northern Lights.


17 Dec 09 – Sometimes, vast curtains of aurora borealis collide, producing spectacular outbursts of light. Movies of the phenomenon were unveiled at the Fall meeting of the American Geophysical Union today in San Francisco.

Colliding auroras photographed by THEMIS all-sky imagers (ASIs) on Feb. 29, 2008. Credit: Toshi Nishimura/UCLA.

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“Our jaws dropped when we saw the movies for the first time,” says space scientist Larry Lyons of UCLA, a leading member of the team that made the discovery.

The collisions occur on such a vast scale that observers on Earth had never noticed them before

NASA and the Canadian Space Agency created the camera network for THEMIS, which consists of five spacecraft launched in 2006.

Earlier this year UCLA researcher Toshi Nishimura completed the Herculean task of assembling continent-wide movies from the individual cameras.

The first movie he showed Lyons was a pair of auroras crashing together in Dec. 2007.

The explosions of light, they believe, are a sign of something dramatic happening in the space around Earth—specifically, in Earth’s “plasma tail.” Millions of kilometers long and pointed away from the sun, the plasma tail is made of charged particles captured mainly from the solar wind. Sometimes called the “plasma sheet,” the tail is held together by Earth’s magnetic field.

The same magnetic field that holds the tail together also connects it to Earth’s polar regions. Because of this connection, watching the dance of Northern Lights can reveal much about what’s happening in the plasma tail.

 


Schematic diagram of Earth's magnetosphere. Earth is the circle near the middle and the plasma tail is denoted in yellow. Credit: Larry Lyons/UCLA

By examining many collisions, Lyons and Nishimura have identified a common sequence of events. It begins with two elements:

(1) a broad curtain of slow-moving auroras and

(2) a smaller knot of fast-moving auroras, initially far apart.
The slow curtain is quietly glowing over the Arctic when
the speedy knot rushes in from the north. The two
auroras collide and an eruption of light ensues.

Lyons believes that the fast-moving knot is associated with a stream of relatively lightweight plasma jetting through the plasma tail. The stream gets started in the outer regions of the plasma tail and moves rapidly inward toward Earth. The fast knot of auroras moves in synch with this stream.

Meanwhile, the broad curtain of auroras is quietly hanging over the Arctic, gently glowing, more or less minding its own business. This curtain is connected to the stationary inner boundary of the plasma tail and is fueled by plasma instabilities there.

When the lightweight stream reaches the inner boundary of the plasma tail—bang!–there is an eruption of plasma waves and instabilities. This collision of plasma is mirrored by a collision of auroras over the poles.

The five THEMIS spacecraft have been able to fly through the plasma tail and confirm the existence of lightweight material rushing toward Earth. (These are the “plasma bullets” reported in a 2008 Science@NASA story “Plasma Bullets Spark Northern Lights.”)

During a magnetic reversal, when the earth’s magnetic field strength drops to only 15 percent of today’s – or even lower – and we lose our shielding, wouldn’t it make sense that these explosions would occur right above our heads … just as I propose in “Magnetic Reversals and Evolutionary Leaps?”

Click here for movies.

Author: Dr. Tony Phillips | Credit: Science@NASA

See entire article:
http://science.nasa.gov/headlines/y2009/17dec_whenaurorascollide.htm?list1350813
Thanks to Hans Schreuder for this link

(THEMIS is short for “Time History of Events and Macroscale Interactions during Substorms.)

 


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