Lights out

By Jonathan Knight in San Francisco A NASA satellite has discovered electrons streaming out into space from the Earth’s poles. These particles may be the cause of the mysterious “dark aurora” that breaks up the curtains of light in auroral displays. The magnetic field lines surrounding the Earth act like guide wires for electrons and positive ions streaming out from the Sun, forcing these charged particles to travel along the magnetic field lines towards the Earth’s poles. Near the poles, where the magnetic field lines converge, the electrons accelerate and become highly energised. “It’s analogous to water speeding up as it is forced through a nozzle,” says Charles Carlson, an astronomer at the University of California at Berkeley. When these electrons hit the atmosphere, about 300 kilometres up, they generate bursts of light—the multicoloured displays known as the aurora. But one thing has puzzled scientists. They know these electrons must leave again somehow, or the poles would become highly charged, but no one has yet figured out how. So to search for the departing electrons streaming back out of the poles, Carlson and his colleagues at NASA designed and built the Fast Auroral Snapshot satellite (FAST), which went into orbit in 1996. FAST picks up electrically charged particles moving in any direction. As Carlson reported at a meeting of the American Geophysical Union in San Francisco last week, FAST has found electrons streaming back out from the poles as well as into them. These electrons effectively complete the auroral electric circuit. Because they don’t reach high speeds until after they leave the ionosphere, they don’t collide with atoms in the atmosphere fast enough to produce light. Observers on the ground occasionally see dark columns, known as dark auroras, breaking up the curtains of light in auroral displays, but nobody is certain what causes them. Carlson says the columns could be a result of the rising electrons moving too slowly to generate electromagnetic radiation. “FAST has given us a very detailed picture of the aurora,” says Thomas Hallinan, a geophysicist at the University of Alaska Fairbanks. Another member of the FAST team, astrophysicist Robert Ergun of the University of Colorado at Boulder, hopes the satellite data will help explain the intense radio waves that emanate from the Earth’s poles as well as from the poles of other planets, such as Jupiter and Saturn. The strong electric fields associated with the auroral electric circuit may power laser-like radio emissions from atoms in the atmosphere,
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