A NASA satellite has discovered electrons streaming out into space from the
Earth鈥檚 poles. These particles may be the cause of the mysterious 鈥渄ark aurora鈥�
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鈥檚
poles.
Near the poles, where the magnetic field lines converge, the electrons
accelerate and become highly energised. 鈥淚t鈥檚 analogous to water speeding up as
it is forced through a nozzle,鈥� says Charles Carlson, an astronomer at the
University of California at Berkeley.
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When these electrons hit the atmosphere, about 300 kilometres up, they
generate bursts of light鈥攖he 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鈥檛 reach high speeds until after they leave the
ionosphere, they don鈥檛 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.
鈥淔AST has given us a very detailed picture of the aurora,鈥� 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鈥檚 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, but no one yet knows how this happens.
