SAN FRANCISCO - The aurora borealis and australis, the eerie, multicolored lights that often streak the sky above the Earth's polar regions, have a dark counterpart invisible to the naked eye, a team of researchers said Tuesday.
Downward flowing electrons, accelerated by the electric fields in the Earth's magnetosphere, cause the visible auroras -- known as the northern and southern lights -- when they collide with the upper atmosphere.
But researchers using NASA's Fast Auroral Snapshot (FAST) spacecraft have detected for the first time a consistent and regular upward flow of electrons as well.
However, the upward-moving electrons are accelerated at altitudes so high there is no atmosphere to collide with -- and thus no magnificent displays that make the visible auroras such a delight. Physicists have dubbed the phenomenon the "black" or "invisible" aurora.
"FAST has opened the door to a whole new world of space physics," said Robert Pfaff, the mission's project scientist at NASA's Goddard Space Flight Center. The results were presented Tuesday during the fall meeting of the American Geophysical Union.
FAST, launched in 1996, uses detectors to sample electrons speeding from all directions with unprecedented resolution -- regardless of the spacecraft's own orientation.
Furthermore, the team reported, the FAST results show that the visible aurora and its invisible sidekick act together to form an enormous electric circuit in the Earth's magnetosphere, or the extent of the planet's magnetic field.
The upward flowing electrons return the current to the Earth's magnetosphere, flowing upward like the beam of a searchlight to complete the giant circuit.
"One of the mysteries before FAST was where does the current go?" said Charles Carlson, of the University of California, Berkeley.
The FAST research has also shed light on how the Earth generates radio emissions backed by 1 million watts of power.
The electrons causing the auroras are accelerated by electric fields oriented parallel to the magnetic field lines that converge at the Earth's poles. As the electrons travel through the funnel-like shape, they are accelerated like water coursing through a narrow nozzle at the end of a garden hose.
Those parallel electric fields in turn generate radio emissions at just below the AM band.
"It makes the Earth look like a giant radio transmitter," said Robert Ergun, of the University of Colorado.
The discovery should help scientists understand phenomena detected beyond Earth by radio astronomers.
"It is a fundamental process seen on other stars and planets throughout the universe," Ergun said.
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