Satellite Unveils a Black Aurora

SAN FRANCISCO--The luminous curtains of color that shimmer in the polar skies have dark companions, a satellite has revealed. As sheets of electrons cascade down into Earth's atmosphere to trigger the northern and southern lights, a "black aurora" streams up into space alongside them. The invisible aurora completes a vast electrical circuit that researchers had suspected was there, but hadn't observed directly.

The aurorae result from a constant barrage of charged particles from the sun. Earth's magnetic field deflects most solar electrons and traps others in doughnut-shaped radiation belts that girdle the planet. But when the sun ejects major blasts of particles in flares and solar storms, these belts overflow and send electrons streaming toward Earth along the looping lines of the magnetic field, which intersect the planet near the north and south poles. The electrons accelerate as they near the surface, where the field grows more intense. Eventually, they crash into the first wisps of the atmosphere, molecules of nitrogen, oxygen, and other gases 100 to 300 kilometers high. The collisions produce the wavy sheets of light seen at high latitudes.

But this is only the base of the ethereal currents that snake through the upper atmosphere. NASA's Fast Auroral Snapshot (FAST) satellite, launched in 1996, now has spied the outgoing electrons that close the loop, according to results shown here yesterday at a meeting of the American Geophysical Union. The satellite, which swoops on an egg-shaped orbit to within 350 kilometers of Earth's surface, detected electrical impulses from electrons coursing upward within charged sheets that shadow the downward flowing auroral electrons. These outbound electrons don't reach peak speeds until they are a few thousand kilometers high, where the atmosphere is so thin that the particles rarely collide with gases and therefore don't glow, says FAST project scientist Robert Pfaff Jr. of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "This is an aurora we can't see," Pfaff says. "It's a whole new world of space physics." The currents are too tenuous to damage orbiting satellites, he notes.

FAST's results also explain the origins of radio waves that flow into space from auroral regions with a power of billions of watts, says atmospheric physicist Robert Ergun of the University of Colorado, Boulder. The parallel sheets of incoming and outgoing current force the electrons to emit coherent, laserlike radiation at radio wavelengths as they spiral along the planet's magnetic field lines, Ergun says. Such emissions are seen on vastly greater scales from aurorae on Jupiter and Saturn, flares on the sun, and binary stars that orbit closely enough to exchange charged gases. "The [Earth's] aurorae are accessible to spacecraft, but they provide a laboratory to help us understand distant astrophysical events," Ergun says.

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