SAN FRANCISCO--Videos are revealing the fine-scale structure of eerie red flashes that dance delicately atop thunderclouds. Known as "sprites" and resembling high-altitude Christmas trees, the ephemeral flashes show a surprising array of patterns, which researchers described here this week at a meeting of the American Geophysical Union. Detailed studies of the images promise to shed light on the weird properties of sprites, which show how electricity courses through the thin upper atmosphere.
First spotted in 1990, sprites often appear when a storm unleashes a rare but powerful bolt of lightning that blasts positively charged current toward the ground. Researchers believe that the cloud top reacts by driving swarms of electrons into the upper atmosphere; along the way, the electrons crash into nitrogen molecules between 50 and 90 kilometers high and make them glow a fiery red. But because sprites are unpredictable and last just thousandths of a second, scientists have been stymied in their efforts to determine exactly what causes them.
To gather these elusive images, a team led by electrical engineer Umran Inan of Stanford University set up a 40-centimeter-wide telescope atop a 3300-meter peak in New Mexico. A fast video camera attached to the telescope recorded hundreds of sprites in July and August, as the telescope peered at the tops of strong storms in northwest Mexico, hundreds of kilometers away. A major surprise included electrified channels of air that crisscross along horizontal paths, whereas the scientists had expected sprites to shoot upward like inverted brooms. In some cases, light flashed repeatedly in the same channel like the lingering trail of a meteor, instead of disappearing with the rest of the sprite. "The complexity is mind-boggling, and there is no theory to explain some of these details," Inan says.
The sprite images reveal structures resembling "fireworks" patterns seen in earlier videos of sprites made from airplanes, says geophysicist Davis Sentman of the University of Alaska, Fairbanks, who runs a sprites site on the Web. Inan's telescope studies bolster the airplane results by watching many more storms and revealing details as small as 10 meters across, he notes.