SAN FRANCISCO--Lightning bolts pack a punch when they hit the ground, but they pale next to sprites--intense flashes that go the other direction, propelling energy into the atmosphere high above storm clouds. New high-speed electronic images of sprites have exposed the finest details yet, including strange remnants of glowing light that slowly ascend long after the sprite has vanished.
A century of anecdotal reports about fleeting bursts of light above the distant horizon were verified in 1990, when atmospheric scientists first caught sprites on film. The discharges accompany rare but powerful strokes of lightning that carry positively charged currents. To counteract those downward pulses, storm clouds blast negatively charged electrons into the ionosphere, 40 to 90 kilometers high. The currents ionize nitrogen molecules, which emit red light. Videos of sprites have revealed fantastically intricate branches and tendrils, but typical video cameras snap 60 frames per second--far too few to catch changes in the short-lived sprites.
The new films, shown here on 7 December at a meeting of the American Geophysical Union, display the birth and growth of sprites at 1000 frames per second. Geophysicist Hans Stenbaek-Nielsen of the University of Alaska, Fairbanks, built the camera to intensify the low levels of light from flickering auroras. But in August 1999, graduate student Dana Moudry used its sensitive electronic detector to monitor the atmosphere over a Nebraska thunderstorm from a perch in southern Wyoming more than 500 kilometers away. Her video frames (see figure) show tendrils darting downward as channels of nitrogen light up, while a fierce spray of current surges high into the ionosphere in a few thousandths of a second. Later frames feature motionless beads and faint "crawlers" of light that linger a tenth of a second and drift upward, like buoyant embers from a firework. No one predicted such aftereffects from earlier videos of sprites, Moudry says.
Moudry's images are "extraordinary," says lightning specialist Walt Lyons of FMA Research in Fort Collins, Colorado. "They're the best images of sprites we've ever gotten, and they illustrate how little we know." The movies should help theorists explain the physics of sprites, Lyons notes. For instance, the beads might arise from dust left behind by meteors and ignited by electric currents.