Catching Sprites by Radio

High above the tumult of thunderstorms, gigantic reddish apparitions called sprites sometimes light the sky for a fraction of a second. So far, the only way to record this fleeting glow has been to monitor storms with video cameras. Now scientists may have an easier way to track the phenomenon: A paper in today's issue of Geophysical Research Letters suggests that sprites have a radio wave signature that standard weather antennas can detect up to 12,000 kilometers away.

Sprites are born when electrons accelerated by lightning slam into nitrogen molecules and excite them to emit red light. The flash lasts just 10 milliseconds, illuminating a swath of atmosphere up to 50 kilometers tall. But spotting sprites is difficult, because they are often blocked by clouds. To see if sprites could be caught by a different signal, Umran Inan, an electrical engineer at Stanford University, and his colleagues videotaped a storm over Kansas in 1996 while taking radio measurements.

Perusing the data, the researchers found that 81 of the 98 sprites formed right after a particular type of lightning discharge--one whose charge flows from cloud to ground more slowly than usual. And 83% of these strikes, they noticed, were accompanied by a low-frequency radio wave of about 10 kilohertz.

The work may allow scientists to detect sprites in tropical regions, where the thunderstorms and lightning have very different characteristics from midwestern storms. To date, sprites have been studied mainly in the midwestern United States, says David Sentman, a geophysicist at the University of Alaska. But many questions remain, such as whether they play a role in climate. "We still don't even know how often they occur," Sentman says.

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