First Estimates Developed Of Lightning-Associated "Sprites" -- Radio Signals Help Scientists Estimate How Many Occur In Thunderstorms

Sprites accompany roughly one in every 200 lightning strikes. They tower up to 55 miles above a thundercloud, occurring simultaneously with a lightning strike, and can be seen with the naked eye, sometimes from as far away as 400 miles. Sprites are electrical phenomena that appear above thunderclouds, reaching the lower ionosphere. These striated, glowing ribbons appear at several-minute intervals. They are found above all the major landmasses of the earth, according to Reising.

"Sprites are spectacular luminous evidence of electrodynamic coupling between the neutral atmosphere in which weather processes occur and the higher altitude (60-90 km) ionized regions of the earth's atmosphere known as the mesosphere and the lower ionosphere," explains Sunanda Basu, director of NSF's aeronomy program, which funded the research. "The importance of the new finding is that the radio signals produced by lightning discharges that lead to sprites are distinctly different from those due to other lightning discharges."

Researchers focused on a thunderstorm which occurred on August 1, 1996, in western Kansas, above which a total of 98 sprites were recorded in a 90-minute period. The team recorded the radio signals emitted by each lightning strike. For each visible sprite, they examined the corresponding radio wave measurements using custom-designed radio antennas and receivers. Researchers found that the lightning strikes that produce sprites also tend to carry a distinctive radio signature. The radio signals the team "read" were emitted by the lightning itself, rather than by its companion sprite. The information gleaned in the study may have a bearing on climate monitoring and atmospheric chemistry.

"This marks the first time that independent measurements not requiring video have been used to estimate the number of sprites produced by a single thunderstorm," said Reising. A typical lightning strike occurs in one-tenth of a millisecond. But those associated with sprites emit a much longer-lived electrical current. "These electrical currents last for at least several milliseconds," explained Reising. "In a relative sense, that's a long period of time, and radio measurements can easily tell the difference. We can't rely on video alone to count all the sprites, because many times, sprites are visually blocked by the clouds." Also, it would be nearly impossible, and extremely costly, to video-monitor every thunderstorm in the hemisphere, or around the world, he added.

Sprites do not interfere with spacecraft launches, aircraft or telecommunications satellites. However, chemical changes could be produced in the atmosphere by sprites. But in order to address that issue, scientists first need a reliable estimate of how many sprites actually occur. "Using four relatively low-cost receivers, you can count the number of lightning strikes and sprites in the Western hemisphere, 24 hours a day, and at very low cost," said Raising. "A storm in Brazil could be monitored by stations in California and Antarctica. You can do this from 12,000 kilometers away - a quarter of the way around the world."