Mar. 31, 1999 WASHINGTON, D.C. -- For the first time, engineers and scientists have a reliable estimate of the number of "sprites" spawned by a single thunderstorm. Sprites, the luminous red glows that are the high-altitude companions of some lightning strikes, are the focus of a new study by Steven Reising of the University of Massachusetts and Umran Inan and Timothy Bell of Stanford University in California. The team's findings appear in the April 1 issue of Geophysical Research Letters (GRL), published by the American Geophysical Union. The research was funded by NASA, the U.S. Air Force, and the National Science Foundation.
Sprites accompany roughly one in every 200 lightning strikes. Studied primarily since 1994, they tower up to 55 miles above thunderclouds, occurring simultaneously with lightning strikes. Sprites can be seen with the naked eye, sometimes from as far away as 400 miles.They are electrical phenomena that appear above thunderclouds, reaching the lower ionosphere. These striated, glowing ribbons appear at several-minute intervals above all the major landmasses of the Earth, according to Reising.
"I really enjoy this field because it's so newly discovered," said Reising."Sprites only began to be studied in detail about five years ago. It's remarkable that a phenomenon that's existed during all of human history essentially went unnoticed for such a long time." He added that for many decades, airline pilots reported seeing sprites, but were met with skepticism. "The pilots were vindicated," he said. Reising, an engineering professor who works with the University of Massachusetts Microwave Remote Sensing Laboratory, is embarking on an extension of the research using new portable radars to study the interior structure of clouds.
In the GRL paper, the researchers focus on a thunderstorm that occurred on August 1, 1996, in western Kansas, above which 98 sprites were observed in a 90-minute period. The team recorded the sprites on videotape, along with the radio signals emitted by each lightning strike. For each visible sprite, they examined the corresponding radio wave measurements, using custom-designed 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, Reising said.
"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. In a relative sense, that's quite 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," he said. Also, it would be nearly impossible, and extremely costly, to video-monitor every thunderstorm in the hemisphere, or around the world. Sprites do not interfere with spacecraft launches, aircraft, or telecommunications satellites, Reising said. However, there is some concern in the scientific community regarding chemical changes that sprites could potentially produce in the atmosphere. But in order to address that issue, scientists first need a reliable estimate of how many sprites actually occur. Another application of the same lightning monitoring technique would be the monitoring of rainfall in remote areas.
"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. A storm in Brazil could be monitored by stations in California and Antarctica," said Reising. "You can do this from 12,000 kilometers away--a quarter of the way around the world."
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