AMHERST, Mass. - For the first time, engineers and scientists have a reliable way to estimate 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, assistant professor of electrical and computer engineering at the University of Massachusetts, along with 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), which is published by the American Geophysical Union. The work 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 a thundercloud, occurring simultaneously with a lightning strike. Sprites can be seen with the naked eye, sometimes from as far away as 400 miles, Reising said. They 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.
"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 noted that for many decades, airline pilots reported seeing sprites, and were met with skepticism. "The pilots were vindicated," he said. Reising works with the University''s Microwave Remote Sensing Laboratory, and is embarking on an extension of the laboratory''s research using new portable radars to study the interior structure of clouds.
In the GRL paper, the researchers focused on a thunderstorm which occurred on Aug. 1, 1996, in western Kansas, above which a total of 98 sprites were recorded in a 90-minute period. The team recorded the sprites on videotape, as well as 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, Reising said. Researchers found that the lightning strikes which 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, Reising said. But those associated with sprites will emit a much longer-lived electrical current. "These electrical currents last for at least several milliseconds," explained Reising. "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." 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, 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."