Radio signals help scientists track sprites

Sprites are electrical phenomena that accompany roughly one in every 200 lightning strikes

April 5, 1999
Web posted at: 4:00 PM EDT

Sprites, the spectacular red and blue flashes of light that extend upward from electrical thunderstorms to altitudes as high as 60 miles, are the focus of a study by researchers Steven Reising of the University of Massachusetts, and Umran Inan and Timothy Bell of Stanford University in California.

The scientific team has found that radio signals can be used to determine the number of sprites spawned by a single thunderstorm. These signals may be used to monitor climate and atmospheric chemistry, according to the researchers.

Up until now, scientists have only been able to obtain visual images of sprites.

The team's findings appear in the April 1 issue of Geophysical Research Letters published by the American Geophysical Union. The work was funded by the National Science Foundation, the U.S. Air Force and NASA.

Sprites are electrical phenomena that accompany roughly one in every 200 lightning strikes. They tower up to 60 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.

Some of the flashes extend up through the ozone layer into the base of the ionosphere, the region of the upper atmosphere where auroras occur. 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."

In a study of a thunderstorm that occurred in western Kansas on Aug. 1, 1996, scientists recorded the radio signals emitted by each lightning strike. They then compared the signals to the appearance of 98 recorded sprites. 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 recorded 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.

According to the research team, 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," said Raising.

For more information, contact Sunanda Basu, NSF, (703)306-1529, email: sbasu@nsf.gov.

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