San Francisco, Calif. -- A Penn State graduate student, sitting in a field in central Pennsylvania, is capturing a record of sprites and elves that were seen over northern Texas. Not the mythical creatures from a storybook, but these sprites and elves are optical phenomena that occur during some thunderstorms.
"One night in August 1995, I sat in my truck in the middle of Penn State's Rock Springs farm and recorded the electromagnetic signals from events being videotaped in Colorado of a north Texas storm," says Lee Marshall, graduate student in electrical engineering. "I was on the cellular phone with Colorado and when a signature sprite waveform appeared on my equipment, I could hear the people in Colorado yell `sprite.' "
Sprites are red, vertically oriented flashes of light caused by ionized nitrogen that appear above thunderheads and can rise from 30 up to 50 miles above the ground. They are 1 to 60 miles in diameter. Sprites last only a few tens of milliseconds.
Elves are diffuse glows of ionized gas molecules that occur just below the ionosphere about 56 mile above the ground. Shorter-lived than sprites, they last for only a few hundred of microseconds.
Sprites and elves can occur individually or together. Sprites can barely be seen with the naked eye and are easily photographed with low-light cameras. However, elves are fainter, and we require light amplification equipment to see them. The electromagnetic signature associated with them can also be recorded and analyzed.
"All electromagnetic recordings of sprites and elves have a slow tail' in the direction that indicates positive' lightning," Marshall explained to attendees at the fall meeting of the American Geophysical Union meeting today (Dec. 16) in San Francisco. "But the difference between sprites and elves, electromagnetically, appears to be very complex."
Not all thunderstorms produce sprites or elves. Small and short-duration, storms like those in Florida do not. It takes a regional- size thunderstorm or mesoscale storm to cause these phenomena, and when they occur, it is on the trailing edge of the storm.
Not all lightning produces sprites and elves. Only 40 percent of lightning is cloud-to-ground discharge and only 5 to 10 percent of cloud-to-ground lightning is positive lightning. Of this small number of positive cloud-to-ground lightning strokes, only about 10 percent create elves and sprites.
Marshall, working with Dr. L. C. Hale, professor emeritus of electrical engineering and Dr. C. L. Croskey, professor of electrical engineering, at Penn State and Dr. W.A. Lyons of FMA Research Inc., Ft. Collins, Colo., is capturing the radio wave pattern of these optical phenomena with a system that automatically triggers an alert when a sprite/elve pattern appears and captures the signal.
"All elves and sprites have a typical wave pattern with a slow tail," says Marshall. "But while all sprites and elves have this pattern, so do other lightning strikes that apparently do not generate elves and sprites."
Researchers do not know how elves and sprites are formed or if the electromagnetic pattern is caused by the cloud-to-ground lightning, the elves and sprites, or a combination of everything.
"There is a lot of electrical noise in the atmosphere," says Marshall. "These really low wavelength signals get caught between the Earth and the ionosphere as if they were in a waveguide and they can propagate around the globe. It is very difficult to take apart the signals."
At frequencies of 150 Hertz, one cycle extends 1,300 miles from Texas to Pennsylvania.
"Because of the distance, exact timing of the parts of the signal is not possible," says Marshall. "Next summer I will go out to Colorado so we can capture the electromagnetic signal up close."
The project is funded by the National Science Foundation.
Hopefully, this proximity will allow Marshall to time the electromagnetic signal to the visual phenomenon closely enough to determine if the lightning or the sprites and elves are the cause.
According to Marshall, some people believe that the occurrences of sprites and elve events will increase with the temperature rise suggested by global warming theories. If so, a method of monitoring all the sprites and elves that occur each day around the globe could help monitor global warming. Because global optical monitoring of all sprites is difficult, even for satellites, Extremely Low Frequency(ELF) radio measurements, such as those being developed at Penn State, may be the best way of monitoring and counting sprites. -30-
Editors: Mr. Marshall may be reached at (814) 865-2361 or firstname.lastname@example.org