We aren't talking about small supernatural beings, although these sprites and elves are elusive. They are bright flashes in the atmosphere, optical occurrences that extend through the mesosphere and stratosphere into the lower ionosphere. Lee Marshall, a graduate student in electrical engineering, and his colleague Walt Lyons, a meteorologist with FMA Research of Colorado, have discovered that they are linked to a particular kind of lightning.
Airline pilots first noticed these flashes while flying over severe thunderstorms in the Midwest. Then they appeared on videos taken from NASA's space shuttles. Lyons is responsible for naming them sprites and elves. "Both are acronyms," explains Marshall. "Sprite" stands for Stratospheric/Mesospheric Perturbations Resulting from Intense Thunderstorm Electrification, and "elve" for Emissions of Light and VLF perturbations from EMP events. Marshall jokes that maybe the names fit a little too well: "You think you see something, and when you look again, it's gone."
Sprites have a distinctive delicate shape, much like a small human form, which flashes for only a fraction of a second in the sky. Elves are more formless, rather like a glowing bluish haze. They occur separately or together, appearing above grand scale thunderstorms that meteorologists call "mesoscale convective systems." Lyons began video-taping these whimsical light flashes in 1993 because they piqued his interest from a meteorological standpoint. He would perch on a Colorado mountainside on a clear night, with a view stretching out as far as Nebraska, the Dakotas, and Texas, and take pictures of them with a low-light video camera.
After hearing Lyons give a presentation on sprites and elves at the American Geophysical Union conference in San Francisco a few years ago, Marshall decided that he and Lyons would make a pretty good long-distance team. Instead of taking visual photographs, Marshall would capture a graphical representation of the electromagnetic (EM) waves associated with the phenomena. When there is an electrical disturbance in the atmosphere, such as a lightning storm, Marshall explains, EM waves travel from the center of the disturbance outward in a movement that he equates with dropping a pebble into a still pond. The ripples lap away from the center, getting continuously smaller and less defined. Yet the waves created by midwestern storms can still be measured in the atmosphere over Pennsylvania.
Sitting one night during the summer of 1995 at Penn State's Rock Springs Research Farm, Marshall hooked up sensitive receiving antennas to an oscilloscope, a machine that can measure and display the electric and magnetic fields from the antennas and graph them on a small screen. Connected to the oscilloscope was a notebook computer, which collected the data for processing. "Lightning makes an interference in the atmosphere—if you were listening to the radio you would hear static, or watching TV, you'd see a flicker." Marshall observed those interferences on the screen of his oscilloscope and compared his data with what Lyons, talking on the other end of a cellular phone in Colorado, was simultaneously videotaping.
Marshall explains, "While I watched the screen, I noticed something neat. Most lightning produces an electric signal that's very sharp. Most lightning signals, technically called sferics, are short, quick, and bipolar." However, with some lightning, the sharp signal is followed by what scientists call a "slow tail," a lasting impression on the screen that resembles a tail. When a slow tail appeared on the screen, Marshall could hear Lyons on the cellular phone shouting, "Sprite!" Sometimes both a sprite and an elve would appear, sometimes one or the other flashed alone. Regardless of which type of occurrence dazzled the night sky, sprites and elves were indeed connected exclusively with lightning that produced a slow tail.
Videotaping or recording electromagnetic waves are not the only ways to observe these elusive occurrences. Marshall has also records the sferics on audio tape—to hear the crackle and pop of sprites and elves flashing in the sky.
After hooking up a tangled mess of wires from one piece of equipment to another in his cluttered office, Marshall sits close to the machine, his ear near the speaker. "OK, hear all those popping and crackling noises? Now . . . there. Did you hear it?" he asks, rewinding the tape to play it again. To an unpracticed ear, the tape sounds like oil frying in a pan or someone squeezing a handful of bubble wrap, but to Marshall the sounds are familiar, and telling. He can pick out a distinctive pop and know it was a sprite, not an elve.
By joining Lyons in Colorado, Marshall plans to collect "up close" EM data, and compare it to computer simulations of sprites and elves. He is hoping to find out what is producing the slow tail signals. Is it the actual sprite or elve, or a signal emitted from the lightning, which then causes the sprites and elves?
Knowing the intricacies of this lightning phenomenon, Marshall says, could affect scientists' attempts to model the global climate or otherwise understand the effects of weather. "The electrical perimeters of the atmosphere can have a strong effect on the chemistry of the upper atmosphere," he explains, "which is where ozone reactions and those involving the greenhouse gases take place. The effects of sprites and elves on the 'global electrical circuit' may be an important variable in these equations."
Lee Marshall is a Ph.D. student in the department of electrical engineering, College of Engineering, 332 EE East, University Park, PA 16802; 814-865-2361; lhm100@alumni.psu.edu. His advisers are Charles C. Croskey, Ph.D., professor of electrical engineering, 304 EE East; 865-2357; and Leslie C. Hale, Ph.D., professor emeritus of electrical engineering, 332 EE East; 505-532-1529. Walt Lyons is a meteorologist with FMA Research Inc., Ucca Ridge Field Station, Ft. Collins, CO ; 970-568-7664. This research is funded by the National Science Foundation.