One night in 1989, as Robert Franz, Robert Nemzek and John Winckler of the University of Minnesota were out testing a sensitive video camera, they accidentally captured a fleeting red glow above a distant lightning storm. The enigmatic light turned out to be an electric discharge connecting thunderclouds with the ionosphere high above—an atmospheric phenomenon new to science that explained many of the strange lights that pilots, among others, had sometimes seen while gazing at the darkened sky. Over the past decade, other investigators have confirmed the existence of such "red sprites" and have carefully studied these curious flashes (along with other eerie nighttime lights called "elves" and "blue jets"). But only recently did they learn that sprites can also form during the day.
Mark Stanley, a graduate student at New Mexico Tech, and three colleagues reported the first detection of daytime sprites in the March 15 issue of Geophysical Research Letters. These flashes of light would have been exceedingly difficult, if not impossible, to detect optically while the sun was up, but Stanley and his colleagues used a completely different tactic. They had built special radio equipment to study ordinary lightning discharges that take place within dense thunderclouds—bolts that might be quite bright but are nevertheless hard to see from a safe distance. Stanley realized that the characteristic radio signature of sprites would allow him to use this same apparatus to identify daytime examples, if they existed at all.
Others had considered this possibility, too, but it was unclear whether sprites could form while the sky is bathed in the sun's light. The difficulty is that solar ultraviolet photons keep the atmosphere ionized to lower levels during the day than at night. Sprites normally start out in very thin air (at an altitude of about 75 kilometers), moments after a huge discharge of cloud-to-ground lightning creates a sizable electric field higher up. Such whopping electric fields cannot arise within the conductive ionosphere, which thus acts as a lid for the electrical goings-on below. Because the base of the ionosphere drops to about 60 kilometers during the day, sprites cannot begin to form at the usual level. And for a sprite to initiate lower down, in more dense air, requires a larger electric field, perhaps one greater than any lightning discharge can create—or so it was thought.
Despite this uncertainly, Stanley and his colleagues decided to search for daytime sprites. They began a concerted campaign in 1997, operating their radio gear whenever promisingly large thunderstorms erupted. Because the radio technique is sensitive to events taking place well over the horizon, they followed electrical storms raging many hundreds of miles away. These physicists looked intently during that entire summer for the fingerprint of sprites during daylight hours. They found nothing.
"I never did look for daytime sprites after that," recalls Stanley, "I pretty much ruled it out." And so he got on with other projects. One was to conduct radio measurements that might provide "ground truth" for a satellite that was launched that summer. The Fast On-orbit Recording of Transient Events (FORTE) satellite detects optical flashes and radio emissions from the atmosphere, primarily to help monitor clandestine nuclear testing. But scientists also intended to use that satellite to make observations of natural atmospheric discharges.
On August 14 of the following year, Stanley turned on his radio equipment to record a fly-by of the FORTE satellite. The orbital geometry was rather unfavorable, with the satellite passing a mere 15 degrees above the horizon. But Stanley noted that storms were brewing in southern Texas, directly below the speeding spacecraft, and he thought it might just be worth collecting measurements. A few minutes later he was rewarded in a way he had not expected—with the characteristic radio signature of a sprite. Stanley was shocked: "Five minutes later, I saw the next one come through, and five minutes later the next. I knew these had to be sprites." Although his apparatus was switched on for only 35 minutes, he had accidentally captured three sprites in a tiny fraction of the time he had spent looking for them during the previous summer.
Careful analysis showed that the lightning discharges that spawned these sprites were indeed exceedingly large. Specialists concern themselves with the so-called charge moment, the product of the current and the height of the discharge. Typical nighttime sprites take place after lightning with moments of less than 1,000 coulomb-kilometers. The daytime sprites Stanley recorded followed discharges with moments about five times that size.
The requirement for exceptionally large lightning discharges explains why daytime sprites are rare creatures—so rare, in fact, that no one has detected any since Stanley's fateful half-hour recording session nearly two years ago. Stanley, for one, has certainly tried again to find more examples, and he points out that Matt Heavner of the Los Alamos National Laboratory, who operates a network of specialized radio receivers capable of detecting such sprites, has also failed.
Why was the 14th of August 1998 so different from other days? The Texas storm that aroused Stanley's curiosity was large but not exceptionally so. "I've certainly seen bigger storms over the Great Plains," he remarks. Stanley speculates that this particular storm might have been special because it carried so much moisture; the areas it flooded made the national news. But what exactly allows a rare thunderstorm to generate just the right conditions for a daytime sprite remains something of a mystery. In any event, atmospheric physicists are pleased that Stanley was lucky enough to turn on his radio gear at just the right moment—and that Franz, Nemzek and Winkler were lucky enough to point their video camera in the right direction back in 1989. Davis Sentman, a leading investigator of sprites at the University of Alaska's Geophysical Institute puts it well: "This whole field is based on serendipity."—David Schneider