Scientists are puzzled over the weirdest atmospheric discovery since cosmic rays: blood-red apparitions with bluish tentacles that flash briefly far above Earth, sometimes near the edge of space.
These so-called "sprites," perhaps bizarre electrical events quicker than the pop of a flashbulb, are as startling to the lucky few who have glimpsed them as the aurora borealis or exploding meteors.Thunderstorms appear to trigger sprites, yet they flicker and flash far higher than ordinary lightning. They dance in the void tens of miles above thunderclouds, with no obvious connection. The biggest sprites are incredibly large, filling thousands of cubic miles of air.
Some scientists worry that sprites and a related phenomenon, "blue jets," could threaten high-altitude aircraft, perhaps by damaging their sensitive electronics. As a precaution, the U.S. Air Force is starting a sprite research project at Phillips Laboratory at Hanscom Air Force Base in Bedford, Mass.
An American Airlines pilot who was flying at 33,000 feet near Panama and witnessed a blue jet called it "the most spectacular and unexpected natural event I have seen."
"About five times, a large discharge of lightning at the top of and within the cloud was followed by a vertical shaft of blue light that propagated from the top of the cloud upward to 100,000 feet," wrote the pilot, John G. Hammerstrom of Tavernier, Fla., in a 1993 letter to Aviation Week & Space Technology.
Blue jets typically occur just above thunderstorms and, as their name implies, have a bluish color. In contrast, sprites - whose name refers to their elusiveness - are far bigger and occur far higher in the upper atmosphere. Some sprites flash more than 50 miles up in the ionosphere, an electrically charged region best known because it reflects radio signals over great distances, allowing AM listeners in places such as San Diego to hear to talk-show chatter in Detroit.
That atmospheric region was once regarded as a dull void, interesting mainly as the gateway to outer space. But today space shuttles fly to orbit, and return to Earth, by passing through the upper atmosphere. It's also familiar to pilots of spy planes.
In a few decades, the upper atmosphere may be teeming with "hypersonic" commercial airline traffic ferrying thousands of passengers from, say, New York to Tokyo in a few hours.
While there have been some measurements and some theory and computer modeling of sprites, no one has done any definitive study of the potential effects of these phenomena on Air Force aircraft systems such as infrared sensors, according to the head of the Air Force sprite project, physical chemist Laila Jeong.
"The best that can be said is there may be potential effects - and those need to be explored," Jeong said.
The exploration is under way as the thunderstorm season starts to rumble across the American heartland.
In a few weeks, scientists from Lawrence Livermore National Laboratory will take a high-speed camera to Colorado in hopes of filming sprites as they spark in the night sky.
With luck, the Livermore researchers, John Molitoris and Colin Price, will snap up to 40 pictures a second of sprites. With luck, frame-by-frame analysis of the pictures could reveal how sprites evolve during their fraction-of-a-second lifetimes.
"This camera is a `Star Wars' spinoff," Molitoris noted. "The Air Force developed it to scan the heavens very fast in order to identify high-speed missiles."
NASA plans to ask amateur observers to report sprite sightings via the Internet, said Rick Howard of NASA's space physics division in Washington. For those with computer access to the World Wide Web, more information on NASA's "Sprite Watcher" program - and copies of the report forms - will soon be available from this Web address: http://elf.gi.alaska.edu-/sprites.html.
Sprites were anecdotally reported up to a century ago. But most atmospheric scientists ignored the sightings until 1989. That's when University of Minnesota scientists accidentally photographed twin sprites - squirting skyward like illuminated fountains above a thunderstorm in Canada.
Since then, sprites have been photographed from the space shuttle. A satellite that searches the heavens for gamma-ray bursts from black holes and other cosmic objects inadvertently detected gamma rays from Earth's atmosphere. The gamma radiation may come from sprites and blue jets, although direct proof is lacking.
Sprites can be visible to the naked eye. The best viewing place is a high point far from a thunderstorm, with a clear night sky directly over the storm. A patient observer might see sprites flickering in the dark upper atmosphere, tens of miles above the thunderhead's anvil dome, Howard said.
But watch closely, he cautioned: "It's a very quick flash. It's there, and then it's gone, and the first thing you'll think is, `What was that? My mind must be playing tricks on me.' "