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This video shows a series of successive sprites that appeared to "dance" across the screen from right to left. A few minutes later, a second series of sprites danced across the screen in the opposite direction. The thunderstorm producing these events was beyond the horizon.
  • It's estimated that sprites occur randomly with only about 1 percent of all lightning strokes. 



    Red sprites like these are very large--but only moderately bright--flashes of red light seen bursting above thunderstorms high in the atmosphere.
  • A cloud-to-ground lightning strike can carry 100 million to 1 billion volts of electricity. 



    This low light level image reveals diffuse sprites stretching horizontally across the sky for 50 to 100 km.

    For years sprites have been the stuff of pilots' stories. They'd report seeing red or blue flashes above thunderclouds, but the scientific community didn't respond with much enthusiasm. Maybe the flashes were reflections of cloud-to-ground lightning. Maybe they were the product of overly energized imaginations.

    But the accidental capture of the first sprites on video changed all that. It's triggered a decade-long avalanche of studies and speculation, as scientists have scrambled to come up with explanations for these phenomena. Competing theories still jostle for supremacy, but researchers agree on one thing: The discovery of sprites caught everyone by surprise.

    "To learn in 1989 that something existed that we didn't see for 14,000 years, well, it's a bit unsettling," says Edgar A. Bering of the University of Houston, who's spearheading a research project to learn more about sprites this summer. "Hundreds of these occurred each week, and we knew nothing about them. What else is there out there that we don't know about?"

    Here's what's known about sprites: They're observed only during very large thunderstorms ("Think half the size of Nebraska," Bering says). They rise up as much as 60 miles above the Earth's surface, although they're usually brightest around 40 miles up. They're exceedingly brief, lasting only about 10 milliseconds. And they take on a variety of shapes -- they've been likened to jellyfish, hourglasses, even picket fences. 

    What's not known about sprites is even more intriguing to scientists: Why do only some storms produce sprites? Why are sprites observed only during positive lightning strokes -- never during the more common negative strokes? What is the precise mechanism that produces this diffuse light? Do sprites carry electric current upward? If so, what happens when that current reaches the ionosphere? And what is the connection between sprites and other upper atmosphere phenomena, including "blue jets" -- bursts of blue light sometimes seen closer to the cloud tops? 

    So Bering will do what weather scientists often do in a situation like this. He's going to send up some balloons


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    Pictures: Courtesy of Dave Sentman/Geophysical Institute/University of Alaska Fairbanks |
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