Mysterious "Sprites" Light Shows Captured on Film
for National Geographic News
|June 18, 2007|
Blink during a thunderstorm and you may miss the unusual phenomenon of "sprites"—resplendent bursts of light that, for less than a second, burn brighter than Venus.
These brief explosions, which can outshine everything except for the sun and moon, are so fleeting, that scientists still don't know much about how they work.
(See related: "Huge Mystery Flashes Seen in Outer Atmosphere" [June 25, 2003].)
So in a recent study, a group of physicists used an ultra high-speed digital camera to record sprites in unprecedented detail—10,000 frames per second.
"We realized that all these branches and long luminous features that we saw in the sprites, that they didn't really exist," said lead author Hans Nielsen, a physicist at the University of Alaska, Fairbanks.
"It's sort of like you take a time exposure on a highway, and then all the tail lights of the cars make long streaks in the image."
Instead, sprites are made up of tiny bright balls that speed through the middle atmosphere at one-tenth the speed of light, said study co-author David Sentman, a physicist at the University of Alaska. These sprite balls shine five times brighter than Venus.
(See a video of the sprites.)
Now You See It ...
Many researchers believe the sprites' short duration allowed them to escape notice for so long.
"You see this flash in the sky and then you think 'No, that can't be!" Nielsen said.
Scientists did know, however, that sprites occur during active thunderstorms. They are electronic discharges that make the air above glow a luminous red, similar to how fluorescent bulbs burn.
The sprites can also reach about 10 miles (16 kilometers) across and are between 32 feet (10 meters) and about 328 feet (100 meters) in length. They travel upward through the middle atmosphere to reach about 62 miles (100 kilometers) into the uppermost layer of the atmosphere, called the ionosphere.
When a lightning bolt strikes down to the ground, it can create an electrical field above the storm that accelerates the electrons in the middle atmosphere to collide with gas molecules and glow.
Sprites were predicted in theory by Nobel laureate physicist C.T.R. Wilson in 1925. Their existence was confirmed in 1989 when University of Minnesota physicist John R. Winkler caught them on video by accident.
Study co-author Sentman at the University of Alaska started calling them "sprites" shortly after and the moniker stuck.
Many pilots saw sprites but never reported them for fear of flunking their flight physicals. No ancient mythologies have been matched up with them, although the "Andes lights" seen dancing over the peaks of the mountain range bear a superficial resemblance.
Sprites are visible with the naked eye, but they're gone so quickly it's usually just a blur.
The most recent sprite images, published in the journal Geophysical Research Letters, look like "a ticker tape parade on Broadway," Nielsen said. "You see all these balls of light falling down from the sky."
Matt Heavner is a geophysicist at the University of Alaska Southeast in Juneau who was not involved in the study.
"People are identifying a lot of different kinds of discharge occurring in the middle atmosphere," Heavner said.
(See related: "Ball Lightning Mystery Solved? Electrical Phenomenon Created in Lab" [January 22, 2007].)
For instance, "elves" are areas about 250 miles (402 kilomters) wide that glow a dim red. They can be found about 60 miles (96 kilometers) above thunderstorms.
"Blue jets" appear as bright blue cones extending from the top of a thundercloud to 25 to 30 miles (40 to 48 kilometers) above Earth. They are more rare than sprites and last up to a third of a second.
"They actually look like whale spouts shooting up out of the storms," Heavner said.
More Than a Light Show
The energy released by a sprite amounts to a fraction of what comes from a lightning bolt. A single bolt can power a light bulb for anywhere from one to three years, lasts just 10-milliseconds.
"It's not like were not talking about atomic bombs going off or anything, but there could there could be as many as ten sprites a minute globally worldwide," said Walt Lyons, a meteorological consultant at Forensic Meteorology Associates in Fort Collins, Colorado.
"So they probably do play some role in the energetics of the atmosphere," he added.
Lead study author Nielsen wonders whether sprite activity can change the composition of the atmosphere in any way, which could potentially impact on the ozone layer and global climate change.
"The jury is still out as to whether sprites are important in the atmospheric system," he said, "or if they are just like the rainbow, pretty to look at but with no further significance."
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