Wed. Dec 8, 1999 

New Clues to Fleeting Flashes High in the Atmosphere

By Robert Roy Britt
Senior Science Writer
posted: 11:14 am EST 
08 December 1999 

If scientists know little about lightning, as most will admit, they know even less about a hard-to-spot electrical cousin that shoots up above a thunderstorm. But a new study sheds precious light on the mysterious high-altitude discharges, showing us they may be far less rare than suspected.

Scientists call them red sprites. They may or may not be related to blue jets and elves. And, yes, we're completely serious here.

The first images of the enigmatic spurts of energy were made by accident, just 10 years ago. Before that, pilots had reported seeing mysterious red flashes above thunderstorms, but their claims were not always taken seriously. Since then, researchers have learned a great deal about the high-altitude discharges, but still they exist in, or rather above, a cloud of mystery.

A new study, to be published in the Dec. 15 issue of Geophysical Research Letters, explains the observation of two red sprites above a 1998 New Mexico storm. The sprites were associated with negative cloud-to-ground lightning. Previously, sprites were thought to be related only to positive cloud-to-ground lightning, a far less common variety. 

The basics of sprites

Red sprites, which frequently occur in clusters of three or more, soar up to 95 kilometers (59 miles) into the atmosphere, above thunderstorms. Their flash is incredibly brief -- just 3 to 10 thousandths of a second. The flashes expand to cover a wide area but are weak in electrical energy. Their brightest portions exist 65 to 75 kilometers up, above which wisps and glowing regions often extend.

(The black & white animation of multiple sprites at the top of this page was captured in October 1997. Credit: New Mexico Tech/NASA.)

Blue tendrils have been spotted extending below the sprites. These tendrils are not to be confused with blue jets, a separate phenomenon that shoots more slowly upward, and not as high. Red sprites have on occasion been preceded by elves, lower-altitude flashes produced by the widespread heating of the atmosphere, caused by lightning.

Grasping the extent of red sprites, and their physics, could bear on the understanding of upper atmospheric chemistry and even on high-energy particles known to affect satellites. Researchers also say there may be similar phenomena that occur above other planets, though interplanetary comparisons have not yet begun.

The mechanics of sprites

With negative cloud-to-ground (CG) lightning strikes, electrons move from the clouds to the Earth (and, by definition, current moves upward). In a positive CG, electrons move upward. While all of this might be interesting to devoted lightning researchers, red sprite aficionados are most curious about the physics happening above the clouds. And the new research, produced by Stanford University's Christopher Barrington-Leigh and colleagues, changes the perception of what's going on up there.

"Our observation of negative CG sprites suggests that the normal mechanism for sprites does not depend greatly on the sign of the charge moved by the lightning, but only on how much charge is moved by the lightning," Barrington-Leigh told space.com. "The scarcity of -CG sprites must then lie in the fact that most negative CG lightning just doesn't often move enough charge around."

Every second of every day, on average, lightning hits Earth about 50 times. Barrington-Leigh points out that the majority of that activity involves negative CG strikes, of the type he's just observed to be associated with red sprites. This raises the possibility that sprites may be far more prevalent than previously suspected.

The possibly higher frequency could affect our understanding of numerous upper-atmosphere phenomena. Some researchers have speculated that sprites might create nitric oxide, which destroys the protective ozone layer. And there are other possible relationships.

"We have seen evidence that sprites, and the electromagnetic pulses from lightning, may be able to both increase and decrease [respectively] the populations of high-energy particles in the Earth's radiation belts," said Barrington-Leigh. "These remarkable effects could thus affect the rate at which satellites in low and middle earth orbit are being bombarded by electrons."

It's also possible that similar discharges might occur on other planets -- Jupiter and Venus being likely candidates. "Differences in how the analogies to sprites and lightning appear in other planetary atmospheres could serve as a diagnostic for the nature of extraterrestrial ionospheres, atmospheres, and electrification processes," Barrington-Leigh said.

"The study is quite significant," said Matt Heavner, an atmospheric electricity researcher at the University of Alaska, Fairbanks. Heavner, noting that we still understand very little about the lightning that comes out of the bottom of clouds, said knowledge of red sprites is growing rapidly, considering researchers have only been on the trail for a decade.

How the discovery was made

Because sprites are so fleeting, the trick to recording them is to use very fast equipment. Barrington-Leigh and his colleagues set up high-speed photometers (which can detect flashes lasting only a few millionths of a second) atop a mountain in New Mexico, then monitored satellite weather images for storm activity.

"If the weather is clear overhead, we uncover our photometric equipment, start up its software, and point the cameras over distant storms," he said. Each evening, all night for two months, they watched and waited, adjusting the equipment based on what they could see. "Sprites often occur in a repeatable location, so if you don't get one right where you want it, you can make an adjustment for the next one."

Such high-speed imaging has only been used for sprite hunting in the past couple of years, Heavner said, adding that results of similar studies are expected to be released in the coming weeks. 

Heavner told space.com there are several lines of additional research that need pursuing. For one, scientists need to arrive at an estimate of the global rate of occurrence. All sprite observations to date have been in North America, he said, and the heaviest concentrations of lightning around the world have not been investigated.

Armed with more knowledge, researchers might one day include red sprites in other atmospheric computer models. 

"The inclusion of sprites in models of the middle atmosphere, and the improved determination of the total energy that a sprite deposits in the middle atmosphere, will be an exciting next step for sprites research," Heavner said.

A red sprite shooting above a thunderstorm with blue tendrils extending downward. Credit: University of Alaska, Fairbanks. Click to enlarge.

Understanding, observing and reporting sprites from the Universi

Related research at Stanford

Related images from NASA

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