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04/25/95

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Scientists study recently discovered atmospheric lights called red sprites

STANFORD -- It's a hot August night. A massive thunderstorm, repeatedly lit with lightning flashes, looms over western Oklahoma and the Texas Panhandle.

Above the anvil-shaped storm clouds, millisecond bursts of blood-red light flash 60 miles into the ionosphere, brushing the edge of outer space. Almost subliminally brief, the flashes appear like amorphous, glowing octopi with red "heads" pointing toward space and purple "tentacles" dangling toward earth.

These strange and beautiful phenomena are "red sprites." Named for their illusory nature, sprites occur during severe thunderstorms and appear to be related to powerful lightning ground strikes. In a single night, a large storm system may emit several hundred sprites.

Red sprites have been discovered by scientists only in the last decade and are currently the subject of intense scientific investigation and speculation around the world, including Stanford.

"The research is being driven by a scientific desire to understand these mysterious, spectacular occurrences that were just recently discovered. We thought we knew the basic physics of lightning and now we find we don't," said Umran S. Inan, professor of electrical engineering and an atmospheric physicist at Stanford's STARLAB (Space, Telecommunications and Radioscience Laboratory).

Stanford research

Inan; Victor Pasko, a doctoral student in electrical engineering; and Tim Bell, senior research associate, proposed one of the first major theories to explain sprites in a paper published in the Feb. 1 issue of Geophysical Research Letters. They assert that red sprites are produced when speeding atmospheric electrons, disturbed by lightning ground strikes, collide with nitrogen in the upper atmosphere.

Following a positively charged lightning ground strike and the sudden dissipation of a thundercloud's electric charge, an electric field is briefly created above the storm, Inan said. Electrons in the mesosphere are accelerated by the sudden electrical change. Their rapid movement produces heat, ionization and red light when the electrons collide with nitrogen molecules.

Inan, Pasko and Bell used a computer model to support their theory. However, the scientists said that their model explains only the sprite's red head, not its dangling purple tentacles. The sprite's lower portions may be formed when cosmic rays are accelerated by the atmospheric electron discharges produced by lightning.

There is still much to learn about sprites. This summer, Inan and his colleagues will conduct an experiment to measure the location, size and structure of the disturbed ionospheric regions around sprites.

From transmitters in Maryland and Puerto Rico, the researchers will beam radio waves to six receivers positioned along the California coast from Stanford to San Diego. The technique, called "very low frequency strip holography," will produce a three-dimensional picture of the atmospheric disturbances caused by sprites.

Sprites may affect certain forms of high-frequency radio signals that are bounced off the ionosphere, like short-wave, or transmissions from satellites that pass through the ionosphere, such as navigation signals that are used by ships and airplanes, Inan said.

History

Red sprites have been reported for decades by airline pilots but have been photographed and studied by scientists only in the last seven years.

The flashes were first photographed by accident on a clear summer night in 1988 by two University of Minnesota physicists experimenting with a low-light video camera. As they were photographing a storm on the distant horizon, the video monitor sparked with the image of a discharge of light rising above the thunderheads. John R. Winckler, one of the physicists, described the incident in the journal Science.

This article inspired researchers at NASA's Marshall Space Flight Center in Huntsville, Ala., to search videotapes taken by orbiting space shuttles. They came upon a tape of a large storm system over northwest Africa that showed eruptions of light high above the clouds, and subsequently found many similar images from other parts of the world. However, these images, photographed from more than 100 miles above the earth, were small and difficult to analyze.

Next, scientists at the University of Alaska's Geophysical Institute in Fairbanks attempted to photograph the sprites from a closer range. In July 1993, Davis Sentman and Eugene Wescott photographed intense midwestern thunderstorms from a specially equipped high- altitude jet aircraft. They captured 19 images of sprites on black-and- white videotape.

The following summer, in July and August 1994, Sentman and Wescott used a pair of NASA jets equipped with color video cameras to record a series of astounding images of red sprites that intrigued scientists around the globe and initiated a flurry of scientific investigation.

Competing theories

The great interest in sprites was evident at last year's meeting of the American Geophysical Union, the world's largest gathering of earth scientists. More than 40 papers were presented that dealt with sprites. Since then a number of scientists have advanced theories to explain the forces that create these unusual celestial lights.

A team of scientists headed by Walter Lyons at the Massachusetts Institute of Technology studied readings from a national system of lightning ground strike detectors and discovered that red sprites correlate with positively charged ground strikes, which make up about 10 percent of lightning and are many times more powerful than more common negatively charged lightning.

Earle R. Williams of MIT argued that the flashes are akin to giant electric sparks. After a powerful ground strike, the electric field above the storm becomes strengthened to the point that it causes "dielectric breakdown," an overload that breaks down the atmosphere's resistance to electrical current flow. The result is an immense red spark in the mesosphere, a sprite. Williams has recreated this situation in a laboratory experiment.

Inan, on the other hand, argued that dielectric breakdown is not required to produce red sprites; the movement of energetic electrons and their collision with nitrogen is enough. He said he does not consider Williams' model sufficient to explain sprite formation.

One NASA scientist proposed that red sprites are excited by ultraviolet glow from cloud-to-cloud lightning. But that does not explain the heat or radio transmissions that sprites produce, Inan said.

In another theory, Yuri Taranenko, who studied with Inan while earning his doctorate at Stanford, and Robert Roussel-Dupre of Los Alamos National Laboratory proposed that cosmic rays strip electrons free from air molecules and that the electric fields above storm clouds accelerate the electrons upward, causing them to rip even more electrons free. The careening electrons bombard nitrogen molecules and emit red light. This is called the "runaway breakdown model."

The Stanford researchers have independently proposed their own runaway breakdown model. According to Inan, this model may explain why some sprites have downward pointing tentacles. The light may follow different cosmic ray paths. Their paper explaining this process has been submitted to Geophysical Research Letters.

Effects

There is speculation among scientists that the bursts of light and energy may affect the ozone layer or contribute to global warming. Some scientists also have suggested that they may pose a threat to high- altitude spy planes or the space shuttle. But at this point these possibilities are not being taken too seriously, Inan said.

The sprites have other interesting effects, however. MIT researchers found that the sprites created the low-frequency radio bursts that have been picked up by instruments around the world for years, but whose source was a mystery.

Large bursts of gamma rays, emanating from the earth rather than space, were found by NASA scientists to originate during thunderstorms and to coincide with sprites. The scientists had puzzled over the gamma ray bursts for several years, ever since NASA's Compton Gamma Ray Observatory, launched in 1991, began recording them as soon as it became operational.

So far, red sprites have had one definite effect: They have brought together scientists from several different disciplines to solve a major atmospheric mystery and have awakened scientific curiosity about lightning 250 years after Benjamin Franklin flew his kite.

Because red sprites reach from the lower atmosphere to the edge of space, the study of sprites has brought together meteorologists and space physicists, who are working in a joint effort to understand the phenomena. Meteorologists generally study the lower, electrically "neutral" atmosphere, while space and ionospheric physicists study charged particles in the upper atmosphere.

"We are learning together and from each other," Inan said. "It is very rewarding for all involved."

-hs-

This article was written by Stanford News Service intern Hudson Sangree.

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