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Glowing doughnuts flash high above storms.

Scientists have discovered a new kind of lightning that flares in the shape of a vast, 400-kilometer-wide doughnut at the outer reaches of Earth's atmosphere. These fleeting flashes-too quick to be seen with the naked eye-join an ever-growing roster of electric fireworks detected in the night sky high above thunderstorms.

Researchers call the newfound bursts elves, short for "emissions of light and very low frequency perturbations due to electromagnetic pulse sources." They blaze for less than one-thousandth of a second in Earth's mesosphere and ionosphere at altitudes of 70 to 100 km, says meteorologist Walter Lyons of Atmospheric Simulation Testing and Environmental Research in Fort Collins, Colo.

"It looks just like a disk that flashes on and off," says Lyons, adding that theory predicts a hole in the center of the disk. He and others documented the existence of elves last summer. The group described its work last week at the annual meeting of the American Geophysical Union in San Francisco.

During the summer experiment, researchers from Japan and from institutions across the United States convened at Lyons' home on Yucca Ridge, Colo. There, they could monitor thunderstorms across the Great Plains from the Canadian border to Mexico. "Virtually every night you can see a thunderstorm out our back door," Lyons says.

The group had planned the experiment primarily to study sprites, jellyfish-shaped blobs that flash bloodred at altitudes of 40 to 80 km. Only in the last few years have scientists discovered these red sprites and much rarer flares, called blue jets (SN: 12/17/94, p.405).

Red sprites, blue jets, and elves (which may be red) appear over the parts of a thunderstorm that produce the most powerful cloud-to-ground lightning. Using video monitors and high-speed photometers, the Yucca Ridge team discovered that elves occur often just before sprites and sometimes on their own. Although elves burn with 10 times the intensity of sprites, the latter look brighter because they last 10 times as long, report Hiroshi Fukunishi and his colleagues from Tohoku University in Sendai, Japan.

The discovery of elves traces back to researchers at Stanford University who observed ionospheric radio disturbances above thunderstorms in the 1980s. In 1992, space shuttle astronauts captured an image of light emissions now thought to be elves. Stanford University's Umran Inan and Yuri Taranenko then theorized that when powerful lightning sparks to the ground, it sends up a pulse of electromagnetic energy that heats the ionosphere, causing atoms and molecules there to glow. The recent observations of elves seem to support that theory, says Taranenko, now at Los Alamos (N.M.) National Laboratory.

While researchers have observed only 50 elves and a like number of blue jets, they have documented thousands of sprites. At the San Francisco meeting, two research groups reported results that help explain sprites. Experiments this summer showed that emissions from excited nitrogen molecules produce the red glow of sprites. Physicists haven't determined how the nitrogen molecules get energized.

Taranenko and Los Alamos colleague Robert Roussel-Dupre proposed last year that an upward-moving avalanche of electrons may cause blue jets and red sprites. Just after a lightning bolt, the leftover charge in storm clouds creates a temporary electric field above the storm. If a stray cosmic ray collides with an air molecule, it knocks loose an energetic electron. Accelerated upward by the field, that electron collides with air molecules and tears more electrons free. The upward cascade of electrons eventually collides with nitrogen molecules, causing them to glow blue at lower altitudes and red at higher ones.

Los Alamos researchers plan to test this theory next summer by sending balloon-borne instruments above thunderstorms in the Midwest.

Lyons predicts even more discoveries of electric surprises now hiding above storms. "Every year we look, we see something new. I'm not sure what we'll see next year, but there are new phenomena up there."
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Title Annotation:Science News of the Week; light emissions above thunderstorms
Author:Monastersky, Richard
Publication:Science News
Date:Dec 23, 1995
Words:642
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