Discoveries Electrify Atmospheric Science
FOR atmospheric scientists, 1994 was literally a ``shocker.'' They had thought Earth's upper atmosphere was a ``quiet, calm, tame, almost boring'' place, to quote geophysicist Davis Sentman of the University of Alaska at Fairbanks. Instead, Dr. Sentman and his colleagues found it to be alive with brightly flashing electromagnetic displays. That's to say nothing of the gamma-ray outbursts and mysterious radio noises other scientists discovered.
As gamma-ray observer Gerald Fishman at the NASA Marshall Space Flight Center in Huntsville, Ala., observed, ``It is becoming apparent that the upper atmosphere is much more electrically active than we ever imagined.''
As that realization sank in over the past year, a classic research scenario unfolded. Scientists studied a subject for centuries - in this case our atmosphere. They thought they had milked it of major discoveries. Suddenly, phenomena they had ignored or never even suspected hit them in the face.
Actually, they shouldn't have been surprised by the brilliant flashes. Occasional observers have reported glimpses of such things for over a century. More recently, aircraft pilots have reported seeing powerful ``lightning'' stabbing upward in the clear air above thunderstorms. But scientists didn't pay attention until a few years ago.
Then, in 1993, Sentman and Eugene Wescott from Alaska and meteorologist Walter Lyons at Mission Research Corporation in Fort Collins, Colo., caught some of the flashes on videotape. Mr. Lyons has recorded hundreds of them since then. Scientists from Pennsylvania State University at University Park and Stanford University in California joined in this research last summer. They captured video and still images of dozens of flashes between June 28 and July 12.
There are two types of flashes. One type - called sprites - is described as looking like blood-red or salmon-red jellyfish with faint blue tendrils. They appear as high as 100 kilometers (60 miles) and are not always directly connected to storm clouds. The other type reaches about half as high. These flashes - called blue jets - shoot up from thunderstorms in geyser-like streams that flair out at the top. Although both types seem connected with thunderstorms, there is no established theory to explain what happens.
The gamma-ray bursts are even more mysterious. Gamma rays are the most energetic form of electromagnetic radiation. Scientists look for them in violent astronomical events. There was no reason to expect an atmospheric source. Then, last May, Marshall Space Flight Center's Gerald Fishman reported that a review of data showed the Compton Gamma Ray Observatory satellite had recorded a dozen atmospheric gamma-ray bursts since 1991. It ``came as a complete surprise,'' he said.
Dr. Fishman's Marshall colleague Steve Goodman told a meeting of the American Geophysical Union in San Francisco last month that they now are observing gamma-ray events above thunderstorms about once a week. William Feldman of the Los Alamos National Laboratory in New Mexico told the meeting that he and his co-workers, Eugene Symbalisty and Robert Roussel-Dupre, have found a burst rate of about four a day over a two-year period in data from the Army Background Experiment Satellite. This satellite operated from its launch in February 1990 until March 1993. The bursts ``lit up'' the area from Iraq east to the Tibetan Plateau, the region east of Indonesia, and the coasts of China and Japan. ``It's clear that something is going on,'' Feldman said. However, he added, ``We found something we really don't understand.''
The radio bursts offer more mystery. There are lots of natural atmospheric radio noises. Unlike the others, these new ones come in pairs and look as though they travel through Earth's electrically conducting ionosphere. Los Alamos radio physicist Daniel N. Holden, who finds the bursts in data from the Department of Energy's ALEXIS satellite, calls them ``the strangest signals we've ever seen from Earth.''
In short, geophysicists suddenly have an unexpected puzzle. Perhaps 1995 will bring them some answers.