Puzzling atmospheric bursts spark interest.

Weather aficionados have watched the skies for centuries, but that hasn't kept modern researchers from finding something new under the sun. Atmospheric physicists have recently detected a number of previously unrecognized or poorly studied phenomena, including pulses of radio emissions and odd flashes of light high above Earth's surface.

Investigators believe these unusual features relate somehow to thunderstorms, although scientists remain unsure what causes such events and have yet to resolve whether a connection exists between the light flashes and radio bursts.

Dan Holden and his colleagues at Los Alamos (N.M.) National Laboratory discovered the radio phenomenon while studying measurements made by the ALEXIS satellite. The craft was launched last year as part of an effort to develop technology for identifying nuclear blasts. Since November, ALEXIS' radio receiver has recorded some 100 pulses of radio energy 10,000 times stronger than the radio noise generated by lightning. Each pulse consists of a pair of emissions separated by 40 millionths of a second.

Researchers have ruled out the possibility that the radio bursts come from another planet or star, because the emissions show a characteristic distortion, caused by passage through Earth's ionosphere -- the layer 200 to 400 kilometers above the planet's surface. Generated below the ionosphere, the radio discharges disperse as they travel toward the ALEXIS satellite orbiting 800 km above the Earth, says Holden.

The satellite thus far has detected most of the events over Africa and the South Pacific, places lacking the background electromagnetic noise generated by radio and television signals common in the United States and Europe. Holden suspects the bursts occur over many parts of the globe, but "the radio noise is so loud over the United States we have a hard time seeing [the pulses] here."

While researchers have not previously recognized such pulses, Holden says he has found some hints that classified military satellites have detected the emissions, which resemble the radio noise from nuclear blasts.

Holden and his colleagues think the pulses have some connection with thunderstorms because ALEXIS most often detects them in the afternoon and early morning, typical times for convective storms. Yet the bursts do not come from lightning because they carry much more energy and are much shorter than the radio releases associated with lightning.

Atmospheric physicists wonder whether the enigmatic bursts relate at all to odd flashes of light recently documented high above thunderstorms. Although researchers have long heard anecdotal reports of such features, mainly from pilots, they have obtained confirming evidence only within the last five years. Last year, scientists reported the first detailed measurements of the flashes, made by investigators on the ground as well as by detectors carried by a NASA airplane.

The observations showed that the above-cloud flashes reached roughly 60 km in altitude and covered a vast horizontal distance 10 to 50 km wide, making them distinct from the narrow channels of regular lightning. The flashes resemble glowing auroras more than bright lightning bolts.

Research teams will spend this summer studying the new phenomena. And this week, Holden worked with NASA to obtain simultaneous observations by the ALEXIS radio receiver and video cameras on the space shuttle mission. Such dual observations should resolve whether the radio bursts occur at the same time as optical flashes within or above thunderstorms.