Storms paint bull's-eyes in stratosphere.

An experimental satellite designed to improve the detection of ballistic missiles has captured images of giant natural bull's-eyes in the sky. These ring-shaped waves are triggered by thunderstorms and rise high up through the atmosphere.

Physicists have long known that thunderstorms could generate these so-called gravity waves, but they lacked satellite sensors specialized enough to capture images of such disturbances in the stratosphere, says Edmond M. Dewan of the Air Force Research Laboratory at Hanscom Air Force Base in Bedford, Mass. He and his colleagues report their discovery in the April 1 Geophysical Research Letters.

"This [satellite] technique uses remote sensing from space as an effective way to study the dynamics of the middle atmosphere, which has been a difficult region to study," says coauthor Robert R. O'Neil, also of Hanscom. The stratosphere (17 to 50 kilometers in altitude) and the mesosphere (50 to 90 km) lie above the observational range of airplanes but below that of rockets.

Dewan, O'Neil, and their colleagues observed the wave patterns using a sensor on board the Midcourse Space Experiment (MSX) satellite. This instrument captures thermal radiation emitted by carbon dioxide molecules about 40 km above the surface.

The researchers detected two cases of bull's-eye temperature patterns measuring many hundreds of kilometers across. The circles developed precisely over spots where a thunderstorm had been an hour or two earlier. These observations confirm geographically limited ones made from the ground during the 1980s that indicated thunderstorm-triggered gravity waves at an altitude of 80 km.

Atmospheric gravity waves resemble waves produced in water when a floating object is pushed down and released. As the object pops up, it overshoots its resting place, then sinks back below the surface. The bobbing motion sends rings rippling outward.

In a growing thunderstorm, hot air rises quickly and surpasses its proper altitude, ultimately falling back down. That action sends a wave rising up through the stratosphere and mesosphere.

Gravity waves can also be caused by winds passing over mountains and by pressure systems in the atmosphere. As the waves rise, they inject energy into the upper stratosphere and mesosphere. "Gravity waves are quite important in the overall thermal structure of the atmosphere--determining how it normally behaves," says C. Russell Philbrick of Pennsylvania State University in State College. The new satellite images "provide a three-dimensional view of gravity waves that we haven't had in the past."

The Department of Defense launched the MSX satellite in part to study what natural patterns exist in the atmosphere--a prerequisite for being able to distinguish the signature of a warm trail left by a ballistic missile.