The Van Allen radiation belts are a pair of doughnut shaped rings of ionized gas (or plasma) trapped in orbit around Earth. The outer belt stretches from 19,000 km (11,500 miles) in altitude to 41,000 km (25,000 miles); the inner belt lies between 13,000 km (7600 miles) and 7,600 km (4,500 miles) in altitude.

For decades, space physicists theorized that the Sun and its solar wind provided most of the high-energy particles found in Earth's radiation belts. But new observations from the International Solar-Terrestrial Physics (ISTP) program and other missions suggest that Earth's own magnetic shell in space, or magnetosphere, is a more effective and efficient accelerator of particles.

According to Dr. Geoffrey Reeves of Los Alamos National Laboratory and an investigator for ISTP, the solar wind and Sun are insufficient sources for the radiation belts. "There are just not enough high-energy electrons in the solar wind to explain how many we observe near Earth," said Reeves, who discussed the findings on December 7 in San Francisco during the Fall Meeting of the American Geophysical Union.

Data from NASA's Polar and SAMPEX spacecraft, as well National Oceanic and Atmospheric Administration (NOAA) and the Department of Defense satellites, show that the radiation belts change in response to a variety of solar events. High-speed streams of solar wind, coronal mass ejections, and shock waves from the Sun all can compress and excite the magnetosphere. But it is the pressure and energy of these events, not the particles buried in them, that energizes the particles trapped inside the radiation belts.

"It is amazing that the system can take the chaotic energy of the solar wind and utilize it so quickly and coherently," said Dr. Daniel Baker of the University of Colorado, an investigator for ISTP and SAMPEX. "We had thought the radiation belts were a slow, lumbering feature of Earth, but in fact they can change on a knife's edge."

Discovered in 1958, the radiation belts have long been treated as a relatively stable and predictable phenomenon. But in studying recent space weather events, space physicists have found that the intensity of the belts can vary by 10, 100, or even 1000 times in a matter of seconds to minutes. "The radiation belts are almost never in equilibrium," said Reeves. "We don't really understand the process, but we do know that things are changing constantly."

For instance, in early May 1998, a series of solar events provoked the most powerful storm in the radiation belts of the current solar cycle. Following a succession of coronal mass ejections and flares on the Sun, several major magnetic storms brought auroras to Boston and Chicago, and ISTP ground observatories in Canada and Antarctica measured electric currents in the ionosphere about 3-4 times the norm. The leading edge of the magnetosphere, which usually sits at 76,000 km (45,000 miles) from Earth toward the Sun, was pushed in to 25,000 km (15,300 miles).

In the wake of this disturbance, the natural gap (or "slot" region) between the two radiation belts was filled by a new radiation belt, as energized particles were trapped where they wouldn't naturally settle. The new belt lasted for nearly six weeks.

"The May 1998 event was a harbinger of what may come during the approaching solar maximum," said Baker. At the height or maximum of the 11-year solar cycle -- predicted for 2000-2001 -- coronal mass ejections and other solar events that disturb the radiation belts are likely to be much more common.

Observations from the May event are prompting researchers and space weather forecasters to reconsider the radiation belt models relied upon by the engineers who design and operate satellites. "We now have a fleet of satellites that gives us a more complete picture of what's going on in the radiation belts," said Reeves. "We are using this data to construct pictures, essentially 'weather maps' of what's going on in the radiation belts."

"Within the research community, there has been continuous progress in modeling the space environment, but very little of that research has made it into the space weather operations community," said Dr. Terrance Onsager of NOAA's Space Environment Center. "Most of the models in use today do a reasonable job of predicting average conditions, but few of them take into account the dynamics and how quickly the system can change."

"Some of the new models that we are developing will allow us to visualize the radiation environment over vast regions of space and then specify and predict the conditions at any location," Onsager added. "We are beginning to synthesize mature models with the new stream of real-time measurements from space in order to give industry and the government the information it needs to work in space."