Results from NASA's Van Allen Probes mission show the acceleration energy is in the belts themselves, as local variations in energy levels kick particles inside the belts to ever-faster speeds approaching 99 percent of the speed of light, the space agency reported Thursday.
Knowing the location of the acceleration within the radiation belts -- there are actually two Van Allen belts -- will help improve predictions of space weather, because changes in the radiation belts can be risky for satellites near Earth, scientists said.
Most satellite orbits are chosen to duck below the radiation belts or circle outside of them, but some satellites, such as GPS spacecraft, must operate between the two belts.
"Until the 1990s, we thought the Van Allen belts were pretty well-behaved and changed slowly," lead study author Geoff Reeves at Los Alamos National Laboratory in Los Alamos, N.M., said. "With more and more measurements, however, we realized how quickly and unpredictably the radiation belts change. They are basically never in equilibrium, but in a constant state of change."