Using a new tool for data selection and retrieval called the Magnetospheric State Query System, Fung and colleagues measured high-speed electrons in the Van Allen radiation belts that encircle Earth. They used raw data from the National Oceanic and Atmospheric Administration's polar-orbiting meteorological satellites from 1978 to 1999.

When they compared data taken during relatively low solar activity periods, called solar minimum, with data from peak solar activity periods, called solar maximum, they noticed the safe zone's location shifted to higher latitudes and altitudes during the solar maximum.

The Van Allen Belts, which were discovered by early satellite studies in the 1960s, resemble a pair of donuts around Earth, one inside the other with Earth in the hole of the innermost donut. The safe zone, called the slot region, would appear as a gap between the inner and outer donut. The belts consist of high-speed electrically charged particles - electrons and atomic nuclei - trapped inside Earth's magnetic field.

The satellite data show the safe zone is created in a region where conditions are favorable for the radiation belts to eject particles. The research is the first indication that the location of this region can change with the solar activity cycle.

The Sun goes through an 11-year cycle of activity, from maximum to minimum, and back again. During solar maximum, increased solar ultraviolet radiation heats Earth's upper atmosphere, called the ionosphere, causing it to expand. This increases the density of the plasma trapped in Earth's magnetic field.

"This discovery helps narrow down the search for the primary wave-particle interaction region that creates the safe zone," Fung said. "Although no known spacecraft uses the safe zone extensively now, our knowledge could help planning and operations of future missions that want to take advantage of the zone."

The research by Fung and colleagues appears in the Feb. 22 online version of Geophysical Research Letters

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Magnetospheric State Query System