Van Allen radiation belts, belts of radiation outside the earth's atmosphere, extending from c.400 to c.40,000 mi (c.650–c.65,000 km) above the earth. The existence of two belts, sometimes considered as a single belt of varying intensity, was confirmed from information secured by launching the first U.S. earth satellite, Explorer I, sent up during the International Geophysical Year of 1957–58. The belts were named for James A. Van Allen, the American astrophysicist who first predicted the belts and then was first to interpret the findings of the Explorer satellite. In 2012 NASA space probes studying the belt tracked the formation of a third belt, between the two belts previously known. This third belt and the outer belt disappeared several weeks after the third belt appeared; the outer belt subsequently re-formed.
The region of the radiation belts has been given the name of magnetosphere to distinguish it from the atmosphere. The high-energy particles of which the belts are composed circulate along the earth's magnetic lines of force extending from the area above the equator to near (but not above) the North and South Poles. The inner belt is mainly protons with some electrons; the outer one mainly electrons. The particles of the inner belt are believed to be produced by the collisions of cosmic rays with atoms in the upper atmosphere. Those of the outer belt are believed to originate both from the atmosphere and from the solar wind; particles from the solar wind become trapped by the earth's magnetic field and are responsible for the aurora borealis seen at polar regions. A part of a belt dips into the upper region of the atmosphere over the South Atlantic to form the Southern Atlantic Anomaly. This can present a dangerous hazard to satellites orbiting the earth.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.