two belts (sometimes considered as a single belt of varying intensity) 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. Their existence 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. The region of external belts has been given the name of magnetosphere to distinguish it from the atmosphere. The charged particles of which the belts are composed circulate along the earth's magnetic lines of force extending from the area above the equator to the N Pole, to the S Pole, and circles back to the equator. These particles are believed to originate in periodic solar flares. Carried by the solar wind, they 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 southern Atlantic Ocean to form the southern Atlantic Anomaly. This can present a dangerous hazard to satellites orbiting the earth.

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