Van Allen radiation belts

Van Allen radiation belts, 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 North Pole, to the South 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 South Atlantic to form the Southern Atlantic Anomaly. This can present a dangerous hazard to satellites orbiting the earth.

---

Van Allen radiation belts

Two doughnut-shaped zones of highly energetic charged particles (see electric charge) trapped at high altitudes in Earth's magnetic field. Named for James A. Van Allen (b. 1914), who discovered them in 1958, they are most intense over the Equator and effectively absent above the poles. The two zones merge gradually, with the flux of charged particles showing two regions of maximum density. The inner one, mostly protons thought to be produced by primary cosmic rays striking the atmosphere, is centred about 3,700 mi (6,000 km) above Earth's surface. The outer region includes some helium ions from the solar wind and is centred about 12,500 mi (20,000 km) above Earth's surface. Intense solar activity (see solar cycle) causes disruptions of the belts, linked in turn with such phenomena as auroras and magnetic storms.