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.

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