The Earth actually has two radiation belts of different origins. The inner belt, the one discovered by Van Allen's Geiger counter, occupies a compact region above the equator (see drawing, which also includes the trajectories of two space probes) and is a by-product of cosmic radiation. It is populated by protons of energies in the 10-100 Mev range, which readily penetrate spacecraft and which can, on prolonged exposure, damage instruments and be a hazard to astronauts. Both manned and unmanned spaceflights tend to stay out of this region.

Cross-section of the two radiation   belts, together with the orbits of   Pioneers 3 and 4 which contributed   early observations of them.

    The outer radiation belt is nowadays seen as part of the plasma trapped in the magnetosphere. The name "radiation belt" is usually applied to the more energetic part of that plasma population, e.g. ions of about 1 Mev of energy (see energy units). The more numerous lower-energy particles are known as the "ring current", since they carry the current responsible for magnetic storms. Most of the ring current energy resides in the ions (typically, with 0.05 MeV) but energetic electrons can also be found.

Next Stop: #12H.  The Radiation Belts--History

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