Two ring-shaped regions around the Earth where there are concentrations of high-energy electrons and protons that have been trapped by the Earth’s magnetic field. They were discovered by the USA’s first successful artificial Earth satellite, Explorer 1, which was launched on 31 January 1958. They are named after James Van Allen, the physicist who led the experiment on Explorer 1. The inner Van Allen belt lies about 0.8 Earth radii above the equator. The main concentration of the outer belt lies between about 2 and 3 Earth radii above the equator, but a broader region, extending from the inner belt out to as far as 10 Earth radii, contains protons and electrons of lower energy, believed to have come primarily from the solar wind. Because the Earth’s magnetic field is offset from the planet’s rotation axis, the inner belt dips down towards the surface in the region of the South Atlantic Ocean, off the coast of Brazil. This South Atlantic Anomaly presents a potential hazard to the operation of artificial satellites. In 1993, a region within the inner Van Allen belt was found to contain particles that have penetrated from interstellar space.