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The Van Allen belts are two roughly-torus-shaped regions encircling the Earth (extending some distance into space, the outer belt to about a 6th of the way to Lunar orbit), containing energetic charged particles consisting of relativistic speed electrons and ions captured from the solar wind by the Earth's magnetic field. The captured particles are harmful to humans and spacecraft when they are within the belt regions, but this capture of the particles protects the lowest Earth orbits and the Earth's surface, reducing cosmic rays as well as the solar particles from extreme solar eruptions (e.g., the Carrington event). Other bodies with magnetic fields, such as Jupiter can have analogous regions, termed radiation belts, but occasionally described as a planet's Van Allen belt.
The South Atlantic Anomaly (SAA) is a region above South America and the southern Atlantic Ocean where the inner Van Allen belt reaches its nearest point toward Earth, about 120 miles altitude. It presents some of the dangers of the Van Allen belts to low Earth orbits. The anomaly is caused by the configuration of Earth's rotation axis in relation to its magnetic field.