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Van Allen radiation belt
A radiation belt is a layer of energetic charged particles that is held in place around a magnetized planet, such as the Earth, by the planet's magnetic field. The Earth has two such belts and sometimes others may be temporarily created. The discovery of the belts is credited to James Van Allen and as a result the Earth's belts bear his name. The main belts extend from an altitude of about 1,000 to 60,000 kilometers above the surface in which region radiation levels vary. Most of the particles that form the belts are thought to come from solar wind and other particles by cosmic rays. The belts are located in the inner region of the Earth's magnetosphere. The belts contain energetic electrons that form the outer belt and a combination of protons and electrons that form the inner belt. The radiation belts additionally contain less amounts of other nuclei, such as alpha particles. The belts endanger satellites, which must protect their sensitive components with adequate shielding if their orbit spends significant time in the radiation belts. In 2013, NASA reported that the Van Allen Probes had discovered a transient, third radiation belt, which was observed for four weeks until destroyed by a powerful, interplanetary shock wave from the Sun.
== Discovery ==
Kristian Birkeland, Carl Størmer, and Nicholas Christofilos had investigated the possibility of trapped charged particles before the Space Age. Explorer 1 and Explorer 3 confirmed the existence of the belt in early 1958 under James Van Allen at the University of Iowa. The trapped radiation was first mapped out by Explorer 4, Pioneer 3 and Luna 1.
The term ''Van Allen belts'' refers specifically to the radiation belts surrounding Earth; however, similar radiation belts have been discovered around other planets. The Sun itself does not support long-term radiation belts, as it lacks a stable, global dipole field. The Earth's atmosphere limits the belts' particles to regions above 200–1,000 km, while the belts do not extend past 7 Earth radii ''RE''.〔 The belts are confined to a volume which extends about 65°〔 from the celestial equator.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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