Van Allen radiation belt

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The Van Allen radiation belts

The Van Allen radiation belts are two or more layers of charged energetic particles that surround the Earth as a result of the Earth's magnetic field.[1] Other planets such as Jupiter and Saturn also have magnetic fields and have similar radiation belts.[2] They are associated with other magnetic phenomena like aurora. The Earth's belts are named in honor of James Van Allen, who discovered them in 1958 using the Explorer satellite.[1] This was in fact the very first spacecraft launched by the United States.[3] The belts are enclosed within the magnetopause of the Earth.

The belts surrounding the Earth are toroidal (doghnut-like) in shape. There are normally two belts called the inner and outer, though during periods of high solar activity, such as a coronal mass ejection, a temporary third belt may form. These two belts do not have well defined boundaries, rather there are two regions containing a high concentration of particles separated by a region of lower concentration. The inner belt is some 1,860 miles (3,000 km) above the surface and the outer thought to be 9,300-12,400 miles (15,000-20,000 km) though estimates vary significantly and some sources suggest it may be as much as 23,700 miles or 38,000 km.[1] The inner belt contains highly energetic protons, with energies roughly 30 GeV.[1] These protons are thought to come from the decay of neutrons created from collisions of cosmic rays with the atmosphere. As neutrons have no charge they are unaffected by the Earth's magnetic field, but the proton and electron produced by the decay are affected and end up trapped by the field.

The outer Van Allen belt contains various charged particles, some of which come from the atmosphere as for the inner belt, but others which come form the Solar wind. The solar wind means that the outer belt contains many helium ions. There are many more particles contained within the outer belt and at a higher concentration though these are less energetic with the greatest energies being around several hundred electronvolts.[1]

The Van Allen belts have an impact on space travel, exposing astronauts and satellites to radiation. When the inner belt expands during large amounts of solar activity, it can include the orbit of the International Space Station.[4]

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