Van Allen belts, also called radiation belts, are two zones of electrically charged particles that surround Earth high above its surface. The belts were named for James A. Van Allen, an American physicist, who discovered them in 1958. He based his discovery on data from the Explorer 1 satellite. The belts surround Earth forming somewhat doughnutlike shapes. The inner belt extends from about 600 to 3,000 miles (1,000 to 5,000 kilometers) above Earth. The outer belt reaches from about 9,300 to 15,500 miles (15,000 to 25,000 kilometers).
The radiation in the belts consists of high concentrations of charged particles, such as protons and electrons. Earth’s magnetic field traps such particles and directs them toward the magnetic poles. The trapped particles spiral along a system of imaginary lines of the magnetic field. These field lines curve from the north magnetic pole to the south magnetic pole. As particles approach either pole, the converging field lines reflect them back toward the opposite pole. This effect keeps particles in the belts bouncing between the poles.
The inner belt traps protons set free from Earth’s atmosphere by cosmic rays, high-energy particles from outer space. In 1992, it was discovered that the inner belt of protons is divided in the middle by a thin layer of electrons. The outer belt acquires electrons from the solar wind, a continuous stream of charged particles from the sun, and from solar flares, violent eruptions on the sun’s surface. Intense solar activity disrupts the belts and leads to magnetic storms. Disruptions of the belts also interfere with radio reception, cause surges in power transmission lines, and produce auroras. In 2011, a team of scientists revealed evidence that an unusually large amount of antimatter fills the space between the two belts. Antimatter resembles ordinary matter but with certain properties of its particles, such as electric charge, reversed. The antimatter particles detected were antiprotons, the antimatter equivalent of a proton.
At times, scientists have observed a third radiation belt. Each time it was temporary, lasting only a few weeks. Scientists are uncertain what caused the third belt to form. However, an unusual change in the solar wind was suspected in at least one instance.
Like Earth, Jupiter, Saturn, Uranus, and Neptune are surrounded by magnetic fields. In the 1970’s and 1980’s, the Voyager space probes found evidence that these planets also have radiation belts. These belts were later studied in detail by other probes, including the Galileo mission to Jupiter and the Cassini mission to Saturn.
