VAN ALLEN RADIATION BELT

doughnut-shaped zones of highly energetic charged particles trapped at high altitudes in the magnetic field of the Earth. The zones were named for James A. Van Allen, the American physicist who discovered them in 1958 using data transmitted by the U.S. Explorer satellite. The Van Allen belts are most intense over the Equator and are effectively absent above the poles. No real gap exists between the two zones; they actually merge gradually, with the flux of charged particles showing two regions of maximum density. The inner region is centred approximately 3,000 km (1,860 miles) above the terrestrial surface. The outer region of maximum density is centred at an altitude of about 15,000 to 20,000 km (9,300 to 12,400 miles), though some estimates place it as far above the surface as six Earth radii (about 38,000 km [23,700 miles]). The inner Van Allen belt consists largely of highly energetic protons, with energy exceeding 30,000,000 electron volts. The peak intensity of these protons is approximately 20,000 particles per second crossing a spherical area of one square cm in all directions. It is believed that the protons of the inner belt originate from the decay of neutrons produced when high-energy cosmic rays from outside the solar system collide with atoms and molecules of the Earth's atmosphere. Some of the neutrons are ejected back from the atmosphere; as they travel through the region of the belt, a small percentage of them decay into protons and electrons. These particles move in spiral paths along the lines of force of the Earth's magnetic field. As the particles approach either of the magnetic poles, the increase in the strength of the field causes them to be reflected. Because of this so-called magnetic mirror effect, the particles bounce back and forth between the magnetic poles. Over time, they collide with atoms in the thin atmosphere, resulting in their removal from the belt. The outer Van Allen belt contains charged particles of both atmospheric and solar origin, the latter consisting largely of helium ions from the solar wind (steady stream of particles emanating from the Sun). The protons of the outer belt have much lower energies than those of the inner belt, and their fluxes are much higher. The most energetic particles of the outer belt are electrons, whose energies reach up to several hundred million electron volts. Studies show that intense solar activity causes disruptions of the Van Allen belts, which in turn are linked with such phenomena as auroras and magnetic storms. See also aurora; magnetic storm.