Van Allen Belts
Van Allen Belts
The radiation belts are enormous populations of energetic, electrically charged particles (plasma)— principally protons and electrons—trapped in the external magnetic field of a planet. Durable radiation belts exist at the planets Earth, Jupiter, Saturn, Uranus, and Neptune but not at Mercury, Venus, or Mars. The radiation belts surrounding Earth are called the Van Allen radiation belts.
Raditation belts were first speculated upon by Greek-American physicist Nicholas Constantine
Christofilos (1916–1972). The first successfully launched satellite of the United States was Explorer I. It was propelled into orbit from Cape Canaveral, Florida, on January 31, 1958, by a four-stage combination of rockets (called a Jupiter C missile) developed by the U.S. Army Ballistic Missile Agency and the Jet Propulsion Laboratory (JPL). The principal scientific instrument within the payload was a Geiger tube radiation detector developed by American space scientist James Alfred Van Allen (1914–2006), a professor at the University of Iowa, and graduate student George H. Ludwig. The instrument’s intended purpose was a comprehensive survey of cosmic ray intensity above Earth’s atmosphere. Lead by Van Allen, the Explorer I mission was the first scientific discovery of the torus-shaped (doughnut-shaped) region of charged particles within Earth’s magnetic field.
The launch of an improved version of the radiation instrument was attempted on March 5 on Explorer II, but an orbit was not achieved because the fourth stage rocket failed to ignite. On March 26, the launch of the improved instrument on Explorer III, including the first magnetic tape recorder ever flown in space, was successful.
The in-flight data from the radiation detectors on Explorer I and Explorer III revealed that there are enormous numbers of energetic, electrically charged particles trapped in the external magnetic field of Earth. This discovery was promptly confirmed and extended later in 1958 with additional space flights by the Iowa group and others, including Soviet investigators on Sputnik III. In subsequent years, the continuing study of this phenomenon has included the efforts of over one thousand scientists in at least 20 different countries. The radiation belts were later mapped by such spacecraft as Explorer IV, Pioneer III, and Luna 1
the populations of energetic particles trapped in the Earth’s magnetic field have come to be known as radiation belts because the doughnut-shaped regions within which they are confined encircle Earth like huge belts. There are two distinct belts: an inner one whose lower boundary is at an altitude of about 250 mi (402 km) and whose less-well defined outer boundary is at a radial distance of about 10,000 mi (16,100 km) and an outer one that extends outward from 10,000 mi (16,100 km) to over 50,000 mi (80,500 km). The outer belt consists of high-energy electrons, measured, on average, at about 0.1 to 10 MeV (where one MeV is equal to one million electron volts [eV]). The inner belts consists of high-energy protons with energies over 30 MeV. Both belts encircle Earth in longitude and have the greatest concentration of trapped particles at its magnetic equatorial plane. The concentration diminishes with increasing latitude north and south of the magnetic equator and falls to nearly zero over the north and south polar caps at latitudes greater than about 67°. Each trapped particle spirals around a magnetic line of force, oscillates between magnetic mirror points in northern and southern hemispheres, and drifts slowly in longitude. This defines a doughnut-shaped region.
The principal source of particles in the outer belt is solar wind, a hot ionized gas that flows outward from the sun through interplanetary space. Some of the electrons, protons, and other ions in the impinging solar wind are injected into Earth’s external magnetic field. These electrons, protons, and ions subsequently diffuse inward and are accelerated to greater energies by natural fluctuations of electrical and magnetic fields induced by the varying solar wind. The solar wind also sweeps back the outer portion of Earth’s magnetic field to produce a long wake called the magnetotail extending on the night side of Earth to a distance of about 4,000,000 mi (6,440,000 km) far beyond the orbit of the moon. In the heart of the outer belt, typical intensities of electrons with energies greater than one million electron volts (1.0 MeV) (the most penetrating component there) are 2,000,000 particles per square inch per second. The numbers of particles of lesser energy and lesser penetration are much greater. The outer belt exhibits marked variations on time scales of hours, weeks, and months.
In the heart of the relatively stable inner belt, some intensities of protons of energy exceeding 30 million electron volts (30 MeV) are 120,000 particles per square inch per second. The residence times of such protons are many years. Penetrating particles in the inner belt are attributable to neutrons from reactions of cosmic rays in the gas of the upper atmosphere. A small fraction of such (uncharged) neutrons decays into protons, electrons, and neutrons as they move outward. At the points of decay, the electrically charged protons and electrons are injected into trapped orbits. A radiation belt of this type would be created around a magnetized planet by cosmic rays even in the absence of the solar wind, though no such example has been found.
The fate of trapped particles is loss into the atmosphere or outward into space. A quasi-equilibrium population of any specified species of trapped particle is achieved when losses are equal to sources.
Radiation belts are part of a more complex system called the magnetosphere, which also contains large populations of relatively low energy ionized gas (plasma). This gas plays a central role in the overall physical dynamics of the system.
Nine artificial radiation belts of Earth were produced during the period from 1958 to 1962 by the injection of electrons from radioactive nuclei produced by United States (U.S.) and Union of Soviet Socialist Republics (USSR) nuclear bomb bursts at high altitudes. These experiments made important contributions to understanding the natural belts. Since 1962, such high altitude bursts have been prohibited by international treaty.
The aurorae, in both northern and southern latitudes (northern and southern lights) at typical but widely fluctuating magnetic latitudes of about 67°,are one of the visible, widely observed manifestations of magnetospheric phenomena. Aurorae are not a direct product of the outer radiation belt but share the solar wind as the primary agent for their creation. Magnetic storms detected by the fluctuations of sensitive magnetic compasses are attributable to electrical currents in the outer radiation belt.
The inner radiation belt imposes an altitude ceiling on the region around Earth within which orbital flights of humans and animals can be conducted without exposing the occupants to excessive or fatal radiation exposures. Prolonged flights of human crews above an altitude of about 250 mi (402 km) are unsafe though rapid traversals of the radiation belts requiring only a few hours (as in the Apollo missions to the moon in the late 1960s and early 1970s) result in moderate exposures. Even satellites that have no human crew are sensitive to the radiation belts. The Hubble Space Telescope, in the 1990s and 2000s, often has its sensors turned off when it interacts with strong radiation. However, space travel beyond the inner belt poses little risk to astronauts.
Contrary to some common statements, trapped particles are not radioactive. Rather they are mainly ordinary electrons and protons such as those accelerated in high energy physics laboratories. Radiation belts do not shield Earth’s surface, though the magnetic field deflects some cosmic rays away from Earth. The atmosphere acts as an effective shield against many solar and other radiations that impinge on it.
In 1958–1959, radio astronomers discovered that the planet Jupiter has an enormous radiation belt of high energy electrons. This discovery provided a powerful impetus for the investigation of Jupiter and the other planets by scientifically instrumented spacecraft.
Beginning in 1962 in situ investigations of the planets have been conducted by American, Soviet, and European spacecraft. The radiation belt of Jupiter has been explored in detail. Enormous radiation belts of Saturn, Uranus, and Neptune also have been discovered and investigated. It has been found that Venus, Mars, and Mercury have no durable radiation belts, but there are significant magnetospheric effects at these planets because of the obstacles that they present to the flow of the solar wind. Such effects have also been observed at the moon and at several comets.
A durable radiation belt at a planet can only exist if that planet is strongly magnetized so that energetic electrically charged particles can be trapped durably in its external magnetic field. Earth, Jupiter, Saturn, Uranus, and Neptune meet this condition by virtue of electrical currents circulating in their interiors to produce huge electromagnets.
Venus, Mars, Mercury, the moon, comets, and asteroids are insufficiently magnetized to retain radiation belts. It is likely that the dwarf planets Pluto and Eris are also in this group.
In addition, pulsars and other distant astrophysical objects have radiation belts. These pulsars, or rotating neutron stars, generate regular pulses of radiation.
See also Magnetosphere.
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Van Allen, James A. Origins of Magnetospheric Physics Washington, DC: Smithsonian Institution Press, 1983. ———. “Radiation Belts Around Earth.” Scientific American 200 (March 1958): 39-47.
James A. Van Allen
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