Radiation Belts of the Earth
Radiation Belts of the Earth
inner regions of the earth’s magnetosphere, in which the earth’s magnetic field retains charged particles (protons, electrons, and alpha particles) possessing kinetic energies ranging from tens of keV to hundreds of MeV; the energy of particles differs in various regions of the belts. The special configuration of the lines of force of the geomagnetic field creates a magnetic trap for charged particles and prevents the escape of these particles from the radiation belts.
Particles trapped by the Lorentz force in the earth’s magnetic trap undergo a complicated motion. This motion may be represented as an oscillating motion in a spiral trajectory along a line of force of the magnetic field from the northern hemisphere to the southern hemisphere and back again, with concomitant but slower displacement (longitudinal drift) around the earth (Figure 1). When a particle spirals toward an increasing magnetic field as it approaches the earth, the radius and pitch of the spiral decrease. The velocity vector of the particle remains unchanged in magnitude and approaches a plane perpendicular to the direction of the field. At a point called the mirror point the particle reflects and begins to move in the reverse direction, toward the conjugate mirror point in the other hemisphere. One oscillation along a line of force from the northern hemisphere to the southern hemisphere is accomplished by a proton with an energy of approximately 100 MeV in about 0.3 sec. The retention time, or lifetime, of such a proton in the geomagnetic trap may be as much as 100 years (~3 × 109 sec), in the course of which the proton accomplishes up to 1010 oscillations. On the average, captured high-energy particles complete at least several hundred million oscillations from one hemisphere to the other. The longitudinal drift occurs at a considerably reduced velocity.
Depending on its energy, a particle makes a complete revolution around the earth in a period ranging from several minutes to several days. Positive ions drift in a westerly direction, whereas electrons drift in an easterly direction. The spiral motion of a particle along a line of force of the magnetic field may be represented as consisting of rotation around the instantaneous center of rotation and the translational motion of this center along the line of force.
Structure. When a charged particle travels in the earth’s magnetic field, its instantaneous center of rotation remains on the same surface, which is called the magnetic shell (Figure 2). The magnetic shell is characterized by the parameter L, whose value in the case of a dipole field is equal to the distance, expressed in earth radii, separating the magnetic shell in the equatorial plane of the dipole from the center of the dipole. For the actual magnetic field of the earth, parameter L approximates this simple physical significance. The energy of particles is related to the value for parameter L; particles with higher energy are found in shells with smaller L. This is explained by the fact that high-energy particles can be contained only by a strong magnetic field, that is, in the inner regions of the magnetosphere.
The earth’s radiation belts are usually divided into inner and outer belts, the low-energy proton belt (ring current belt), and the particle quasicapture zone (auroral radiation zone; Figure 3). The inner radiation belt is characterized by the presence of high-energy protons (20–800 MeV); maximum density of the flux of protons with energy ℰp > 20 MeV is 104 protons/cm2-sec-stere at a distance L ~ 1.5. In the inner belt, electrons are also present with energies of 20–40 keV to 1 MeV. The density of the flux of electrons with ℰe ≳ 40 keV is maximally ~ 106–107 electrons/cm2-sec-stere.
The inner belt is found around the earth at equatorial latitudes (Figure 4). This belt is bounded externally by a magnetic shell with L~ 2, which intersects with the surface of the earth at geomagnetic latitudes of ~45°. The inner belt most closely approaches the surface of the earth (at a height of 200–300 km) near the Brazilian magnetic anomaly, where the magnetic field is greatly weakened. Above the equator, the lower boundary of the inner belt is 600 km above the surface of the earth over America and as much as 1,600 km over Australia. At the lower boundary of the inner belt, particles frequently collide with atoms and atmospheric gas molecules, lose their energy, and are dispersed and “absorbed” by the atmosphere.
The outer radiation belt is contained within magnetic shells with L ~ 3 and L~ 6, with maximum density of particle flux at L ~ 4.5. Electrons with energies of 40 to 100 keV and maximum electron flux of 106–107 electrons/cm2-sec-stere are characteristic of the outer belt. The average lifetime of particles in the outer radiation belt is 105–107 sec. During periods of increased solar activity, electrons with higher energies (1 MeV or more) are present in the outer belt.
The low-energy proton belt (ℰp ~ 0.03–10 MeV) extends from L~ 1.5 to L ~ 7–8. The zone of the quasicapture of particles (zone of auroral radiation) is found beyond the outer belt and has a complex spatial structure as a result of the deformation of the magnetosphere by solar wind, that is, the flux of charged particles from the sun. The major components of the particles in the zone of quasicapture are electrons and protons with energies ℰ < 100 keV. The outer belt and low-energy proton belt most closely approach the earth (at heights of 200–300 km) at latitudes between 50° and 60°. The zone of quasicapture, which is the region in which auroras most frequently occur, projects at latitudes above 60°. During certain periods, the existence of narrow belts of high-energy electrons (ℰe ~ 5 MeV) is observed in magnetic shells with L~ 2.5–3.0.
The energy spectra of all particles in the radiation belts of the earth are described by functions of the type N (ℰ)~ℰr, where N (ℰ) is the number of particles with a given energy ℰ, or N (ℰ) ~ exp - ℰ/ℰ0, with characteristic values of γ ≈ 1.8 for protons in the energy range of 40 to 800 MeV, ℰ0 ~ 200–500 keV for electrons of the outer and inner belts, and ℰ0 ~ 100 keV for low-energy protons.
History of discovery. The first radiation belts to be discovered were the inner belt, which was discovered in 1958 by a group of American scientists headed by J. Van Allen, and the outer belt, which was discovered in 1958 by a group of Soviet scientists headed by S. N. Vernov and A. E. Chudakov. Particle fluxes of the radiation belts were recorded by Geiger counters placed aboard artificial satellites. Strictly speaking, the radiation belts of the earth do not have sharply delineated boundaries, because each type of particle forms its own specific radiation belt in accordance with its energy; it is thus more correct to speak of the earth’s having a single, unified radiation belt. The division into inner and outer radiation belts, which was made in the first stage of study and has been retained until the present because of differences in the belts’ properties, is essentially arbitrary.
The theoretical possibility of a magnetic trap in the earth’s magnetic field was shown by the calculations of C. Störmer (1913) and H. Alfvén (1950). It required experiments conducted on artificial satellites, however, to demonstrate that the trap actually exists and is filled with high-energy particles.
Filling of the radiation belts with particles and the mechanism of particle loss. The origin of captured particles with energies considerably exceeding the average energy of the thermal motion of atoms and molecules in the atmosphere is related to the action of several physical mechanisms. These mechanisms include (1) the decay of neutrons created by cosmic rays in the earth’s atmosphere, causing protons to form that fill the inner radiation belt, (2) the “pumping” of particles into the belts during magnetic storms, which is primarily responsible for the presence of electrons in the inner belt, and (3) the acceleration and slow exchange of particles of solar origin from the outer to the inner regions of the magnetosphere, which accounts for the filling of the outer belt and the low-energy proton belt with electrons.
Particles of solar origin can penetrate the radiation belts through special points in the magnetosphere called daylight polar cusps (Figure 5) and through the neutral sheet in the tail of the magnetosphere (its night side). In the regions of the daylight cusps and in the neutral region of the tail, the geomagnetic field is reduced sharply and is not a significant barrier to charged particles of the interplanetary plasma. The radiation belts are also filled partially as a result of the capture of the protons and electrons of solar cosmic rays that penetrate the inner regions of the magnetosphere. These sources of particles are apparently sufficient to establish radiation belts around the earth with the characteristic distribution of particle fluxes.
In the radiation belts of the earth, a dynamic equilibrium exists between the processes of filling the belts and the processes of particle loss. Particles escape from the belts partly because of the loss of energy in ionization; this factor limits the presence of protons of the inner belt in the magnetic trap to a time τ ~ 109 sec. Another basic cause of particle loss is the dispersion of particles upon mutual collision and dispersion in magnetic irregularities and plasma waves of varied origin. Dispersion may reduce the lifetime of electrons in the outer belt to 104–105 sec. These effects lead to a breakdown in the conditions for steady-state motion of particles in the geomagnetic field (adiabatic invariants) and to a “spray” of particles from the radiation belts into the atmosphere along the lines of force of the magnetic field.
Relationships of processes in the radiation belts of the earth with other processes in near space. The radiation belts undergo certain changes with time. The inner belt, which is closer to the earth, has greater stability and undergoes insignificant variations, whereas the outer belt experiences more frequent and pronounced changes. Small variations over the 11-year cycle of solar activity characterize the inner radiation belt. The outer belt markedly changes its boundaries and structure when there are even slight perturbations of the magnetosphere. The low-energy proton belt occupies an intermediate position in this regard.
Especially strong variations of the radiation belts are produced during magnetic storms. Initially, the density of low-energy particle flux increases sharply in the outer belt, and simultaneously a significant fraction of high-energy particles is lost. This is followed by the capture and acceleration of new particles, which causes particle fluxes to appear in the belts at distances closer to the earth than they would under quiescent conditions. After the compression phase, the radiation belts slowly and gradually return to their original state. During periods of high solar activity, magnetic storms occur frequently, causing the effects of individual storms to be superimposed on one another; the maximum of the outer belt is closer to the earth during these periods (L~ 3.5) than during periods of minimal solar activity (L~ 4.5–5.0).
The particle spray from the magnetic trap, especially in the zone of quasicapture of particles, leads to an increased ionization of the ionosphere, and an intense spray results in auroras. The reserve of particles in the earth’s radiation belts is not sufficient to sustain a prolonged aurora, however, and the relation of auroras to variations in the fluxes of particles in the radiation belts indicates only the general nature of these variations; in other words, it indicates that during magnetic storms particles are pumped into the radiation belts and sprayed into the earth’s atmosphere. Auroras continue for the duration of these processes—sometimes a day or more.
Radiation belts may be created artificially when nuclear devices are exploded at high altitudes. They may also be created by the injection of artificially accelerated particles, for example, through the use of an accelerator aboard an artificial earth satellite. An artificial radiation belt may result from the dispersion of radioactive substances in near space, when the products of these radioactive substances are captured by the magnetic field. Artificial belts were created as a result of nuclear explosions in 1958 and 1962. After an American device was exploded on July 9, 1962, about 1025 electrons, with energies of approximately 1 MeV, were injected into the inner belt; this was by two to three orders of magnitude greater than the intensity of the electron flux of natural origin. The residues of these electrons were observed in the belts for approximately ten years afterward.
The radiation belts of the earth pose a serious danger when long flights are made in near space. Fluxes of low-energy protons may cause malfunctioning of solar batteries and lead to fogging of fine optical coatings. A prolonged period of time spent in the inner belt may result in radiation injury to living organisms within spacecraft because of the action of high-energy protons.
In addition to the radiation belts around the earth, there are thick radiation belts around Jupiter and possibly around Saturn. The radiation belts of Jupiter, which were studied by America’s Pioneer 10 space probe, are considerably more extensive than the earth’s radiation belts and have particles of higher energies and higher particle flux density. The radiation belts of Saturn were discovered by radio-astronomical methods. Soviet and American spacecraft showed that Venus, Mars, and the moon do not have radiation belts. Mercury was found to have a magnetic field by the American spacecraft Mariner 10 in a flight past the planet, and thus it may have a radiation belt as well.
REFERENCESVernov, S. N., P. V. Vakulov, and Iu. I. Logachev. “Radiatsionnye poiasa Zemli.” In Uspekhi SSSR v issledovanii kosmicheskogo proslransiva. Moscow, 1968. Page 106.
Kosmicheskaia fizika. Moscow, 1966. (Translated from English.)
Tverskoi, B. A. Dinamika radiatsionnykh poiasov Zemli. Moscow, 1968.
Rederer, H. Dinamika radiatsii, zakhvachennoi geomagnilnym polem. Moscow, 1972. (Translated from English.)
Hess, W. Radiatsionnyi poias i magnitosfera. Moscow, 1972. (Translated from English.)
Shabanskii, V. P. Iavleniia v okolozemnom prostranstve. Moscow, 1972.
Gal’perin, Iu. I., L. S. Gorn, and B. I. Khazanov. lzmerenie radiatsii v kosmose. Moscow, 1972.
IU. I. LOGACHEV