Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
ON THE GEOPHYSICAL AND GEONUCLEAR SIGNIFICANCE OF THE EARTH'S RADIATION BELTS James A. Van Allen State University of Iowa Introduction An immense region around the earth has been found to be populated by energetic protons and electrons, temporarily trapped in periodic orbits in the earth's magnetic field [Van Allen, McIlwain and Ludwig (1959), Vernov, Chudakov, Vakulov and Logachev (1959), Van Allen and Frank (1959a). (1959b), Van Allen (1959)]. The region is found to be subdivided into two distinctly different belts or zones--the inner zone and the outer zone. An annular "slot" or region of minimum intensity of charged parti- cles separates the two regions of high intensity. (Figure 1. ) During the some two and a half years (February 1958 to present) since discovery of this phenomenon with the Explorer I satellite, a large variety of experimental and theoretical work has been conducted by workers in the United States and Russia. Salient features of present knowledge are as follows: 1. The intensity-structure of the inner zone is stable in time to a known accuracy of some 20 percent. 2. There are enormous temporal fluctuations of the intensity- structure of the outer zone. The intensity of radiation at a specified point in space has been observed to vary by as much as three orders of magni- tude (1000/1) as measured with a given detector. Also the outer zone often exhibits a complex and time-varying detailed spatial structure. These fluctuations are, broadly speaking, correlated with geomagnetic storm activity and hence with solar corpuscular radiation. 3. A typical time history of the outer zone on the occasion of a magnetic storm comprises a marked "dumping" or depletion of detectable radiation accompanied by aurorae, and enhanced atmospheric airglow, recovery of the intensity with a characteristic time of the order of a day to a value exceeding (perhaps by manyfold) its pre-storm value, then gradual relaxation toward a quasi-steady value characterizing the general period of time of the observations. 63
GEOMAGNETiC AXIS Figure 1. Intensity-structure of the trapped radiation around the earth. The diagram is a section in a geomagnetic meridian plane of a three- dimensional figure of revolution around the geomagnetic axis. Con- tours of constant intensity are labeled with numbers 10, 100, 1000 and 10, 000. These numbers are the true counting rates of an Anton Type 302 Geiger tube carried by Explorer IV and Pioneer III. The linear scale of the diagram is relative to the radius of the earth-- 6371 km. The outbound and inbound legs of the trajectory of Pioneer in are shown by the slanting, undulating lines. See references for further discussion. 4. There seems to be very little doubt that the outer zone is the result of intrusion of solar plasma into the geomagnetic field, of its tem- porary trapping there and of "local acceleration" of the charged particles therein. 5. The composition at the heart of the outer zone on an occasion of high intensity [Van Allen and Frank (1959b)] was as follows: Electrons of energy greater than 20 kev: Omnidirectional intensity ~1 x l011/cm2 sec. Electrons of energy greater than 200 kev: Omnidirectional intensity <1 x l08/cm2 sec. Protons of energy greater than 60 Mev: Omnidirectional intensity <102/cm2 sec. 64
The typical level of intensities in the outer zone is at least an order of magnitude less than given in this example. There remains an especially important lack of observational knowledge of the intensity of charged particles of lesser energies than those specified. 6. The composition at the heart of the inner zone (more-or-less time-stationary during the past 30 months) has been estimated by the author as follows: Electrons of energy greater than 20 kev: Maximum unidirectional intensity ~ 2 x 109/cm2sec sterad. Electrons of energy greater than 600 kev: Maximum unidirectional intensity â¢-1 x 107/cm2sec sterad. Protons of energy greater than 40 Mev: Omnidirectional intensity ~2 x 104/cm2sec. 7. It now appears very probable that the high energy proton com- ponent of the inner zone is due to the decay of a small fraction of the neutron albedo which emerges from the top of the Earth's atmosphere as a result of cosmic ray interactions with atmospheric constituents. The origin of the electron component in the inner zone is not as clear but is likely the same. Geonuclear Effects of the Trapped Radiation In estimating quantitative geonuclear effects of the trapped radiation, it is appropriate to take the effects of the cosmic radiation as a standard of comparison. Present evidence indicates that the intensity of trapped electrons (in either the inner or the outer zone) of sufficiently high energy (>10 Mev) to produce nuclear disintegrations in the atmosphere is many orders of magnitude too small to be of interest in geonuclear processes. Only the protons in the inner zone are of potential interest in this connection. But if the neutron albedo hypothesis of their origin is indeed correct, it follows that their contribution to geonuclear effects is only a trivial fraction of the total of cosmic ray effects. The argument is as follows: The fraction of cosmic ray energy carried away from the top of the atmosphere by energetic neutron albedo does not exceed 10-1. Furthermore, only some 10-3 of neutrons of energy greater than 0. 5 Mev decay within the region of the geomagnetic field in which they are effec- tively trapped. Thus the total source strength for trapped protons of possible geonuclear interest is energetically much less than 10-" of the total source strength for all cosmic ray processes. Trapped particles enjoy a vastly greater geometric path length than do cosmic ray secondaries 65
which move downward into the atmosphere. But their physical path length (i. e., amount of material penetrated before coming to rest) is unchanged. Hence, it seems safe to conclude, without more detailed discussion, that trapped protons from cosmic-ray neutron albedo are of little or no geo- nuclear interest. Nonetheless, an object exposed to the bombardment of trapped pro- tons in the inner zone may be expected to become radioactive [Evans (1958)]. Indeed, observational evidence was obtained [Vernov and Chudakov (1960)] that the NaI crystal and the material surrounding it in Sputnik EI developed a significant level of radioactivity characterized by a half-life of the order of one hour and by other half-lives of the order of weeks or more. A quantitative study of the production of nuclear stars in emulsion by protons in the lower fringe of the inner zone [Yagoda (1960)] has been made. The reader is reminded that the underlying assumption of the pre- ceding discussion is that the trapped protons in the inner zone are the decay products of neutron albedo generated in the atmosphere by cosmic ray bombardment. It is conceivable, though perhaps unlikely, that they arise from geomagnetic capture of solar protons. But even in this case the geonuclear effects of trapped protons are found to be negligible. (The direct effects of incoming solar protons require separate consideration.) The approximate agreement of the World's inventory of C14 with that ex- pected from cosmic ray production alone [Libby (1955)] provides inde- pendent, over-all confirmation of the more detailed estimates given above. Geophysical Effects of the Trapped Radiation In contrast to the negative assessment of geonuclear effects, it is reasonably certain that the trapped radiation is intimately associated with, and in some cases essential to, a wealth of geophysical (i. e., atomic, molecular and electronic) effects. The inner zone is of secondary im- portance in this connection. But the outer zone participates to an important degree in many local and worldwide geophysical phenomena. Indeed its existence provides an important new element in the understand- ing of a variety of problems of long standing. A comprehensive discussion of such matters is not attempted here but the following listing of pertinent phenomena indicates the scope of current investigations: 1. high latitude aurorae 2. low latitude aurorae 3. airglow 4. geomagnetic storms and quiescent geomagnetic effects 5. atmospheric heating, particularly in the auroral zone 6. ionospheric effects 7. generation of radio noise and whistlers 66
Arnold: The time required for six passages of Sputnik III through the inner zone should be a day or so, and consequently we might expect nuclides to have been produced with lifetimes of the order of a few hours to a few days. None of the aluminum isotopes falls in this range--they all have half-lives of a few seconds or minutes, except for A12° which is 106 years. I126 has a half-life of 13. 3 days and could result from a (p, pn) reaction on the iodine in the scintillation crystal. [There are no obvious candidates among the light isotopes which might reasonably be formed from Al27, Mg24-26, O16, or N14, having the right half-life and emitting either positrons or 0. 5 Mev gammas.] Cameron: In view of the large radiation belts which we infer for Jupiter from the radio noise and polarization observations, we might expect Saturn to have substantial radiation belts as well. The fact that it apparently does not could be attributed to the presence of rings â the dust or ice particles would simply absorb the protons and electrons. It seems doubtful that Saturn could maintain its rings for 4. 5 billion years under these conditions; if not, then the rings must be steadily regenerated. REFERENCES Evans, T. C. (1958) private communication to the author. Libby, W. F. (1955) Radiocarbon Dating, 2nd edition, 175pp., University of Chicago Press. Van Allen, J. A. (1959) "The Geomagnetically-Trapped Corpuscular Radi- ation," J_1_GeophysJ_^es_. 64, 1683-1689. Van Allen, J. A., and Frank, L. A. (1959a) "Radiation Around the Earth to a Radial Distance of 107, 400 Km." Nature (London), 183, 430- 434. Van Allen, J. A., and Frank, L. A. (1959b) "Radiation Measurements to 658, 300 Km with Pioneer IV, " Nature (London), 184, 219-224. Van Allen, J. A., Mcllwain, C. E., and Ludwig, G. H. (1959) "Radiation Observations with Satellite 1958*. " J. Geophys. Res. 64, 271-286. Vernov, S. N., and Chudakov, A. E. (Moscow 1960) "Investigations of Radiation In Outer Space, " in Proceedings of the Moscow Cosmic Ray Conference, pp. 19-29, Vol. III, ed. S. I. Syrovatsky, Inter- national Union of Pure and Applied Physics. 67
Vernov, S. N. , Chudakov, A. E. , Vakulov, P. V. , and Logachev, Yu. I. (1959) "Study of Terrestrial Corpuscular Radiation and Cosmic Rays during Flight of the Cosmic Rocket." Doklady Akad. Nauk. SSSR 125, 304-307. Yagoda, H. (1960) "Star Production by Trapped Protons in the Inner Radi- ation Belt," Phys.Rev.Letters 5, 17-18. 68
