Figure 34. Magnetospheric bow shock as revealed by space-probe measurements. Magnetic field data from orbit 11 of Imp 1. The magnetopause is at 13.6 earth radii. The second transition at 20 earth radii to an ordered field outside is the location of the bow shock wave. C. S. Scearce and J. B. Seek, Journal of Geophysical Research 69 (1964): 3531–69; copyright American Geophysical Union, 1964. that the particles were accumulated in the trapping regions of the magnetosphere and then dumped or dribbled into the auroral zones to produce the auroras also ran into difficulties. Although both Soviet and U.S. measurements showed that the fluxes at the altitudes from which the particles could spiral along the field lines into the auroral regions were adequate to produce an aurora, the quantity of radiation was too low. The particles would be drained away in a few seconds, whereas auroras often lasted for hours. 25 Brian O'Brien observed, however, from instruments in Injun satellites of the State University of Iowa that trapped electrons in the radiation belt, electrons precipitated into the atmosphere of the auroral zone, and auroral light emissions all increased simultaneously.26 One could conclude that the disturbances ultimately causing the auroras somehow also replenished the radiation belt, perhaps in this way making it possible to sustain a long-duration auroral display. Whether these additional electrons were inserted into the radiation belt from outside or came from lower energy electrons already existing within the belt and accelerated by some mechanism to the necessary higher energies was not known. Indeed, while many clearcut relationships between auroras and radiation belt activity had been established, at this stage the actual mechanism producing the auroras remained a mystery. Also unexplained was the immediate cause of the main phase of magnetic storms. A ring current around the earth continued to be the most likely candidate, but how such a current was generated remained a puzzle. It could be shown that charged particles in the magnetosphere, in addition to spiraling around magnetic field lines bouncing back and forth between northern and southern reflection points, would also tend to drift longitudinally, the electrons drifting eastward and the protons westward.27 Thus, these drift motions produced in effect a ring current, which S. Fred Singer suggested as the cause of the main phase of magnetic storms.28 By the end of 1964, however, no spacecraft measurements had been able to locate the postulated ring current. By the mid-1960s a very detailed, though by no means complete, picture of what the magnetosphere was like had evolved, as illustrated in figure 35. In the magnetospheric paradigm of 1964 the existence of the solar Figure 35. The magnetosphere as visualized in the mid-1960s. Space-probe measurements have provided a wealth of detail. The principal research problems are shifting from describing the phenomenon to explaining the relationships and processes. wind had been established. The wind consisted of protons mostly, with some alpha particles (helium nuclei), both of which had been observed. To be neutral the wind had to include equal numbers of electrons, but these had not been detected as yet. Embedded in the solar wind was an interplanetary magnetic field pulled out of the sun by the solar wind plasma. Near the earth the interplanetary field intensity was between five and six gammas. Blowing against the earth's magnetic field, the solar wind produced a huge shock wave sweeping around the earth much as an aerodynamic shock wave accompanies a supersonic airplane. But, whereas an aerodynamic shock wave is produced by compression of a gas consisting of air molecules all colliding with each other, the magnetospheric shock wave was set up by deflection of the individual plasma particles by the earth's magnetic field and was referred to as a collisionless shock wave. Behind the shock was a region of turbulence. Here the magnetic fields became highly disordered; particle velocities, which in the solar wind were usually confined to a rather narrow range, suddenly varied widely. Closer to earth this transition region was bounded by the magnetopause enclosing the geomagnetic field now grossly distorted from the simple dipole configuration that would have existed in the absence of a solar wind. Some of the field lines that would otherwise have lain on the sunward side of the earth were swept backward in the antisolar direction and along with field lines on the night side were extended into a magnetospheric tail. The magnetic field lines that still enveloped closed regions near the earth contained the Van Allen Radiation Belt, which paradoxically appeared to be more limited in extent on the night side of the earth than on the daytime side, where the field was compressed by the solar wind. On the dayward side, toward the poles, where some of the field lines were swept out into the tail, appeared a cusp or dimple in the magnetopause. It was thought that where magnetic field lines of opposite direction came together near the equatorial plane of the tail, they might cancel each other producing a neutral sheet. Along this neutral sheet one could envision charged particles leaking from interplanetary space into the zones closer to earth, where they could then be steered by the field toward the poles. In the steady state this magnetospheric configuration drifted slowly around the earth, always keeping the tail away from the sun as the earth revolved around the sun. The nose of the shock wave was about 14 earth radii from the center of the earth, and the nose of the magnetopause typically at about 10 earth radii. The extent of the magnetospheric tail was a matter of speculation, but it appeared certain to reach at least to lunar distances. At times when the sun was disturbed, the magnetosphere and the radiation belts were affected. The spatial extent of the magnetosphere varied appreciably and trapped radiations were enhanced following solar storms. There was a question as to whether during these disturbed conditions new particles were injected into the radiation belt or energy was transferred by hydromagnetic waves from the interplanetary plasma to particles already in the magnetosphere. Many problems, of course, remained unsolved. An explanation of the auroras appeared tantalizingly close, yet elusive. The immediate cause of the main phase of magnetic storms was still to be found. How energy and particles were inserted from the interplanetary medium into the magnetospheric regions had yet to be explained. The existence of the neutral sheet had not been established, nor had its precise role in magnetospheric physics been described. How the field lines in the magnetospheric tail closed again also had yet to be described. Did they perhaps connect with magnetic field lines in interplanetary space, as some surmised? Related questions concerned the sun. How did the sun manage to eject the streams and clouds of highly energetic particles and magnetic fields that from time to time upset the normal conditions in the solar wind? There was reason to suppose that solar magnetic fields were the ultimate source of the energy conveyed to these clouds, but there was as yet no generally accepted explanation. Most of the early research on the magnetosphere was directed toward describing it. As the subject became more familiar, more and more attention was devoted to achieving a coherent explanation of the magnetosphere and its relationship to the sun and interplanetary medium on the one hand, and to terrestrial phenomena on the other. By 1964 the major interest of the scientists lay in trying to understand the various processes in magnetospheric physics. There was, of course, still much to learn about what the magnetosphere and its most important phenomena were. But enough of the what had been learned that now investigators could profitably spend much of their time on the how, the immediate and ultimate causes of the auroras, magnetic storms, radiation belts, and the magnetospheric tail, and on the processes that related causes with effects. To understand these processes would be the principal objective of magnetospheric research in the years ahead. SIGNIFICANCE Clearly the discovery of the earth's radiation belt and the subsequent description developed for the magnetosphere constituted a major scientific achievement. It is natural, then, to ask what the significance of the achievement might be. Was magnetospheric physics really a new field of research, as some claimed? Did Van Allen's discovery set in motion a scientific revolution, or was the unveiling of the magnetosphere simply normal science? The attempt to answer these questions provides a good illustration of the difficulties in Kuhn's concepts of paradigm, normal science, and scientific revolution. As to whether magnetospheric physics was a new field of research, certainly before the discovery of the radiation belt no one was consciously working on investigating the magnetosphere, since the existence of such a region was unknown. Following Van Allen's experiments, scores of researchers began to investigate the magnetosphere. One could then legitimately argue that here was indeed a new field of research, not being pursued before, now being pressed vigorously. But this seems too shallow a conclusion. For research on the earth's magnetic field, the auroras, sunearth relationships, and cosmic rays had been of long standing when Explorer 1 went aloft. From this, magnetospheric physics appears more as simply one aspect of those other fields-a remarkable and hitherto unforeseen aspect, to be sure, but integrally related. Did, then, the unveiling of the magnetosphere constitute a scientific revolution in the related scientific fields? Certainly the magnetospheric paradigm that emerged from the first half-dozen years of satellite and space-probe research was new and unpredicted. One is tempted, then, to argue that the emergence of this entirely new paradigm was evidence of a scientific revolution. But again the quick answer may be too superficial. True, the trapped radiations and the magnetosphere as it was revealed were unpredicted. But that is not the criterion of a scientific revolution. One must ask instead whether the radiation belt and the magnetosphere were unpredictable from the existing paradigm in the sense of being fundamentally inconsistent with it. The answer to this question may well be no. In fact, the work of Størmer and others, based wholly on the existing paradigm, had provided an adequate basis for predicting the existence of trapped radiations in the earth's magnetic field. In this light the new magnetospheric paradigm appears as a straightforward extension of the previously existing paradigm, requiring no changes in fundamental principles or concepts. From this perspective, then, the magnetospheric research of the early 1960s was normal science-exciting, productive, important, yet normal science. But magnetospheric physicists are likely to consider the above perspective too broad. Norman Ness, one of the key figures in magnetospheric research, regards the progress made in the half-dozen years following the discovery of the radiation belts as revolutionary. In this assessment Ness considers the emergence of a new magnetospheric paradigm and the fact that no one predicted it as of primary significance.29 One major implication of the research on the earth's magnetosphere— which was immediately recognized—was that the way in which the interplanetary medium affects a planet depends strongly on whether the planet has a magnetic field. In a period when the idea of comparative planetology was being emphasized by the availability of spacecraft to carry scientific investigations to the different planets, scientists previously interested in sun-earth relations were beginning to talk about sun-planetary relations. It had already appeared as though the moon produced a detectable wake in |