Particles and Fields in the Magnetosphere

The most extensively studied space discipline of the post-Sputnik period is that of the "particles and fields" in the upper environment of the earth. Some 700 instruments on over 100 spacecraft have been used to make measurements of these phenomena. Consequently, over one billion relevant measurements have now been made in space, and an appreciable proportion of these were made during the period of the quiet sun, i.e., during the IQSY. However, because of long lead times and uncertain launch dates in space flights, it is difficult to isolate IQSY activities per se. This review, therefore, is intended to summarize the present knowledge of magnetospheric particles and fields. The need for an international coordinated program (such as the IQSY) has of course been recognized on several occasions. This need can be shown to be particularly strong in studies of auroral and magnetospheric phenomena, since these phenomena are neither controllable nor reproducible, as are many laboratory measurements. Thus a concerted worldwide study of individual phenomena-such as was permitted and encouraged by the IQSY-can yield far more worthwhile scientific results than the equivalent number of uncoordinated studies of separate "events." Historical General Review.--In the 1930's, Chapman and Ferraro (1932), after noting that great "worldwide" auroras occurred a day or two after a very large solar flare, speculated that the energy that sustained the auroras was supplied by corpuscular radiation emitted in the solar outburst. They considered only the rare "great" auroras, and envisaged the geomagnetic field as hollowing out a cavity in this fast-streaming flow of solar particles. Subsequently, Parker (1959) suggested that this solar-corpuscular flow might be continuous and due simply to the immersion of the earth in the supersonically expanding hot atmosphere of the sun. This plasma flow came to be called the "solar wind." It was also recognized that auroras occur continuously (O'Brien, 1967) and so the relationship of auroras and the solar wind as its ultimate source of energy seemed even closer, although no direct theoretical causal links were established. Figure 1 illustrates crudely the comparison between the magnetospheric concepts of the 1950's and one of those of the 1960's. Theoretical ideas and direct space measurements have developed a concept shown in more detail in Figure 2. The supersonic solar wind, travelling at around M\ach 5, is obstructed by the geomagnetic field, so that an enormous collisionless shock front is set up (see Fig. 2). Behind the shock front the solar wind particles are thermalized. Due to the fight for control between plasmas and local magnetic fields, the thermalized solar wind sweeps back the geomagnetic field into a cometlike tail behind the earth (see Fig. 2). The length of this tail has been estimated theoretically, and estimates differ by factors of 1000 or more, but it does appear that the tail reaches beyond the orbit of the moon. Auroras: Auroras are among the most spectacular of geophysical phenomena, with their tremendous variations in shape, brightness, color, and time and height. During the IGY, rocket flights into visual auroras demonstrated that most of their energy came from the kinetic energy of electrons of some 1 to 10 keV as they bombarded and were absorbed in the atmosphere at altitudes of some 100 to 150 km.