Particles and fields in the interplanetary medium.

M\Ian's knowledge of the properties of interplanetary space has advanced radically since 1962, the major part of this advance occurring since the commencement of the International Years of the Quiet Sun. The IQSY has, in fact, been quite unique in that it has seen the augmentation of the extensive synoptic studies of both geophysical and solar phenomena such as were mounted during the IGY and its predecessors, the Polar Years of 1882 and 1932, by essentially continuous in situ studies of the interplanetary medium by detectors flown on interplanetary spacecraft. Thus, since late 1963, no less than ten interplanetary spacecraft belonging to the Interplanetary -Monitoring Platform (DIP), Eccentric Geophysical Observatory (EGO), -Mariner, Pioneer, and Zond series have provided almost continuous surveillance of interplanetary phenomena, using secondand third-genleration detection devices. The fact that most of these spacecraft have made simultaneous measurements of a nIumber of the properties of the interplanetary medium has provided detailed information on the causal relationships between the several interplanetary parameters and geophysical phenomena, at a tinme when the relative lack of solar activity resulted in an attractive simplicity in the observed data. As' a consequence, the IQSY has provided an unprecedented series of observations of the causal relationships between solar, interplanetary, and geophysical phenomena, which will be of major importance in understanding the more complicated situations observed during periods of appreciable solar activity. In this review, I attempt to summarize the 1967 understanding of the interplanetary medium, this understanding being based very heavily on work performed during and since the IQSY. The "Pecking Orcder" of Interplanetary Phenomena. First, briefly, let us review the "pecking order" in the interplanetary phenomena. The most important single feature is the solar wind, a tenuous, collisionless plasma which originates in the solar corona, and which is moving at a velocity of about 300-600 km sec-' at the orbit of earth. The kinetic energy density of the solar wind is quite considerable (typically a X 103 eV per cubic centimeter at the orbit of earth), and dominates the energy densities of other phenomena in interplanetary space. The solar wind provides the ultimate energy source for most geophysical phenomena (e.g., the aurora, magnetic storms, etc.). The solar wind, being a magnetohydrodynanaic fluid, transports the solar magnetic fields throughout the solar system, the lines of force of the interplanetary field being, on the large scale, in the form of Archimedes spirals (see Fig. 1). The magnetic fields, typically of field strength 5 X 10oersted, are in turn strong enough to dominate completely the motion of any charged particle of energy less than 1011 eV, this energy range embracing 99.9 per cent of the cosmic radiation. Thus, in summary, the solar wind determines the behavior of the interplanetary magnetic field, which in turn is completely dominant in determining the behavior of the cosmic radiation. The Solar Wind.-Extensive studies of the solar wind by instruments on the Mariner, LWIP, and I'ioneer spacecraft by research groups at MIIT, and the Jet Propulsion Laboratory and Ames laboratories of NASA, have shown that the solar wind persisted throughout the whole period of solar minimum.1' 2 The wind ex-