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Particles and Fields in the Interplanetary Medium M PARTICLES AND FIELDS IN THE INTERPLANETARY MEDIUM By KENNETH G. -MCCRACKEN UNIVERSITY OF ADELAIDE, ADELAIDE, AUSTRALIA 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 geo- physical 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 space- craft. Thus, since late 1963, no less than ten interplanetary spacecraft belonging to the Interplanetary -Monitoring Platform (DIP), Eccentric Geophysical Observa- tory (EGO), -Mariner, Pioneer, and Zond series have provided almost continuous surveillance of interplanetary phenomena, using second- and 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 under- standing 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 inter- planetary field being, on the large scale, in the form of Archimedes spirals (see Fig. 1). The magnetic fields, typically of field strength 5 X 10- oersted, 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- 2149 Downloaded by guest on October 2, 2021 2130 N". A. S. SYMPOSIUAI: K. G. AcCRACKEN PROC. N. A. S. hibited velocities typically ill the vicinityr of 300 km see-', rising to 600 km sec' and more for periods of a few days at a time. The fluctuations in velocity reappear every 27 days,3 4that is, after each complete rotation of the sun, often being seess for as maniy as six solar rotations. Stated differently, the faster solar wind is associated with specific, localized regions of the solar corona. The observation of the faster plasma is preceded by the observation of enhanced plasma densities, due to the piling up of the slower plasma in front of the fast plasma. Ultimately, a hydromagnetic shock will develop at the interface between the slow and fast plasma. The plasma density normally lies in the range 1-20 atomis cm-3, and both hydrogen and helium ions have been observed in the quiet-time plasma. The plasma is relatively cold, the kinetic energy of bulk motion exceeding that of random motion by a factor > 10. SOLAR WIND BLOWING RADIALLY SUN SUN sv AWAY FROM INTERPLANETARY MAGNETIC FIELD LINES OF FORCE COSMIC RAY SPIRALING ABOUT MAGNETIC LINES OF FORCE FIG. 1. The Interplanetary Magnetic Field.-Our knowledge of this property of the inter- planetary medium has increased immensely since 1964. While this has been largely due to a series of experiments on the I\IP and Pioneer spacecraft by Norman Ness and co-workers at the NASA Goddard Space Flight Center,5 important information has also been obtained by P. J. Coleman and co-workers from instrumentation flown on the 'Mariner spacecraft.6 The interpretation of cosmic-ray studies has also aided in exploring the properties of the fields presently inaccessible to space- craft. First, the direct measurements confirmed the theoretical prediction7 that the large-scale magnetic field would be in the form of an Archimedes spiral. Wilcox and Ness then demonstrated a feature of paramount importance, namely, that the field exhibits a well-defined "sector structure," the interplanetary field having the same polarity over a range of azimuths which is typically 90° wide. It has been shown that the sector structure is sometimes very long-lived: ill two cases, at least, sectorial boundaries exhibited lifetimes in excess of two years. By a study of the magnetic fields within the sectors, it, has been shown that there are small-scale irregularities in the field; Coleman further demonstrated that the power spectrum Downloaded by guest on October 2, 2021 VOL. 58, 1967 N. A. S. SYMPOSIUM: K. G. MCCRACKEN 2151 of the irregularities varies as the reciprocal of their scale size.' Since the number and size of the irregularities in the interplanetary field determine the diffusion coefficients for charged particles in the interplanetary field, the power spectrum of the field irregularities is an experimental quantity of paramount importance in understanding the phenomena of cosmic rays and other charged particles. By comparing the observations made by IMP III and Pioneer VI, when separated by a distance of 3.3 X 106 km, Ness was able to demonstrate a filamentary nature of irregularities in the interplanetary magnetic field, and to demonstrate the "co- rotation" of this field9 (i.e., the field behaves as if it were rotating with the sun). It is pertinent to observe that many of the above features, of greatest significance in the study of charged-particle propagation processes, could not be inferred from observations performed on the surface of the earth. Thus, these spacecraft experi- ments were crucial to the advance of many aspects of the study of the interplanetary medium. Energetic Particles.-The cosmic radiation and fast charged particles generated on the sun are normally grouped together as "energetic particles." During the IQSY, considerable advances have been made in both observational details and the theoretical understanding of the propagation of such particles, the studies now having reached a stage of considerable sophistication and precision. First, a carefully coordinated, worldwide network of "super" neutron monitors was established to provide detailed continuous records of the cosmic radiation of medium energy ( > 109 eV). The detector used, being an evolution of the design used during the IGY, was developed to a high degree of perfection by Hugh Carmichael and co-workers at the Chalk River Laboratories of the Atomic Energy of Canada, Ltd. Then, to calibrate the energy response for such detectors, Car- michael measured the neutron monitor counting rate as a function of position on the earth's surface, ranging between northern Canada and southern Mexico, and between Hawaii and Boston. With an accurate spherical harmonic expansion of the earth's magnetic field to calculate the minimum energy required to permit a cosmic ray to reach each point of observation, it was then possible to determine the neutron monitor counting rate as a function of primary cosmic-ray flux for particle energies 109 < E < 2 X 1010 eV. This calibration, which is invariant with respect to time, will permit accurate energy-spectra studies using "super" neutron monitor data throughout the next cycle of solar activity. The new "super" neutron monitors have permitted definitive studies of the diur- nal variation of the cosmic radiation, and of the anisotropic nature of all forms of cosmic-ray variations. The diurnal variation observations exhibit impressive agreement with theories developed independently in 1964 by W. I. Axford'0 and E. N. Parker," in which it is argued that the cosmic rays will corotate with the sun. The theoretical and observational work exhibit such impressive agreement that apparently the old problem of the origin of the diurnal variation has at last been solved. The corotation of the cosmic radiation with the sun appears to be a com- mon feature of the cosmic-ray propagation process at particle energies of less than 1011 eV, having been further demonstrated by comparison of the data obtained by widely separated spacecraft,'2 and from a study of the degree of anisotropy of the cosmic radiation.'3 Contrary to expectation, a number of experiments have demonstrated that even Downloaded by guest on October 2, 2021 2152 N. A. S. SYMPOSIUM: K. G. McCRACKEN PROC. N. A. S. during solar minimum, the sun frequently generates low-energy cosmic radiation in solar flares. Detailed studies have shown that the cosmic radiation is highly anisotropic, and indicative of unimpeded "guiding center" motion consistent with an Archimedes spiral magnetic field throughout the whole sun-earth region.
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