M\Ian's knowledge of the properties of interplanetary space has advanced radically

Transcription

1 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 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 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 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 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 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

2 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 AWAY FROM SUN sv 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 interplanetary medium has increased immensely since 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 spacecraft. 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

3 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 "corotation" 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 experiments 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, Carmichael 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 diurnal 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 common 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

4 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. J. A. Van Allen, K. A. Anderson, and co-workers, have made the initial identification of electrons generated in solar flares, and since the gyroradii of these electrons are greatly different from those of the protons generated in the same flare, comparison of both species of data is a fruitful source of information regarding the field irregularities throughout the sun-earth region. Throughout the solar cycle, a cosmic-ray detector observes intensity variations in galactic cosmic rays which are roughly out of phase with solar activity. A topic of considerable relevance in the study of the energetics of the cosmic radiation in the galaxy, as a whole, has therefore always been whether the cosmic-radiation intensity observed at solar minimum is a true sample of the interstellar cosmic-ray population and flux, or whether there is still some residual modulation (depression) of the cosmic-ray intensity by the interplanetary magnetic field. The cosmic-ray spectrum at the orbit of earth and the continual flow of the solar wind throughout solar minimum indicated that there was, in all probability, some residual modulation; this subject was, however, settled unambiguously by J. A. Simpson'4 and co-workers when they measured the cosmic-ray flux as a function of distance from the sun, using instrumentation flown on Mariner IV during A very appreciable cosmic-ray density gradient was still evident, indicative of the fact that at no time during the solar minimum of did the earth lie outside of the sphere of influence of the sun's magnetic field. Confirmatory evidence for this conclusion is provided by the fact that cosmic-ray energy spectra and cosmic-ray anisotropy measurements indicate that the cosmic radiation observed at the orbit of earth for energies in the vicinity of 107 ev was primarily of solar origin, even during sunspot minimum. Conclusions.-The highly definitive experimental measurements made during the solar minimum of have already unambiguously confirmed a number of theoretical models, and have also added much quantitative detail unobtainable heretofore. For the first tinme, we have definitive information on the phenomena of maximum importance in determining the physics of the interplanetary medium, and may therefore expect a much greater understanding to emerge as these facts are studied theoretically throughout the next few years. The definitive data obtained during the period of relatively simple and slowly varying phenomena observed during the IQSY will permit an understanding of the phenomena to be observed during the forthcoming sunspot maximum which, from our experience during the IGY, might be undecipherable without the prior experience of simple well-behaved phenomena. The author believes that the IQSY was an extremely worthy successor to the previous international endeavors in geophysics, namely, the first and second Polar Years of 1882 and 1932, respectively, and the International Geophysical Year of In a sense, it may well represent the point at which history will decide that the new interplanetary spacecraft technologies came of age; they certainly augmented the older technologies with a wealth of unique and otherwise inaccessible data. From this point of view, the IQSY may well go down in history as the most

5 VOL. 58, 1967 N. A. S. SYMPOSIUM: K. G. McCRACKEN 2153 important endeavor since the first introduction of systematic geophysical observations during the nineteenth century. I Wolfe, J. H., R. W. Silva, D. D. McKibbin, and R. H. Mason, "The compositional, anisotropic and nonradial flow characteristics of the solar wind," J. Geophys. Res., 71, (1966). 2 Lazarus, A. J., H. S. Bridge, and J. Davis, "Preliminary results from the Pioneer 6 MIT plasma experiment," J. Geophys. Res., 71, (1966). 3 Snyder, C. W., M. Neugebauer, and U. R. Rao, "The solar wind velocity and its correlation with cosmic ray variations and with solar and geomagnetic activity," J. Geophys. Res., 68, (1963). 4 Neugebauer, M., and C. Snyder, "Mariner 2 observations of the solar wind," J. Geophys. Res., 71, (1966). 5 (a) Ness, N. F., C. S. Scearce, and J. B. Seek, "Initial results of the IMP I magnetic field experiment," J. Geophys. Res., 69, (1964). (b) Wilcox, J. B., and N. F. Ness, "Quasistationary corotating structure in the interplanetary medium," J. Geophys. Res., 70, (1965). (c) Ness, N. F., and J. M. Wilcox, "Solar origin of the interplanetary magnetic field," Phys. Rev. Letters, 13, (1964). (d) Ness, N. F., C. S. Scearce, and S. Cantarano, "Preliminary results from the Pioneer 6 magnetic field experiment," J. Geophys. Res., 71, (1966). 6 Coleman, P. J., L. Davis, E. J. Smith, and D. E. Jones, "Variations in the polarity distribution of the interplanetary magnetic field," J. Geophys. Res., 71, (1966). 7 Parker, E. N., Interplanetary Dynamical Processes (New York: Interscience, 1963). 8 Coleman, P. J., J. Geophys. Res., 71, 5509 (1966). 9 Ness, N. F., "Simultaneous measurements of the interplanetary magnetic field," J. Geophys. Res., 71, (1966). 10 Axford, W. I., "Anisotropic diffusion of solar cosmic rays," Planet. Space Sci., 13, (1965). 11 Parker, E. N., "Theory of streaming of cosmic rays and the diurnal variation," Planet. Space Sci., 12, (1964). 12 Fan, C. Y., J. E. Lamport, J. A. Simpson, and D. R. Smith, "Anisotropy and fluctuations of solar proton fluxes of energies Mev measured on the Pioneer 6 space probe," J. Geophys. Res., 71, (1966). 13 McCracken, K. G., U. R. Rao, and R. P. Bukata, "Cosmic ray propagation processes, I, A study of the cosmic ray flare effect," J. Geophys. Res., 72, (1967). 14 O'Gallagher, J. J., and J. A. Simpson, "The heliocentric intensity gradients of cosmic ray protons and helium during minimum solar modulation," Ap. J., 147, (1967).