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Chapter 4 PARTICLES, FIELDS, AND RADIO PHYSICS The study of fields and particles has provided direction and stimulus to a large fraction of the scientific effort in the U.S. space program since its inception. It has been demon- strated that interplanetary space involves phenomena that are of basic importance to our understanding of solar- planetary relations, of a wide variety of astrophysical problems, and of the sun itself. Missions to the outer planets will inevitably require that extensive observations be made of particles and fields within and in the vicinity of the magnetospheres of these planets. Furthermore, in any such missions, many years will be spent in interplanetary space en route to the planets; hence to guarantee some scientific results and to maintain a balanced program, observation of particles and fields in the outer solar system should be included in all such missions. For these reasons, and also because the flyby missions offer the possibility of out-of-ecliptic and interstellar trajectories, we must con- tinue to pay a great deal of attention to the design of effective particles and fields experiments, with regard to both interplanetary and planetary observations. SOLAR WIND Near the earth's orbit, typical parameters of the solar wind are a radial velocity of 400 km/sec, a density of 5 protons per cnr } an ion temperature of l00,000 K, an electron tem- perature several times as high, a flow direction (corrected for aberration) that is a couple of degrees east of the sun- earth line, and noticeable temperature anisotropies with the random ion velocity being greatest parallel to the magnetic field. All these quantities, plus the embedded magnetic field and chemical composition, vary substantially over intervals that range down to a few seconds and up to at least a week and perhaps a month. The identification of velocity streams, sector structure, 46
47 waves, and convected filaments or other structures has intro- duced some order into this chaos, but our understanding of the physical nature of some of these components, their origin, and their mode of evolution is incomplete in spite of much fruitful theoretical work. The ion temperature is higher and fluctuates more with time, and the anisotropies are less than is suggested by most theoretical models. The physical basis for the high ion temperature is presumably partially the thermal conductivity of the electrons and also a substan- tial conversion to thermal energy of the mechanical energy by wave-particle interactions. It seems likely that some- where beyond 3 to l0 AU most of the velocity fluctuations would have been smoothed out and the waves damped out except as regenerated by plasma instabilities perhaps associated with the anisotropic expansion. Observations over the space between the orbits of Venus and Mars are consistent with the basic theory. The average radial velocity should be essentially independent of radius. The average density should be inversely proportional to the square of the distance from the sun. The ion temperature should decrease as in adiabatic expansion, but over this range this effect cannot be disentangled from the fluctuations, effects of electron conductivity, and wave damping. The same theory should serve for these average properties out to the orbit of Jupiter and beyond to where the transition from solar system to interstellar regime becomes noticeable. But the theories governing the fluctuations, and our identifica- tion of the nature of the fluctuations are seriously deficient. Observations of as many as possible of plasma and magnetic properties as a function of distance from the sun out to 5 or l0 AU should form the basis for a vastly improved understanding of the rich diversity of plasma physics involved. On this basis, we should then be better able to extrapolate the fluctuations and structures in the solar wind upstream toward the sun to understand their origin and down- stream to understand the interaction with the galactic magnetic field and interstellar plasma. The basic discipline involved is collisionless plasma theory. Many of the proc- esses that can be understood by a comparison of theory and observation in the solar system should play important roles in the formation of stars, in phenomena produced by super- novas, in galactic and extragalactic radio sources, perhaps in quasars, and in all the other areas of astrophysics where plasmas are important.
48 The solar wind cannot continue to flow outward from the sun at hundreds of kilometers per second to indefinite distances. Somewhere at an estimated distance of 5 to 300 AU from the sun, it must begin to interact with the interstellar gas and the galactic magnetic field. The nature of this transition is of great interest. Quite probably it involves a shock across which the wind velocity drops substantially and outside which the flow is subsonic. Eventually there must be mixing with galactic plasma. The region within which the plasma is distinguishable as of solar origin is called the heliosphere. There is no reason to think in terms of a smooth, symmetrical model; there should be irregularities in plasma properties, magnetic fields, and fluxes of energetic particles over a vast range of scales. The region near the ecliptic, where the large-scale interplanetary field is mainly in the azimuthal direction and normal to the flow, may be very different from the sun's polar region, where even at very large distances the interplanetary field and the flow are expected to be nearly parallel except for the magnetic fluctuations generated by the probably inevitable instabili- ties. The sun and solar system probably have a velocity of the order of 20 km sec~l with respect to the surrounding galactic gas and magnetic field. In the direction of this relative velocity vector, the transition to the galactic region should be closest to the sun. The zero-order estimate of this direction is the direction of motion of the sun with respect to the fixed stars and 2l-cm HI background, which is approx- imately at 270° ecliptic longitude, +30° ecliptic latitude (i.e., roughly the longitude of Neptune in l980). This estimate is very tentative since the motion of the galactic gas and magnetic field may be anywhere from 0 to 20 km sec~*- in an unknown direction. Any mission to or beyond 5 AU should include both the capability of detecting the shock if it should unexpectedly be so close that it is traversed and of searching for any clues available on the transition region. Cosmic-ray observations might provide such a clue, as might a slight decrease in solar wind velocity with radius that could be ascribed to momentum lost by charge exchange between solar wind protons and incoming neutral atoms from the inter- stellar gas. Direct detection of the neutral particles would also be significant. An indication in an early flight of the best direction in which to go would be invaluable in planning later flights to explore the transition. It should be
-=- emphasized that our knowledge of plasma physics is so limited and our history of predicting such effects is so spotty that we must be prepared for a wide range of possibilities, and almost any observations at large distances from the sun will be valuable in restricting the number of possible models and in stimulating plasma theory. Although the missions considered here are to the outer planets, radio propagation observations can be made that provide information on the solar corona and interplanetary plasma. The coronal information is important in itself and also for its bearing on the origin of the solar wind. Radio links between the earth and the spacecraft can be used during the cruise phases of the flights to study certain properties of the solar corona and the interplanetary plasma. These could be the same links that are used for communications and tracking, but they should have the capability of measuring range rate, range, dispersion, absorption, scattering, and polarization rotation. On the long flights to Jupiter and beyond, the spacecraft will repeatedly go through superior conjunction, so that the radio paths will pass near, and perhaps be occulted by, the sun. Dispersion and polarization measurements conducted simultaneously will make it possible to separate effects of the plasma density of the solar corona from magnetic field effects, so that one may study space and time variations of the coronal plasma density and of longitudinal components of the coronal magnetic field. Range measurements, corrected by the results on dispersions, would provide accurate determi- nations of the 30-km apparent change in range due to the general relativistic effect of the solar gravity field, perhaps making it possible to test alternative theories. Amplitude and spectral measurements would provide information on irregularities and mass motions in the solar corona. The average plasma density between the earth and the spacecraft could be measured to very high precision. Radio propagation experiments on Pioneer A through Pioneer D show the expected values of the interplanetary electron density, with marked changes due to pulses of plasma from the sun, for regions near the orbit of the earth. These investigations could be extended to great heliocentric distances using outer-planet spacecraft. Differential measurements to several such spacecraft would be particularly sensitive to
50 the very low densities expected at great distances from the sun and should be able to detect a possible increase in density and subsequent drop-off at the limits of the solar- wind region. Observation of the galactic radio emission at frequencies down to l-l0 kHz may provide independent evidence for the large-scale structure of solar-wind plasma at the boundary of the heliosphere. Galactic radio emission in this frequency range will not be accessible to any other kind of observation than from outer-planet spacecraft. Its measure- ment will yield two results: clarifying both an important galactic phenomenon related to particles and fields in interstellar space and also the structure of the solar wind. INTERPLANETARY MAGNETIC FIELD The solar wind convectively transports the solar magnetic field into interplanetary space. Here its presence controls the motion of more energetic charged particles originating from transient events on the sun, such as solar flares, as well as the continual flux of extrasolar origin. The solar cycle modulation of galactic cosmic rays is due to the existence of the interplanetary magnetic field permeating the outward flowing solar plasma. Assuming a spherically symmetrical expansion of the solar corona into interplanetary space permits the theoretical pre- diction of the variation with distance (r) of the magnitude and direction of the "frozen-in" magnetic field. Near the solar equator, the radial component is given by Br = Bo(a/r) and the azimuthal component is given by B<b = BQ(a ft/rV), where BQ is the radial solar magnetic field close to the sun (on a surface of radius a), ft is the equatorial angular velocity of the sun, and V is the velocity of the solar wind. In this simplified model, it is also assumed that the solar wind velocity is constant beyond r = a. These two formulas represent geometrically an Archimedean spiral centered at the sun. Thus with typical solar-wind velocities of 300-500 km sec~l, the angle between a field line and a radius vector from the sun will be ~45° at l AU. However, the "spiral" becomes much more tightly wound beyond this distance until at 5 AU, the orbit of Jupiter, this angle is ~80°. Hence the field direction in the context of
5I the outer planets is expected to be principally transverse to the local solar-wind velocity. Direct measurements of the interplanetary magnetic field have been made, mainly by the IMP series of satellites at l AU and the Pioneer and Mariner series of space probes between 0.7 and l.5 AU. From statistical analyses of these data it has been shown that at l AU the average magnitude is 60 uG with short excursions to values as large as 400-500 uG, while the average direction is close to the expected spiral angle of 45°. The sense of the field along the spiral direction, either outward or inward, is referred to as posi- tive or negative polarity, a terminology consistent with that used in the study of the solar magnetic field. The radial variation of the magnitude of the field has been found to be consistent with the simple formulas given above. The variation in direction around the spiral angle is sufficiently large that it obscures the detection of the anticipated small variation in angle between 0.7 AU and l.5 AU (from 35° to 56°). At the orbit of Jupiter, the field magnitude is expected to be ~8 nG and to decrease as l/r beyond the orbit. However, nothing is known directly about either the geometry or magnitude of the magnetic field beyond l.5 AU and a major objective of the exploration of the outer planets should include a definitive study of the interplane- tary medium and its imbedded magnetic field. The observed magnetic field of the sun shows a complex variety of patterns and characteristic behavior. The most significant feature relative to the large-scale structure of the interplanetary magnetic field is the existence of large unipolar regions which persist for several to many solar rotations, although changing their size and location contin- uously. These unipolar regions appear to be directly reflected in the "sector" structure of the interplanetary magnetic field in which the "sense" of the field is observed to be constantly outward or inward for several days when observed between 0.7 and l.5 AU. Evidence for the existence of a sectored structure of the interplanetary magnetic field should be detected at distances > l.5 AU, although the terminology of outward- and inward-directed fields obviously is no longer appropriate. Instead, the positive polarity will refer to fields directed
52 in a sense opposite to the solar rotation and negative to that parallel. The time delay between the passage of successive sector boundaries should not vary with radial distance, but the lag between central meridian passage and the space probe should increase linearly with distance. The correlations of these measurements with those conducted by earth-orbiting satellites will be an important method of studying the large- scale structure and dynamics of the interplanetary medium. As the sector and filamentary magnetic structures in the solar wind are swept far out from the sun, the spiral direction becomes nearly the azimuthal direction at low solar latitudes and there must be many interfaces between tubes in which the field runs in opposite directions. To some extent there should be reconnection of field lines across these interfaces to form loops. These are pulled by magnetic forces in the azimuthal direction. Thus plasma experiments should search for localized regions of denser than normal gas that is moving azimuthally as well as radially. Magne- tometer data would be relevant. Low-energy cosmic rays should be accelerated in these structures, and the flow of low-energy galactic and solar cosmic rays into and out of the solar system may be substantially modified. The small-scale fluctuations of the interplanetary field have been studied and reveal that transverse perturbations are the most prevalent. In addition, a unique feature of the interplanetary medium is the presence of a copious number of discontinuities, surfaces across which the properties of the plasma-magnetic field change suddenly, on a time scale < 30 sec. These surfaces, when observed simultaneously by satellites between 0.7 and l.5 AU, appear to separate regions of magnetized plasma which are being convectively transported outward from the sun. However, the origin of these disconti- nuities is not known, either the sun or the interaction of fast and slow plasma streams being the principal contending sources. Characteristically, the field magnitude does not change across these surfaces, although the direction does change significantly. The measurement of the spectrum of the interplanetary magnetic field fluctuations is made difficult due to the supersonic solar-wind flow. This transports the interplane- tary field past the satellite at velocities many times larger than wave-propagation velocities. Thus, explicit time
53 variations of the field, as observed by satellites and space probes, in fact represent principally spatial variations of the field. Due to the large numbers of discontinuities present, the spectrum shows the expected characteristic dependence f~a(l < a < 2) for frequencies (f) less than l Hz. This is well above the Doppler-shifted ion-gyro frequency. Variations in the coefficient and the amplitude of the spectrum are dominated by the size and distribution of the discontinuities. Thus any attempt to study these spectra must properly assess the relative contribution of discontinuity surfaces. The distance to which discontinuity surfaces and small amplitude fluctuations extend is not known, and no theory yet treats this problem. Thus a fundamental experiment in the study of the interplanetary field is to determine the variation of its microstructure with distance from the sun. The discontinuity surfaces and the small fluctuations may eventually decay beyond l.5 AU, but exactly how and why is not known. Depending on how the solar-wind anisotropies and inhomogeneities vary with distance from the sun, the medium may become more or less intrinsically unstable to certain disturbances. The eventual merging of the solar with the galactic plasma may not occur at a sharply defined surface, and the identification of the extent of the heliosphere may depend on the gradient of the microstructure of the interplanetary medium. Accurate vector measurements of the interplanetary magnetic field and its variations for frequencies less than l Hz are of fundamental significance in a study of the interplanetary medium and the dynamics of the solar-system plasma. GALACTIC AND SOLAR COSMIC RAYS Knowledge of the composition and energy spectrum of galactic cosmic rays is of considerable astrophysical importance. Such knowledge is crucial in understanding the origin of cosmic rays and the major nonthermal process of astrophysics including x- and gamma-ray astronomy, processes occurring in supernovae envelopes and, perhaps, radio sources and pulsars. It is also essential in understanding the propagation of cosmic rays through the galaxy with their important influences on the galactic magnetic field and gas structures. Until we
54 have a sure knowledge of the flux of low-energy cosmic rays (below l GeV per nucleon), their influence in a variety of astronomical problems, including the heating in the interior of dense clouds, their effect on dust grains, and the produc- tion of rare isotopes, will remain uncertain. Measurements near the earth do not provide the necessary information, since low-energy cosmic rays are partially screened out of our part of the solar system by the irregular magnetic fields convected outward by the solar wind. This effect is called modulation, because the magnitude of the effect is a function of solar activity. Careful measurements of the mean intensity and anisotropy of different cosmic-ray nuclei (e.g., e~, e+, ^H, ^D, -*He, ^He, Li, Be, B, C, N, and 0) as a function of distance from the sun out to 5 or l0 AU should allow the modulation process to be understood and the essential characteristics of the unmodulated cosmic rays in our part of the galaxy to be observed directly or at least deduced with considerable con- fidence. It is plausible that the outer boundary of the region of modulation may be somewhat closer to the sun at high solar latitudes and hence that cosmic-ray observations in this region may be the best way to get this essential data. Existing theories of galactic cosmic-ray modulation are quite rudimentary, although the general features of the phenomenon at the orbit of the earth are reasonably well established. At the higher energies (greater than l0^ GeV), modulation is essentially absent, and the anisotropy appears to result mainly from the proper motion of the solar system and possibly from some net streaming of cosmic rays within the galaxy. At intermediate energies (greater than about 200 MeV for protons), the degree of modulation does not amount to more than a factor of ~2 in the intensity, and the intensity appears to behave as if the particles were being influenced by a heliocentric force field. In this energy range, the anisotropy has a small radial component associated with the modulation and a much larger (~0.47») azimuthal anisotropy resulting from the tendency for the particles to corotate with the interplanetary magnetic field structure. At low energies (l to l0^ MeV per nucleon) the anisotropy is essentially radial but, depending on the form of the energy spectrum, may be outwards or inwards. The degree of modulation in this low-energy range is quite large, but since we do not yet understand the modulation process properly,
55 it is not possible to do better than make an informed guess at the unmodulated intensity (a factor of l0 or more might easily be involved). Since solar modulation occurs at all energies up to at least l0 GeV, there must be a radial gradient of the cosmic- ray intensity somewhere beyond the orbit of earth, and the full intensity is presumed to be reached beyond ~l0 AU. However, measurements of the gradient in the region between Venus and Mars (0.7 to l.5 AU) have given quite contradictory results. It is possible that most of the modulation of the lower-energy particles involved in these measurements takes place in a thick shell of turbulent plasma at 3 to 5 AU, and that the interplanetary medium near l AU is relatively smooth so that the radial gradients are small. The determination of the radial gradient is of fundamental importance to our understanding of the modulation problem, and, accordingly, the experiment must be carried out with due care. In order to establish a base level, identical detectors should be placed on spacecraft near the earth. Furthermore, since the radial gradient is intimately related to the radial anisotropy, the shape of the particle energy spectrum, and the spectrum of interplanetary magnetic field fluctuations, some provision should be made for measuring each of these quantities concurrently. Experiments designed to measure the interplane- tary magnetic field should be such that they provide an adequate power spectrum of fluctuations up to frequencies of the order of lO'l Hz. It is a great advantage to be able to observe the behavior of different components of the galactic cosmic rays concurrently, since the modulation (and hence the radial gradient and anisotropy of the particles) is expected to depend on the particle charge-to-mass ratio. Furthermore, in the case of cosmic rays of secondary origin (notably positrons, but also ^Ee, Li, Be, and B), the unmodulated spectrum can be estimated directly from the spectra of the primary particles, and hence observations of the behavior of these particles at various heliocentric distances should provide a very good direct test of modulation theories. There have been as yet no in situ measurements of the interplanetary magnetic field or plasma at high heliographic latitudes. Comet tail observations suggest that the characteristics of the solar wind in the high-latitude region
56 are essentially similar to those found in the ecliptic. Observations of the scintillation of small-diameter radio sources are consistent with the comet tail results, although some preliminary analyses indicate that the solar-wind speed might be higher at higher heliographic latitudes. It is expected that the interplanetary magnetic field is more radial at high latitudes than in the ecliptic since the geometrical factors that produce the spiraling are different. Thus there are reasons for believing that degree of modulation of galactic cosmic rays at a given distance from the sun varies with heliographic latitude, and indeed there is some indirect evidence that this is the case. Clearly a mission to high heliographic latitude could provide a great deal of new information on these effects. Energetic particles (solar cosmic rays) are released from active regions on the sun, especially following solar flares. These events provide a useful means of probing the interplanetary medium along the path followed by the particles as they move from the sun to the point of observation. It should be noted that in order to interpret the observations properly, it is necessary to measure both the intensity and anisotropy of the particle distribution. A solar cosmic-ray event observed in the vicinity of Jupiter, for example, should have quite different characteristics from the same event observed in the vicinity of the earth. The time delay must, of course, be very much greater, because the particles have to travel l0 to 20 AU (depending on solar-wind velocity) along the spiraled interplanetary magnetic field lines, the anisotropies are likely to be correspondingly less pronounced, the direction of arrival must be typically from 80° west of the sun, and the relationship of the event to observed solar flares might not be clear. Furthermore, since the distribu- tion of magnetic field fluctuations along the field line connecting the sun to the space probe (and beyond) might be quite complicated, the temporal behavior of the intensity and anisotropy could be noticeably different from that seen at the earth. It would be interesting to be able to observe events occurring when the earth and the space probe are magnetically linked. This will occur at various radial distances, depend- ing on the relative solar longitudes of the two bodies. Observations of solar electron events would be especially interesting in this regard. In the case of particles which
57 are being emitted almost continuously by an active region on the sun (and therefore produce recurrent events), correlated observations of this nature should yield useful information on the diffusion coefficient perpendicular to the mean direction of the magnetic field. Furthermore, since the particles move essentially parallel to the interplanetary magnetic field lines, it might be possible to make use of observations of their direction of arrival as a means of correcting the magnetometer measurements if the spacecraft fields become noticeable. PLANETARY STUDIES Plasma Flow Past Planets and Their Satellites Three modes of flow of the magnetized solar plasma past a dense body in the solar system have been identified. In the case of the earth, which has a large intrinsic magnetic field, the entire interaction is dominated by this field, which supports the magnetopause (the boundary separating the geomagnetic field and plasma from the solar- wind plasma) at about l0 earth radii above the subsolar point. Outside of this, at about l4 earth radii on the sunlit side, is the bow shock, inside of which heated solar plasma flows around the magnetopause. Behind is a tail produced by the part of the geomagnetic field that is swept back to very large distances, greater than lOORg. In the case of the moon, which has essentially no intrinsic field, no ionosphere, and a very low conductivity, the solar wind flows unimpeded into the surface, leaving a nearly empty cavity behind. The interplanetary magnetic field passes through the moon and through the cavity with small and sometimes undetectable perturbations. In the case of Venus, which has little, if any, intrinsic field but does have a highly conducting ionosphere, the magnetic field of the solar wind cannot quickly penetrate the ionosphere and the solar wind cannot flow unimpeded into the atmosphere. There is a bow shock at about l.3 Venusian radii above the subsolar point; the shocked solar wind flows around Venus between this shock and a bounding surface (called the anemopause or wind shield) just above and supported by the
58 ionosphere. On the dark side there is some indication of the presence of a wake filled with relatively stagnant plasma whose density is hundreds of ions per cubic centimeter and extending to at least 2 Venusian radii. It may be speculated that the wake is bounded by interplanetary field lines that are entangled in the Venusian atmosphere or ionosphere in the front and swept back by the wind at the sides. It seems of great interest to learn whether the solar- wind flow past other planets and their satellites (when the latter are exposed to the wind) can be classified as one of these three types or whether there are new and surprising further modes of interaction. Also, the interaction of the Jovian magnetosphere and lo (as well as other satellites) needs clarification. According to most estimates it is unlike any of the three types discussed above. Studies of these phenom- ena will substantially further our understanding of the behavior of collisionless plasmas, a subject that lies at the heart of many problems in astrophysics. Magnetospheres Of all the outer planets, only Jupiter has thus far been identified as possessing a magnetic field and a significant radiation belt. This conclusion is based on the discovery, study, and analysis of radio emission in various frequency ranges. This permits estimates to be made of the topology and magnitude of the planetary magnetic field. Present estimates of relevant parameters suggest a model in which the dipole and quadrupole moments, magnetic centroid, axes, rotation rate, and field direction have fairly specific values. Observational verification or modification of this model would put the explanation of the wealth of radio astronomical data that can be obtained from Jupiter on a much firmer basis. The interaction of the solar wind and the Jovian magnetic field most probably leads to the formation of a magnetosphere, magnetosheath, and bow shock similar to the earth's. The above-mentioned estimates of the Jovian magnetic field and extrapolated measurements of the solar-wind flux at 5 AU lead to a magnetopause subsolar distance of 50Rj and a bow-shock distance of 70Rj from the center of the planet. The flow in the magnetosheath of thermalized (or shocked) magnetized solar plasma should be similar to that
59 for the earth. Finally, there may exist a huge tail to the Jovian magnetosphere if the wind extends the polar field lines in the antisolar direction. It is possible that, due to the large angular velocity and large magnetic moment of Jupiter and the distribution of plasma within the magnetosphere, the entire magnetosphere corotates with the planet, drastically modifying the tail and the rest of the magnetosphere. Only by direct measurements of the distant Jovian magnetic field will it be possible to determine the correct configuration of the tail -- if it exists. There is almost no knowledge of or reliable estimate for planetary magnetic fields and magnetospheres of the other planets. There is some inconclusive evidence (only one tentative observation) for perhaps intermittent radio emission from Saturn, but it is clear that it is not nearly so spectacular a radio source as Jupiter and hence that its radiation belts must be far less significant even though, of all the planets, it most resembles Jupiter in size, rotation rate, and composition. However, if the rings of Saturn are an effective absorber of charged particles, it could still possess a significant magnetic field. Thus only by direct, in situ measurements can the existence, geometry, and magnitude of a possible field be determined. There are essentially no clues in the case of Uranus and Neptune; we should approach their magnetic exploration with no preconceived ideas. Planetary Radiation Belts Only two planets are known to have radiation belts, composed of electrically charged particles moving in temporarily trapped orbits in the external magnetic field of the planet. The radiation belts of earth were discovered by in situ observations with a Geiger-Muller tube flown on the first American satellite, Explorer I. Those of Jupiter were suggested shortly after as an explanation of the nonthermal decimetric radio noise of that planet. The absence of radiation belts at Venus and at Mars has been established by direct observation on close flyby missions by the United States and the Soviet Union. Also, the moon has been found to have no radiation belt. There is no comprehensive quantitative theory of the
60 origin of planetary radiation belts. Only those of earth have been studied in detail. The following appear to be the rudimentary conditions for the existence of planetary (or satellite) radiation belts: (a) The dominant physical mechanism for the creation of a radiation belt is the electrodynamic interaction of the solar wind with the intrinsic magnetic field of the planet. (b) The magnetic field of the planet must have approxi- mate rotational symmetry about some axis and the strength of the external field B must be such that the hydromagnetic stagnation condition nmv2 = B2/8jt = M2/8itr6 (l) (n is the number density of charged particles in the solar wind of mass m and velocity v, and M is the equivalent dipole moment) must be satisfied at a distance r from the center of the planet that is greater than the radius of the effective "top" of its appreciable atmosphere. The foregoing condition is met at r = l0R£ for the earth and probably at r = 50Rj for Jupiter. The absence of radiation belts at Mars, Venus, and the moon is presumably due to the failure to satisfy condition (b) for r exceeding RM, RV, and R^on, respectively. Tenta- tively, it is thought that the existence and intensity of Jovian radiation belts certify that the directed flow of solar plasma (the solar wind) persists to at least 5 AU and that Jupiter has a magnetic moment in the range l0^ to l0^ cgsu. The theory of planetary magnetism, incomplete as it is, requires two basic properties: a fluid electrically conduct- ing core and a rotating body, though definitive quantitative criteria are not known. Jupiter and earth are very likely to possess both properties. Both Mercury and the moon rotate slowly and also, probably, are solid throughout. Mars has a rotational period similar to that of the earth, but it proba- bly has a solid interior. Venus may well have a fluid core but rotates very slowly (245-day rotational period). Hence, the body of present evidence is internally compatible with the two sets of rudimentary conditions given above. However, nonthermal radio noise has not been detected from Saturn, Uranus, Neptune, or Pluto despite systematic searches. Saturn may well be regarded as a special case on the grounds that directly trapped energetic particles cannot exist in the
region of its rings of particulate matter. Hence, the absence of nonthermal radio noise cannot be adopted as definitive evidence against the intrinsic magnetization of Saturn. Because of the large sizes and high rotational rates of Saturn, Uranus, and Neptune, it would be a matter of astonishment and fundamental significance if each of these planets does not have a magnetic moment comparable with that of Jupiter. The absence of detectable nonthermal radio noise from Uranus and from Neptune may be simply due to their great distances from the earth or perhaps to some change in the character of the solar wind at heliocentric distances greater than, say, l5 AU. It is clear from the foregoing discussion that the following investigations are of significance in illuminating the detailed physical character of the earth's radiation belts and, more broadly, in clarifying the basic physical conditions for planetary magnetism and planetary radiation belts: (a) Detailed study of the external magnetic field of Jupiter (b) Detailed study of the charged-particle populations in Jupiter's magnetosphere (c) Search for magnetic fields and radiation belts of Saturn, Uranus, Neptune, and Pluto and detailed investigation if positive findings occur in exploratory studies (d) Study of solar-wind flow characteristics to large heliocentric radial distances (out to or beyond 40 AU) Exploratory missions to the outer planets will require particle detectors having a wide dynamic range, simply because we have no knowledge of the nature of their radiation belts. It is clear from the observation of Jupiter's nonthermal radio emission that large fluxes of relativistic electrons exist in its magnetosphere. Some information could also be obtained from observations at the earth of x rays from Jupiter; at present, no positive measurements of x rays have been made, but the sensitivity of the detectors used to date could be substantially improved. RADIOPHYSICS IN THE EXPLORATION OF THE OUTER PLANETS Radio astronomy works in a spectral region where surprises are the rule rather than the exception. At low frequencies,
62 electromagnetic emissions from astronomical objects represent plasma physical effects that are novel in today's physics. To support this point of view, we can cite the dynamical radio phenomena of the solar corona and atmosphere, the existence of cosmical masers in the interstellar hydroxyl radical and in the water vapor molecule, the pulsars, and, finally, the extraordinarily intense decametric emissions from Jupiter. (Recent source size measurements set an upper limit of < O'.'l to the source size and correspond to an equivalent temperature brightness of l0l9 K.) Only the last of these phenomena is accessible to in situ observations from spacecraft. Its relation to other planetary parameters, such as the magnetic field, the thermal and nonthermal particle populations, and the different forms of wave motion in Jupiter's magnetosphere and ionosphere will surely clarify the at present still schematic suggestions about its physical origin. That novel plasma physical effects are involved appears to be an implication of the strong modulation of decametric emission by the first Galilean satellite lo (similar effects have not been detected for Europa, Ganymede, or Callisto). (For many reasons it seems unlikely that the actual emission occurs at the satellite; confirmation of this conclusion by observation of signals leaving Jupiter not in the plane of the ecliptic is highly desired and exceedingly difficult if not impossible from the ground.) lo acts at a distance. It is probably unreasonable to suppose that magnetospheric parameters vary so rapidly with distance that, say, Europa, the next Galilean satellite, fails to create a disturbance comparable with lo's. Therefore, we should conclude that not only lo's disturbance, but those of the three other Galilean satellites, plus Amalthea's and the other smaller satellites, are present and sloshing about in a complicated pattern within Jupiter's magnetosphere. In situ measurements of particles and fields may in this case represent an extremely complex superposition of phenomena originating at different source points distributed over Jupiter's magnetosphere. The observational problem is to identify and connect these waves and particles to decametric emissions in a meaningful way.
63 The needs for radio physical studies of Jupiter seem clear in this context. First, measurements of the highly intense, highly directive, and variable decametric emissions should be made directly in Jupiter's vicinity where direction of arrival effects are easier to observe on account of the large angle subtended by Jupiter and the absence of inter- planetary or terrestrial phase shifts. Furthermore, we note that, at Jupiter, decametric emissions can be directly related to wave phenomena. The alternative of ground-based or near- earth observation of the emission simultaneously as a particle- and-fields (without radio) vehicle flies through the magneto- sphere suffers the difficulty that the richness of the wave phenomena will not be easy to sort out. Finally, the radiation beams into solid angles, either cones of 3° or 4° half-angle or sheets of less than l° thick- ness, which do not intersect the position of the spacecraft and the direction of the earth in any clear way. As a consequence, we may expect, for example, that the Europa decametric emissions lie in some other direction than the extremely limited range of angles about the ecliptic to which our earth-based data correspond. To discover the decametric emissions connected to the other Galilean satellites, especially Europa, requires, we infer, a vehicle orbiting Jupiter at high inclination. Simultaneously, we might hope to disentangle the complex of wave phenomena observed at the vehicle by their relation to the (then) observable radio emissions associated with the other satellites. The result of these radio astronomical studies, carried out synoptically over a range of orbiter positions with respect to the major satellites and to the rotational aspect of Jupiter, should be an adequate physical explanation of one of the most intense emissions known to astronomy. Galactic Low-Frequency Radio Noise The local plasma density of the solar wind near the earth limits observations of galactic radio waves to frequencies above ~30 kHz. In interstellar space, the plasma cutoff may be as low as l to l0 kHz. The spectrum of galactic emission turns over somewhere near 2 MHz but is still observed from space outside the plasmasphere down to frequencies of a few
64 hundred kHz. On vehicles passing out through the solar system, the local plasma density should decrease monotonically; observations of the low-frequency cutoff in galactic radio emission could in principle identify the plasma frequency as well as provide new information on galactic vlf emissions. The data could be simple total flux measurements on several radio frequencies observed with antennas with hemispheric directivity. Very-Low-Frequency Emissions from Jupiter Jupiter's magnetosphere very likely also contains another radiophysical phenomenon even more closely related to the plasma than the decametric radio emission. Below l00 kHz, waves in the earth's magnetosphere stand in an intimate connection with basic properties of the trapped-particle radiation, such as its energy density, loss mechanism, and radial and pitch angle distributions. The origin and loss of electrons and protons in the belts depend on these electro- magnetic phenomena. Clearly, a rational study of Jupiter's magnetosphere also requires instrumentation in situ to detect and establish the properties of vlf emissions. Since the propagation properties of these waves require close inter- relations between plasmas and fields, they are unlikely to escape from the magnetosphere. It is therefore important that they be studied from orbiters as well as flybys. Planetary The same radio links between the spacecraft and earth dis- cussed earlier in this chapter as a means of determining some of the plasma parameters of the corona and solar wind can also be used to provide accurate plasma measurements near the planets, for study of their magnetospheres and ionospheres and their interactions with the solar wind. Occultation would be desirable, although the upper reaches of magneto- spheres might be studied even without occultation. Measure- ment accuracies are such that electron number densities of a few ten's per cubic centimeter, or possibly a few per cubic centimeter, could be detected. Polarization measurements would add magnetic field information over regions not measured by on-board magnetometers. Spacecraft radio receivers could make useful measurements of noise from trapped particles in the magnetospheres over regions not covered by direct sampling of particles with on-board instruments. The receivers could
65 detect weaker radiation, to higher spatial resolution, than can be obtained by use of radio astronomy facilities on the earth. Ground-Based Radar The sensitivities of ground-based radars proposed for the future are sufficient to obtain echoes from the Galilean satellites of Jupiter and from Titan, the large satellite of Saturn. Interplanetary measurements to Jupiter and Saturn, and occultation measurements at Jupiter, could thus provide data of the types described above. These would not have the sensitivity and range of the measurements based on links to the spacecraft but could be done without the use of a spacecraft. RECOMMENDATIONS We recommend the following as the major scientific objectives in the study of particles and fields and radio physics of the outer solar system: Interplanetary l. Unmodulated (interstellar) values of the cosmic-ray flux and distribution as a function of rest mass for 1 < Z < 30 energy in the range 1-l0^ MeV should be obtained. This requires observations over a solar cycle. (This objective is of considerable importance in high-energy astrophysics as well as in the study of cosmic rays observed near the earth.) 2. The properties of the solar wind and interplanetary magnetic field at great heliocentric distances should be investigated, both at low and high (>80°) heliographic latitudes, and an attempt should be made to study the termi- nation of the solar wind. Planetary l. A detailed study of the external magnetic field and of the charged particle population in the magnetosphere of Jupiter should be undertaken. In particular, the nature of Jupiter's nonthermal radio emissions should be studied both from spacecraft and from the earth.
66 2. A determination should be made of the mode of solar- wind interaction with the major outer planets and the inter- actions of Jupiter's satellites with its magnetosphere. A search should be made for magnetic fields and radiation belts associated with Saturn, Uranus, Neptune, and Pluto, and if the findings are positive, detailed investigations should follow. We recommend that full use be made of the cruise mode of planetary missions to carry out interplanetary research. We recommend that first priority be given to a balanced program that combines planetary and interplanetary objectives and that smaller purely interplanetary missions to the outer solar system be used only if their scientific objectives cannot otherwise be met. To give a reasonable balance between the first explora- tion of new regions and extensive investigation of the most significant problems, we recommend the following order of importance of the missions (which is not the recommended chronological order): l. The l974 Jupiter flyby test mission with an orbit that brings it back over the sun at high heliographic lati- tude, with the inclusion of a deep atmospheric probe if at all possible 2. The l976 earth-Jupiter-Saturn-Pluto grand-tour mission, with the hope that at least one of the two vehicles could drop a beacon that would be occulted by Saturn 3. The l979 earth-Jupiter-Uranus-Neptune grand tour 4. The l978 Jupiter orbiter with a periapsis of about l.2R and an apoapsis of from 75 to l00 Rj 5. A Uranus atmospheric probe 6. Saturn and Neptune orbiters and probes 7. A mission to Halley's Comet Further Recommendation We recommend that careful attention be given to the cali- bration of cosmic-ray experiments in order that results obtained at different places and times may be compared with confidence.
