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Space Plasma Physics: The Study of Solar-System Plasmas (1978)

Chapter: Magnetospheric Plasma Waves

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Suggested Citation:"Magnetospheric Plasma Waves." National Research Council. 1978. Space Plasma Physics: The Study of Solar-System Plasmas. Washington, DC: The National Academies Press. doi: 10.17226/18481.
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MAGNETOSPHERIC PLASMA. WAVES Stanley D. Shawhan 1014

1015 ABSTRACT A brief history of plasma wave observations in the Earth's magnetosphere is recounted and a classification of the identified plasma wave phenomena is presented. The existence of plasma waves is discussed in terms of the characteristic frequencies of the plasma, the energetic particle populations and the proposed generation mechanisms. Examples are given for which plasma waves have provided information about the plasma parameters and particle characteristics once a reasonable theory has been developed. Observational evidence and arguments by analogy to the observed Earth plasma wave processes are used to identify plasma waves that may be significant in other plane- tary magnetospheres. The similarities between the observed charac- teristics of the terrestrial kilometric radiation and radio bursts from Jupiter, Saturn and possibly Uranus are stressed. Important scientific problems concerning plasma wave processes in the solar system and beyond are identified and discussed. Models for solar flares, flare star radio outbursts and pulsars include elements which are also common to the models for magnetospheric radio bursts. Finally, a list- ing of the research and development in terms of instruments, missions, laboratory experiments, theory and computer simulations needed to make meaningful progress on the outstanding scientific problems of plasma wave research is given.

1016 1. INTRODUCTION The term plasma wave is used to denote all waves which are genera- ted in a plasma or which have their wave characteristics significantly modified by the presence of the plasma. These waves may be electro- magnetic, electrostatic or magnetosonic and are generated by the con- version of plasma electrical and particle kinetic energy into wave energy through a variety of plasma-particle processes. In turn, these waves may interact with the particles and modify the particle popula- tions within the plasma. Plasma waves are generated in many natural plasma systems: planetary ionospheres and magnetospheres, the solar wind, the solar at- mosphere, interstellar space, stellar atmospheres, pulsars, flare stars, galaxies and quasars. An upper limit to the frequency range of plasma waves generated in these different systems is given approximately by the maximum frequency for which the plasma can respond to the presence of the wave fields -- the electron plasma frequency. Within the solar system this electron plasma frequency is 10 kHz for the solar wind, ~ 10 MHz for planetary ionospheres, and 10 GHz for the solar corona. In laboratory plasma devices it may range from 10 MHz to 10 GHz. Radio astronomy techniques are used to study plasma waves that have escaped solar and cosmic plasma systems from above the solar wind plasma fre- quency ~ 10 kHz to the millimeter wavelength range ~ 1000 GHz. Space

1017 plasma wave instruments measure plasma waves generated locally in the plasma below the electron plasma frequency. The emphasis of this paper is placed on the review and interpre- tation of plasma wave observations made in the Earth's magnetosphere. Observations of plasma waves from other planetary magnetospheres and cosmic plasma systems are discussed and interpreted in analogy to the Earth's processes. In situ space instruments ha-ve provided a survey of the wave phenomena, plasma parameters and energetic particle character- istics throughout most of the Earth's magnetosphere. With this informa- tion it has been possible to classify plasma wave phenomena as to fre- quency range, wave mode, and region of occurrence within the magneto- sphere for both trapped and escaping waves. Some of the associated plasma and particle information with which to develop detailed theories of wave generation and propagation and of wave-particle interactions is also available. Gaps in the wave and particle information are to be filled in part by the ISEE Missions and the proposed Pynamics Explorer Missions. Less extensive in situ measurements are becoming available for the solar wind and the other planetary magnetospheres. Presently, however, most deductions about plasma wave processes in solar system plasmas other than the Earth's magnetosphere and in cosmic plasma systems are based on observations of escaping plasma waves and characteristics of IR, optical, UV, x-ray and Y~ray emissions. Therefore, we take the approach that the interpretation of plasma wave phenomena in the Earth's magnetosphere together with existing experimental evidence, laboratory experiments, computer plasma simulations, and theoretical extensions can be used to evaluate plasma wave phenomena in other plasma systems. Also,

1018 the magnetosphre is itself a convenient plasma laboratory of cosmic scale in which to perform experiments, active and passive, related to plasma wave processes. In this paper a brief history of plasma wave observations in the Earth's magnetosphere is recounted and a classification of the identi- fied plasma wave phenomena is presented. The existence of plasma waves is discussed in terms of the characteristic frequencies of the plasma, the energetic particle populations, and the proposed generation mechan- isms. It is pointed out that plasma waves can provide information about the plasma parameters and particle characteristics once a reasonable theory has been developed for the plasma wave process. Observational evidence and arguments by analogy to Earth are used to identify plasma wave processes that may be significant in other planetary magnetospheres and cosmic plasma systems. Important scientific problems concerning plasma wave processes in the solar system and beyond are identified and discussed. Finally, a listing of the research and development in terms of instruments, missions, laboratory experiments, theory and computer simulations needed to make meaningful progress on the out- standing scientific problems is given.

1019 2. THE MENAGERIE OF PLASMA WAVES 2.1. Brief History of Observations Naturally occurring plasma waves associated with the Earth's ionosphere and magnetosphere have been observed and studied with ground- based instrumentation since the late l800's. ' These phenomena in- clude whistlers, hiss, chorus, discrete emissions and geomagnetic micro- pulsations (see Table 1). In 189U Preece reported hearing what may have been whistlers and chorus associated with an auroral display by using a telephone receiver connected to a telegraph line. Rapid variations in the Earth's magnetic field, as measured with a magnetometer, were re- ported by Terada in 1917* Interpretation of these early observations lead to the realization that the Earth had an extensive magnetic field, significant ionization to great distances and a population of energetic particles (at least after enhanced solar activity). In the years encompassing the International Geophysical Year (IGY), networks of magnetometers and of receivers in the VLF (3-30 kHz) fre- quency range were set up at various places in the world to make syste- matic measurements of these wave phenomena in terms of local time, latitude, magnetic conjugacy, solar activity and geomagnetic distur- bances. From this intensive effort the detailed morphologies of the observed micropulsations, whistlers and VLF emissions were established and theories for interpreting these plasma waves were developed. Once the frequency-time dispersion relationship for whistlers was understood,

1020 for example, it was possible to use whistlers to determine the electron density in the equatorial plane. From whistler measurements it was found that the radial electron density profile exhibited an abrupt order of magnitude decrease in the range of 3-6 earth radii. This region of density decrease is now identified as the plasmapause (PP in Figure l) and the high density region at lower radial distances as the plasmasphere (PS in Figure l). After the IGY it was well known that whistlers were generated by lightning discharges in the Earth's atmosphere and the dispersion charac- teristics were a result of propagation through the magnetospheric plasma approximately along magnetic field lines. The source mechanisms for micropulsations and the other VLF waves were not well understood but these waves were thought to be generated by energetic particles (elec- trons and ions) interacting with the plasma and plasma waves in the mag- netosphere. With the advent of scientific sounding rockets and of scien- tific satellites in 1957 it became possible to make measurements of the plasma waves (frequency spectrum, wave mode, wave vector), the plasma (density, ion species, temperature) and the energetic particles (energy spectrum, pitch angle distribution, ion species) within the magnetosphere itself. From these measurements it became possible to develop better theories of wave generation, propagation and interaction with particles. Vanguard III in 1959 carried a magnetometer with frequency re- sponse to 6 kHz with which the first satellite observations of whistlers and other VLF waves were made. 7 Since that time a variety of magneto- meters and plasma wave receivers have been carried to nearly all regions

1021 of the Earth's magnetosphere and into the solar wind between 0.3 and 1 83 astronomical units. " Most of these instruments have measured the spectral characteristics of the waves using a single axis sensor - search coil, magnetic loop or electric dipole antenna. From the spectral information and the change of orientation of the sensor with respect to the magnetic field, it has been possible to classify the observed waves as to frequency range and occurrence probability, and to deduce if the wave vector is nearly along the geomagnetic direction or not. On a few satellites (i.e., FR1, OGO 5, Injun 5, IMP 6) wave measurements have been obtained for more than one wave component (of the possible 3 electric and 3 magnetic) simultaneously. From these measure- ments it has been possible to specify the wave normal vector direction, the wave polarization or the component of the Poynting vector along the 93 geomagnetic field to distinguish between upgoing and downgoing waves. ~ These methods have been applied particularly to micropulsations, hiss, chorus, saucers, upstream whistler mode waves and whistlers. For the non-thermal continuum and kilometric radiation which are not constrained to propagate along the geomagnetic field direction, it has been possible to use the nulls in the antenna pattern rotating with the spacecraft (i.e., RAE 1, IMP 6, IMP 7, Hawkeye l) and the lunar occultation technique (RAE 2) to actually locate the source regions. 2.2 Locations and Characteristics Based on the extensive rocket and satellite measurements, dis- tinct plasma wave types have been identified in addition to those ob- served on the ground. Figure 1 gives a noon-midnight cross-sectional

1022 view of the magnetosphere with the magnetospheric regions identified. A detailed discussion of the magnetospheric configuration is given by Juan Roederer ("The Earth's Magnetosphere") in this report. Indicated in Figure 1 are the regions having the highest probability for observing the various types of plasma waves. In general, the noise types are similar in the north and south half of the magnetosphere (although not shown in Figure l), however, the types of noise that occur in the different magnetospheric regions are somewhat distinct. This distinction is understandable because a particular plasma wave type is generated by a particular particle population under particular plasma conditions and these populations and conditions vary between regions in the magnetosphere. More detailed information about each type of plasma wave identified in the Earth's magnetosphere is given in Tables 1, 2, 3- Each table lists the noise type, its location in the magnetosphere, the frequency range for which it is observed, the characteristic frequency of the plasma associated with the waves, key literature references, a brief description of the important wave characteristics and a statement of the proposed source mechanism. Table 1 includes the electromagnetic plasma waves — waves with a detectable magnetic field component. The geomagnetic micropulsations category includes a large class of phenomena -- both magnetohydrodynamic waves and transient disturbances — associated with variations in the geomagnetic field. Because the wave periods and wave- lengths are long, these phenomena are best observed from the Earth. However, the ATS geostationary series of spacecraft have yielded meaning- ful observations near the equatorial plane. All the other wave types, except the trapped continuum and UHR noise are classified as whistler-

1023 mode waves because the wave characteristics can be described by the whistler dispersion relation for a plasma with a magnetic field. These waves tend to propagate along the magnetic field lines although in certain frequency ranges the waves can propagate across the field to populate regions remote from the source. Many of these waves can be observed only within the magnetosphere itself. The electron whistler classification includes a variety of types such as nose whistlers, multiple hop ducted whistlers, ion-cutoff whistlers, subprotonospheric O-j whistlers, magnetically reflected whistlers and walking trace whistlers. These are distinguished by their frequency-time characteristic due to propagation differences. In Table 2 are listed the identified electrostatic plasma waves — waves with an electric field but without a detectable magnetic field U3 component. ~ These types of plasma waves are generally found in regions where collisions between the plasma and the neutral gas are important (Farley instabilities) or where there is a mixture of hot and cold plasma. These waves are entirely trapped within the plasma and generally are observed close to their region of generation because they generally have a low group velocity and are strongly damped. Two types of plasma waves have been observed to escape from the magnetosphere. These waves, listed in Table 3, are generated near the electron plasma fre- quency and are observed to be propagating in an electromagnetic mode (perhaps by coupling from an electrostatic mode) that can escape the plasma. Because of these waves, the Earth exists as a significant (l()9 UU watts) source of radio emission within the solar system

1024 Not all the waves observed in the magnetosphere are natural. Three types of waves given in Table U are man-made. The power system harmonic radiation is accidental but it apparently leads to wave growth and increased precipitation of trapped electrons within the magnetosphere. These induced effects may have some long term consequences as discussed by P. A. Sturrock in this report ("Impacts of Solar System Environment on Man and Man in the Environment"). The other waves are stimulated by a VLF transmitter and by a rocket-borne electron accelerator in order to study wave-particle interactions in the magnetosphere under somewhat controlled conditions.

1025 3- GENERATION, PROPAGATION AND INTERACTION OF PLASMA WAVES Interpretation of simultaneous in situ plasma wave, plasma, and particle measurements in terms of the particular plasma wave processes is complicated by a number of factors: i. The observed plasma waves may be a combination of waves locally produced and of waves that have propagated from remote source regions, ii. Waves that have propagated to the observation point may have wave characteristics different from those in the source region due to reflection, refraction, wave-wave interactions and wave-particle interactions, ill. Waves and particles in the source region may be interacting strongly at the saturation limit of the instability, so that the particle distribution function has been modified into a nearly stable state. It is therefore difficult to associate the final state with the wave mode and the parti- cle distribution that caused the instability during the linear growth phase, iv. The observed particle distributions may be a superposition of the final state distribution and the distribution of particles newly injected into the source region,

1026 v. In any series of observations with a single spacecraft it is usually difficult to separate the temporal and spatial variations of the measured parameters, and iv. A given satellite may not be suitably instrumented to measure all the desired parameters with sufficient dynamic range and time resolution. However, by modeling the plasma to solve for the possible wave modes and propagation characteristics, by collecting the data from a variety of instruments on various spacecraft and by comparing the measured features to theoretical predictions, computer simulations and laboratory experiments, it has been possible to identify certain wave modes, to classify the most likely wave generation mechanism, and to interpret the observed wave-particle interactions. 3.1 Characteristic Plasma Wave Frequencies A plasma having electrons, ions and neutrals of finite tempera- tures and permeated by a magnetic field can support a variety of elec- tromagnetic, electrostatic and magnetosonic wave modes that cannot exist in free-space. The range of frequencies for which these various modes are observed to exist can be understood in terms of the frequencies characteristic of the plasma. To a first approximation these frequencies can be obtained from solutions to the wave equation for an infinite, homogeneous plasma with cold electrons and ions and a static 12 102 magnetic field assuming plane waves of the form exp (i (k • x - art)). ' '

1027 A representation of this solution is given in Figure 2 in terms of the index of refraction of the plasma n = c|k|/uj where k is the wave vector and uu is the angular wave frequency (ou = 2nf) for the wave frequency f. With a magnetic field, the plasma is aniso- tropic so that the solution depends also on the angle between the wave vector k and the magnetic field which is called the wave normal angle. Those waves below the plasma frequency with the wave normal angle near 0°, have the wave energy primarily in the wave magnetic field and the waves are circularly polarized. Waves with wave normal angles near 90° tend to have their energy primarily in the electric field component and to be linearly polarized. The various resonance (n -» oo) and cutoff O (n -» 0) frequencies can be expressed in terms of the plasma wave fre- quencies related to the electron density (N), ion species (mass = M.) 12 37 102 and magnetic field strength (B): ' ' f~ , electron plasma frequency «*• 9 x 10^ (N, electrons/cm"-5) ' Hz f* , ion plasma frequency °* f~ (\/^} ' Hz f , electron gyrofrequency =* 2.8 X 10 (B, gauss) Hz O * , ion gyrofrequency °* f~ (M /M.) Hz o 6 6 1

1028 fTm, . low hybrid resonance frequency = 1 ) 1- 9' J e (f-) fTTt_ . upper hybrid resonance frequency UrlH Representative curves for the variations of some of these character- istic frequencies with radial distance in the Earth's magnetosphere are shown in Figure 3 for an auroral field line and for a cut along the equatorial plane. Within the magnetosphere these characteristic frequencies can vary over three orders of magnitude. This variation accounts for the wide frequency range observed for a given plasma wave phenomenon. The cold plasma dispersion relation has been successful in describing the propagation characteristics of the plasma wave types listed in Tables 1 and 3- These are also listed on Figure 2. However, the magnetospheric background plasma has a finite temperature which may be different for electrons and ions and may range from 500°K to 10'°K. When the thermal velocity becomes comparable to the wave phase velocity (c/n), the cold plasma description is no longer valid. Introducing thermal effects into the dispersion relation allows the possibility of electrostatic modes which occur near harmonics of the electron and ion gyrofrequencies and near the ion plasma frequency. Waves related to these modes are included in Table 2 and are listed in parentheses on Figure 2.

1029 3.2 Generation Mechanisms A number of excellent books and articles exist which review 33, 3U, U3, our present deductions concerning wave generation mechanisms. U?, 51. 88, 89, 107 Also, for each of the wave phenomena listed in Tables 1, 2 and 3 the proposed generation mechanism is given along with references to representative observational and theoretical papers. The energy source for ion cyclotron and electron whistlers is well identified with lightning discharges in the upper atmosphere. This energy is derived from solar heating of the atmosphere and the Earth's rotational motion which drives the wind systems to effect the differential charging between clouds and the Earth's surface. The momentary discharge current has frequency components extending to tens of megahertz in frequency. Under certain conditions some of this energy couples into the magnetosphere through the ionosphere and pro- pagates in plasma wave modes associated with the electrons and with the ions. These whistler waves may be amplified or damped through electron and ion cyclotron resonances with the energetic particle popu- lations. The energy for plasma waves generated internal to the magneto- sphere comes ultimately from the solar wind flow past the Earth and the Earth's rotation. This energy is dispersed throughout the magneto- sphere in terms of electric fields and particle kinetic energy and converted to wave energy through changes in the particle distribution functions. Particle distributions unstable to wave growth may exist at a given location in the magnetosphere due to processes such as plasma diffusion and convection across magnetic field lines, selective pitch

1030 angle scattering into the atmospheric loss cone, energy selective gradient drifts, energy selective acceleration, resistive energy dissipation or mixture of hot and cold ionosphere and solar wind 88 plasmas. Such processes are discussed more in detail in the papers by Roederer, Lyons, Haerendel and Sonnerup (this report). The resulting non-Maxwellian distributions may be continuously unstable or perturbed into an unstable state under geomagnetic storm conditions. For the purpose of discussion in this paper we divide the proposed plasma wave generation mechanisms into two general categories: gyroresonance and streaming. 3.2.1 Gyroresonance Instabilities Particle distributions that contain free energy in the component transverse to the magnetic field direction (anisotropic temperature distributions T > T,, or loss cone distributions, for example) can give up energy to waves (and vice-versa) under the condition f - kj, v,,/2TT + sf- = 0, s = +1, +2, In this expression k,, and Vn are the wave vector and component of particle velocity parallel to the magnetic field direction, f is the wave frequency and f the ion or electron gyrofrequency around the O field line, and s gives the harmonic number. Interacting particles undergo pitch angle diffusion which may cause them to be precipitated into the atmosphere (see Lyons) or energy diffusion which results in a harder spectrum for locally trapped particles.

1031 Waves associated with the ion cyclotron frequency (also called gyrof requenciy ) are thought to be created just inside the plasmapause boundary where energetic ions from the magnetotail are injected into the cold plasma of the plasmasphere. Even though these waves have not been definitely observed indirect identifications have been made, for o example, from ATS data. Strong theoretical arguments for their exis- 56 tence can also be made. . It may be that the higher frequency micro- pulsations (Pel and Pil) are also generated by this process just out- 2"3. 51 side the plasmapause boundary. ' Lion's roar seems to be explained by this ion cyclotron resonance due to 10 keV protons streaming through 98 the magnetosheath particularly during geomagnetic storm periods. Also ion cyclotron waves associated with streaming ions were observed in the polar cusp region with OGO 5« ELF hiss, chorus and discrete VLF emissions are electromagnetic wave phenomena which seem to result from the electron gyroresonance but under different conditions. ELF hiss is observed nearly continu- ously throughout the plasmasphere but is thought to be generated by the trapped electron population in the outer radiation zone just in- 106 side the plasmapause. Chorus, however, is generated in the region between the plasmapause and the magnetopause principally on the dayside of the Earth .during increased magnetic activity by electrons in the range of 5-150 keV and very near the equatorial plane on the night- 109 side. The sporadic occurrence of the chorus tones suggests that the necessary generation conditions are met sporadically in time or that the propagation paths are spatially distributed. The large

1032 amplitude and burst-like nature of chorus suggests a non-linear treat- ment of the emission process. Discrete emissions are rather rare and their generation seems to be similar to chorus but to require special conditions so that particle bunches moving along the field lines can interact with the waves for an extended period and then repeat the U7 interaction periodically. Both ELF hiss and chorus are associated with electron precipitation process (see Lyons). At the plasmapause boundary and beyond, electrostatic plasma waves with frequencies near 3/2 f~, 5/2 fg, etc. have been observed particularly during magnetic storm time. Also, similar electrostatic electron cyclotron harmonic waves have been occasionally observed near U5 the plasma sheet in the tail.- These waves can occur in regions for which the particle distribution function increases with the perpen- dicular velocity. This condition can be met by a loss-cone pitch angle distribution, by a beam or by large temperature anisotropy and a ratio of f^/f~ > 1. It may be that these waves control the precipi- * o tation of moderate energy (1-10 keV) electrons in the outer magneto- sphere. . Direction finding measurements on the nonthermal continuum radiation found throughout the outer magnetosphere suggests that the source region is near the plasmapause boundary on the morning side. In some cases it is closely associated with the electrostatic electron cyclotron noise suggesting that the continuum electromagnetic noise has coupled from this electrostatic noise. It has also been suggested, however, that this noise is due to gyrosynchrotron radiation from the high energy radiation belt electrons (in analogy to the Jovian decimetric radiation).

1033 71 A doppler shifted electron cyclotron emission mechanism has been proposed for the kilometric radiation observed to be emitted along auroral field lines. ' This emission is thought to result from the same energetic electrons which cause the auroral optical emissions. Both emission and amplification of the waves is required to explain the observed power levels representing 10$ of the particle t| IX. 71 energy flux. Waves emitted by this process would be righthand polarized. 3-2.2 Streaming Instabilities Under this category we include the types of instabilities which are classed as current driven, beam, two-stream and drift wave. Common to these instabilities is a distribution function for which at least one component of the plasma is streaming through the other thermal and energetic components. As an example, the distribu- tion may have a "bump on the tail" — this distribution function, compared to a Maxwellian, has an enhanced population at some velocity different from the average velocity. This sort of distribution is unstable, according to the Penrose criterion, if there is a minimum in the distribution function F(v) at a velocity v* such that f° Zjr dv > 0 . -00 (v-V*) If this distribution is unstable then energy is extracted from the enhanced population with v > v*. In general, plasma waves are created with phase velocities near v*, the streaming velocity:

1034 v = c/n % v* . P ' Note that for frequencies near resonances in the plasma (electron and ion cyclotron frequency, lower and upper hybrid fre- quency and plasma frequency) the index of refraction goes toward infinity giving a low phase velocity making possible the interaction with low energy particles. a) Current Driven. At the bow shock and magnetosheath, in the boundary layer of the tail plasma sheet and along auroral field lines, the few magnetometer and plasma particle probe measurements indicate the presence of field aligned currents and the presence of streaming ions (which may be carrying the current). The observed bow shock turbulence, tail broadband electrostatic noise and auroral field line turbulence have common features and may all plausibly be generated by an electrostatic ion cyclotron or an ion acoustic in- stability associated with the streaming ions. The electromagnetic bow shock plasma waves and the magnetic noise bursts observed on auroral field lines and near the neutral sheet in the tail are in- timately associated with the presence of the electrostatic noise. Either this electromagnetic noise is coupled from the electrostatic noise through a wave-wave interaction or this noise is generated directly by an electromagnetic instability associated with the field aligned 33, U3,eo, 88, 89 currents. Instabilities related to currents moving across the field lines are possible in the ionosphere in the presence of collisions. As an example, collisions in the E-region provide sufficient electron

1035 conductivity to support transverse electric fields and to cause the electrons to stream at an E/B drift velocity with respect to the neutrals (and ions). When this drift exceeds the ion-acoustic velo- 77 city, an instability occurs producing observable electrostatic waves, and corresponding density fluctuations which are seen as echos on backscatter radars (see chapter by Farley). b) Beam. Particularly in the auroral regions, the precipitating energetic electrons (^ 15 keV) often constitute a field-aligned electron beam. This beam can emit plasma waves by the incoherent Cerenkov and cyclotron processes and the emitted waves can be further amplified by a coherent beam process. Noise bands are 6k observed in the magnetosphere at the lower hybrid frequency frr™ and 7k the upper hybrid frequency fITtrr) UllK It seems that these noise phenomena can be understood as in- 7k 105 coherent Cerenkov radiation due to rather low energy electrons. Also, it may be that at least the LHR noise band is due in part to 21 trapped and dispersed electron whistler waves. VLF hiss and saucers (see Table l) are found in nearly the same region of the low altitude (~ 2000 km) auroral zone as illustrated with data from Injun 5 in Figure k. The VLF hiss is associated with an intense flux of precipitating auroral electrons with energies in excess of 10 keV and also with an ELF noise band which is similar to oQ lion's roar. The saucers, however, have no apparent association with 52 particles above 5 eV. With Injun 5 the VLF hiss is found to be down- ward propagating waves whereas the saucers are upward. It has been shown

1036 that the VLF hiss spectral characteristics and power levels can be explained by amplified Cerenkov radiation due to this auroral electron beam at or below the region where the electrons are accelerated (3000 - 69 20,000 km). ' Saucers can be explained by assuming a source of supra- thermal electrons of < 5 eV beamed upward. This beam produces amplified 52 Cerenkov radiation at altitudes above 1000 km. The kilometric radiation, emitted from auroral field lines , may also result from the auroral electron beam and current system. A beam driven electromagnetic instability which operates in the presence of ion wave turbulence has been proposed. The instability saturation levels are consistent with the large observed power levels. Waves 79 generated by this mechanism-would be lefthand polarized. c) Two Stream. At the bow shock, solar wind ions and electrons are reflected to produce a counter-streaming (two-stream) distribution. These reflected ions may be the source of the whistler mode noise observed just below the ion gyrofrequency upstream of the bow shock in the solar wind. The noise band just below the electron gyrofre- quency is identified with the reflected electrons as is the electro- static oscillations at the electron plasma frequency. The dynamics of the bow shock is discussed in detail by Greenstadt and Fredricks in this report. d) Drift Wave. Plasma can be given directed energy in regions where strong temperature or density gradients exist. Ion cyclotron waves are reported in the polar cusp region during a magnetic storm. They may be caused by a current driven ion cyclotron instability or by a density 33 gradient drift instability. Fredricks suggests that the theoretical

1037 work on electrostatic drift waves beyond the plasmapause leads to a source mechanism for Pc5 micropulsations. The Kelvin-Helmholtz in- stability, resulting from a shear in the ion velocity parallel to the magnetic field lines at the cusp boundarfe s, may generate micropulsa- 2U tions in the .07 to 30 Hz range. Farley (this report) discusses the role of gradient drift instabilities in the ionosphere. 3.3 Wave Propagation Characteristics Plasma waves observed in the magnetosphere may have propagated away from the source region. The path of propagation depends on the detailed plasma and magnetic field distributions, on the initial wave normal direction k, on the wave mode and on the wave frequency. To illustrate the characteristics of plasma wave propagation, ray paths (direction of energy flow) for components of a magnetospherically reflected electron whistler are shown in Figure 5& and for continuum radiation trapped between the plasmapause and the magnetosheath in Figure 5b. From these figures it is seen that waves are not necessarily constrained to follow the magnetic field lines as energetic particles do. (However, whistler-mode waves may be trapped in field-aligned iora.zation ducts.) Therefore, waves generated by one particle population in one source region can propagate to an entirely different particle population region where they can interact to modify that population. To obtain the wave characteristics in the generation and wave- particle interaction regions it is desirable to measure the wave characteristics at the point of observation and to trace the rays

1038 forward and backward. The wave characteristics for simple waves have been determined from the amplitudes and phases of the various wave 75 93 components. ' "—' These characteristics include the wave normal vector k and Poynting vector S. However, in most cases the wave field is a mixture of many different waves so that it is necessary to obtain a wave distribution function F(k, S, u)) to fully describe the waves in analogy to the particle distribution functions. So far the problem 103 has only been formulated but not developed to a useful stage. 3iU Consequences of Wave-Particle Interactions Detailed considerations of the magnetospheric plasma wave inter- actions with the particle populations is given in the papers by Lyons, Greenstadt and Fredricks, Haerendel and Sonnerup in this report and in 33, 3U, U7, 88, 89, 107 89 a number of review articles. Scarf and Russell have developed a list of magnetospheric plasma processes which are known to involve or which probably involve plasma waves. This list is pre- sented here to illustrate the variety and significance of wave-particle interactions: 1. Mechanisms for ring-current decay and for precipitation of electrons and ions to form the diffuse aurora, 2. Energy transfer and heating at the collisionless bow shock and at the field-aligned current regions within the magneto- sphere, 3. Mechanisms that provide microscopic coherence and lead to the enormous electromagnetic radiation levels from regions above auroral arcs,

1039 k. Dissipation mechanism in field-line merging regions, 5. Source of anomalous resistivity or instabilities leading to potential double layers which support field-aligned electric fields responsible for the auroral particle acceleration process to form bright auroral arcs, 6. Cause of viscous interactions that support the formation of the geomagnetic tail and the overall plasma convection pattern, 7. Formation of SAR arcs, 8. Cause of electron loss to form the energetic electron slot between the trapped radiation zones, and 9. Scattering of particles from open cusp field lines to closed magnetic field lines. An example of one wave generation and wave-particle interaction system is illustrated in Figure 6. During geomagnetic storm times a hot ion ring current plasma is injected from the tail into the cold plasma- spheric plasma. Ion cyclotron waves may be generated causing some of the ions to be precipitated into the atmosphere. The ion waves are damped as they propagate downward giving rise to a downward heat flux which may produce a stable auroral red (SAR) arc in the lower ionosphere. In order to further test the collection of possibilities for wave generation and wave interaction processes, controlled experiments within the magnetosphere are being carried out. A few details about U8 U9 the Siple VLF transmitter ' and the Echo rocket electron gun and argon ion source ' experiments are given in Table h. The scheme for carrying out the Siple transmitter experiment is illustrated in

1040 Figure 7 including the use of satellites such as the Dynamics Explorer pair. Modulated waves in the range of 2 to 16 kHz are transmitted from Siple, Antarctica. These waves enter the ionosphere and propagate along flux tubes between L = 3 and L = 5- If an energetic electron population exists in the equatorial region, the waves may be amplified or may organize the electrons to emit a new wave at a slightly different frequency which changes with time. Wave growth can exceed 30 dB at the rate of 100 dB sec . The interacting electrons may be scattered into the loss cone and precipitate into the atmosphere. Satellites along the field line can be used to make careful measurements of the wave characteristics and of the particle distribution functions to provide details of the interaction.

1041 U. DIAGNOSTICS WITH PLASMA WAVES Observations of naturally occurring plasma waves resulting from processes that are reasonably well understood have been used to obtain information on local plasma parameters and on remotely occurring plasma processes which is complementary to that obtained from other instrumentation. Measurements of ion and electron whistlers and of LHR and UHR noise have provided estimates of plasma density, ion species and temperature. Micropulsations are used as an indicator for changes in the structure of the plasmapause and magnetopause. The source regions for saucers, VLF hiss and kilometric radiation are thought to be associated with the regions of auroral particle acceleration and the presence of kilometric radiation is a good index of bright auroral arc activity. In addition active wave experiments such as performed with the topside sounders, Siple and Echo, have been used to determine plasma parameters and to study wave-particle interactions. More sophisticated active wave experiments are planned. l4..! Plasma Parameters Compared to techniques using instruments such as Langmuir probes and mass spectrometers, determination of plasma parameters using wave techniques are less affected by satellite potential and sheath effects since the wave characteristics are established over a volume on the order of wavelength cubed which exceeds 1 km . However, one must wait

1042 for the wave phenomenon to occur or must stimulate and receive the appropriate waves. 4.1.1 Ion and Electron Whistlers Ion cyclotron whistlers are observed to occur in the midlatitude ionosphere. From the frequency of coupling between the electron and ion whistler trace and the observed ion gyrofrequency it is possible to obtain the fractional concentration of the ions. From the shape of the ion whistler trace it is possible to estimate the ion number density 3 Q2 and the ion temperature. The time of occurrence with respect to the causative lightning discharge and the frequency of the "nose" for the electron nose whistlers observed on the ground at mid- to high lati- tudes are used to drive the electron density distribution in the equa- torial plane. This technique is useful for locating the plasmapause as a function of local time nearly continuously. An example of the plasmapause location for three days in July 1963 is shown in Figure 8. 4.1.2 Magnetohydrodynamic Waves Recently it has been demonstrated that measurement of the period and identification of the field line for geomagnetic micro- pulsations (at frequencies below the ion cyclotron frequency, see Table l) can also give an estimate of the equatorial electron density near the plasmapause. It is found that an external driving force (possibly the solar wind) can cause the field lines to resonate and that this resonance frequency is dependent on the electron density distribution along the field line. The deduced density values are in good agreement with those 112 from simultaneous nose whistler measurements.

1043 U.1.3 LHR and UHR Noise Bands The lower hybrid resonance frequency depends on the effective ion mass, f~ and f~. LHR noise bands are found at altitudes ~ 1000 km P g in the mid- to high latitudes. With measurements or estimates for f~ and f~, the mean ion mass, which depends on the fraction ion con- fr o centration, has been derived over a wide latitude range. ' The kp non-thermal continuum noise may be generated at the UHR frequency . The observed low frequency cut-off of this noise is identified as the local plasma frequency f~. From measurements of this cutoff with IMP 6

1044 it has been possible to obtain the total electron density in the hot plasma region outside the plasmapause. In fact the comparison of total density from the plasma wave measurements and the suprathermal ion density (88 to 38000 eV) shown in Figure 9 indicate that the plasma is almost entirely suprathermal. U.l.U Wave Sounders By driving an antenna with a sweep-frequency signal and measuring the amplitude and time delay of the received waves it is possible to excite resonances in the local plasma and to observe waves reflected from distant density gradients. From this information, the local density and temperature as well as the density profile toward increasing density can be determined. Such topside sounder experiments have been performed with Alouette and ISIS, for example. Similar experiments OS" are planned for the ISEE Mission and for AMPS Shuttle paylbads. k.2 Plasma Processes Observations of propagating plasma waves can provide information on plasma processes remote to the point of observation: U.2.1 Micropulsations The period, amplitude and polarization of geomagnetic micropulsa- tions have been used to deduce information about the interaction of the solar wind with the magnetosphere, the dimensions of the magnetosphere, and about streaming particles within the magnetosphere. Micropulsations in the Pel range (.02-5 sec. periods) are probably caused by cyclotron instabilities outside of the plasmapause. Micropulsations i-n the Pc2 to Pc5 range (5-600 sec. periods) depend both on particle instabilities

1045 108 and field line resonance conditions in the vicinity of the plasmpause. For example, the location of the plasmapause boundary can be determined from a chain of micropulsation stations by measuring the polarization and period of Pc3 and Pck pulsations. The period increases with the increased plasmasphere extent because the length of the source field 66, 78 line is increased giving a longer resonant period for these waves. k.2.2 Electron Whistlers By observing changes in the characteristics of nose whistlers as observed in Antartica and Canada on the time scale of ~ 15 minutes, the cross-L plasma motions near the plasmapause have been inferred. Plasma velocities of ~ 100 m/sec corresponding to electric fields of 17 ~ 0.1 mV/m are obtained. ' U.2.3 Saucers and VLF Hiss Both of these phenomena have a V-shaped frequency-time charac- teristic as seen in Figure k and illustrated in Figure lOa. Ray paths between the source and the satellite that explain this shape are shown in Figure lOb. By modeling the auroral region and calculating ray paths similar to those in Figure lOb it has been possible to determine the approximate saucer source altitude and source region dimensions (10 km vertically by 0.5 km horizontally). The altitude distribution 52 is seen to be above 1000 km and up to the satellite altitude. A similar analysis of many VLF hiss events has not been carried out but one case from Injun 5 indicated a source region near UOOO km oQ altitude. Since these waves are thought to be due to locally accel- erated electrons, it is tempting to associate the acceleration region

1046 with the noise source region. Saucers and VLF hiss are indicators of the acceleration process for which the acceleration region must be below or at the saucer source and above or at the hiss source altitude. U.2.U Kilometric Radiation The presence of auroral kilometric radiation on the nightside of the earth has been shown to have a high correlation with bright auroral arc activity (but not with diffuse aurora) as exhibited in Figure 11. Also, the apparent source position of the kilometric radiation as ob- served by lunar occultations with RAE2 is illustrated in Figure 12. These source locations trace out an auroral field line region with an altitude range of 1 to 8 earth radii. Since the source mechanism for 71 79 kilometric radiation may also involve the auroral accelerated electrons, ' the presence of kilometric radiation may be used as an indicator of the acceleration region. U.2.5 Wave-Particle Experiments The Siple VLF transmitter and Echo particle gun experiments (see Table U and Section 3«*0 are representative of active experiments which generate waves that carry information about the wave-particle plasma process. For example, the amplitude, growth rate and frequency changes of the waves stimulated by the Siple transmitter contain in- formation about the energetic electron distribution function and Il8 bunching along the field line.

1047 U.3 Plasma Waves Associated with Auroral Particle Acceleration As an example of plasma waves associated with significant plasma processes, we consider the auroral region. Based on the source region determinations for VLF hiss, saucers and kilometric radiations as well as their observed associations with field aligned currents, field aligned fluxes of keV electrons, and bright auroral arcs in the night- side auroral region, these phenomena may all be interpreted as part of the auroral plasma system as depicted in Figure 13. One interpretation is as follows: An emf drives a current along these auroral field lines which closes through the ionosphere. On the field lines for which electrons are driven downward, an electric field parallel to o the field line may develop due to one or more processes -- anomalous resistivity, potential double layers, hot/cold plasma mixture or simply due to magnetic mirror forces on the electrons. These parallel elec- tric fields can accelerate a fraction of the low energy electron population to keV energies which are observed below 3000 km and which cause the auroral arcs (see Haerendel, this report). Waves generated as part of the acceleration mechanism or by the electron beam escape upward as kilometric radiation and downward as VLF hiss. The return current (~ 10 amps) is carried by large numbers of suprathermal elec- trons (^ 5 eV) on adjacent field lines and this low energy beam generates the upward propagating saucer waves. Further verification of this model may be carried out with the Dynamics Explorer spacecraft -- one at low altitudes at the foot of the field lines and the other moving approximately along a field line through the acceleration/generation regions.

1048 The most significant plasma wave emission from this auroral process is the kilometric radiation. This radiation covers a frequency range of 20 kHz to 2 MHz which escapes the magnetosphere with a maximum at ~ 200 kHz. This frequency of maximum emission is approximately the electron gyrofrequency at the observed source region (see Figure 3a) The radiation is beamed into a solid angle about the field line of ~ 3 steradians with a total power of ~ 10^ watts which is ~ 10$ of the electron beam energy flux and ~ 1% of the energy flux supplied by the solar wind. The noise is emitted as a superposition of short bursts in noise storms lasting up to several hours (see Figure ll). As will be discussed in Section 5«1»2 many characteristics of the kilometric radiation are similar to those of the Jovian decametric radiation.

1049 5. PLASMA WAVES IN OTHER MAGNETOSPHERES AND COSMIC SYSTEMS Plasma wave instruments have been carried beyond the Earth's magnetosphere into the solar wind in the vicinity of the Earth by many spacecraft and to a distance of 0.3 AU by Helios 1 and 2. However, no plasma wave instruments have been included on either US or Soviet fly-by and lander missions to the planets (Mercury, Venus, Mars and Jupiter). Plasma wave instruments are included in the forthcoming Pioneer Venus and MJS (Jupiter, Saturn and possibly Uranus) Missions and are suggested for the Jupiter Orbiter Mission. At present the nature of plasma waves trapped in the magnetospheres of other planets as well as those is cosmic plasma systems must be inferred from the escaping waves observed by radio astronomy techniques and from plasma parameters that can be deduced from IR, optical, UV, x-ray and y-r&y emissions. For the Jovian magnetosphere, we do have plasma energetic particle and magnetometer measurements performed with Pioneer 10 and 11. Using the detailed interpretations of plasma wave processes in the Earth's magnetosphere along with the existing observational data, and results of laboratory experiments, computer simulations and theoretical developments it is possible to make predictions about plasma wave phenomena in other planetary magnetospheres and cosmic plasma systems.

1050 5.1 Jupiter Jupiter is an intense radio emitter in two frequency ranges: the decimetric range (100 MHz - 10,000 MHz) due to synchrotron radiation from the trapped relativistic electrons and the decametric/hectometric range (500 kHz - UO MHz) due in part to the motion of the moon lo through the Jovian magnetosphere probably stimulating emission similar to the terrestrial kilometric radiation. The existence of trapped plasma waves is inferred from those at the Earth and from the observed energetic particle characteristics. 5.1.1 Decimetric Radiation A radiointerferometer map of the Jovian radiation at 21 cm wavelength is shown in Figure lU. This emission pattern is consistent with this emission being due to synchrotron radiation by relativistic electrons trapped in a dipole magnetic field. Northrop and Birmingham used the electron fluxes and magnetic field directly observed by Pioneer 10 for the first time to calculate the expected synchrotron flux. This result agrees with the observed flux to within a factor of 62 two. Klein , however, demonstrates that the synchrotron flux is not constant but varies by ~ 30$) over a fifteen year period and the flux variations at two wavelengths are different, as seen in Figure 15. The variations also do not follow the 10.7 cm solar flux varia- tions (indicator of solar activity) nor the square of Jupiter's distance from the sun suggesting that the relativistic electron flux is not controlled by the solar wind directly but probably is determined by

1051 plasma processes within the magnetosphere itself. Some of these pro- cesses may involve plasma waves * (see Section 5«1«3)« 5.1.2 Decametric Radiation Ground-based observations of the Jovian decametric (5-U0 MHz) radio bursts cover a period of 20 years. Recently satellite-borne receivers have extended the frequency range down to hectometric wave- lengths (0.5-5 MHz). The many observational characteristics of this radio noise phenomenon have been discussed by a number of authors. ' ' 19 HI '* Only a summary of the most prominent features is given here with some interpretation. Bursts of radio noise from Jupiter are observed over the fre- quency range of 0.5 to kO MHz with a peak at ~ 9 MHz, as shown in Figure 16, although each burst may have an instantaneous bandwidth as narrow as 100 kHz. Bursts at different frequencies are not generally correlated. These bursts have durations in the tens of seconds range (L-bursts) and in the millisecond to tenths of seconds range (S-bursts) although burst structure on the microsecond scale has been observed. The L-burst duration is thought to be due to interplanetary scintillations (due to solar wind density irregularities) whereas the S-burst structure may represent the coherence time for the plasma instability or the time for a beam from the source to cross the Earth. Probably the most striking feature is that the probability of observing an intense burst (with a power of ^ 10 watts) depends jointly on the longitude of Jupiter facing the Earth (Central Meridian Longitude, ^TTT) and the position of the moon lo in its orbit around Jupiter with respect to the Earth

1052 (cpT f where 180° places lo between the Earth and Jupiter). Figure 17 is a plot of this probability which indicates the pronounced lo- associated emission for the regions cp ~ 90% X ~ 110° (Source B); <p ~ 2UO% X ~ 2UO° (Source A); and <p ~ 2U0% \ ~ 330° (source C). Also a significant, but less probable, lo-independent source at X ~ 250° is apparent. It has been shown that the gross frequency-time character of a noise storm (lasting several hours) is surprisingly reproduceable for a given set of $ and X over time periods of ~ 12 years. This reproduceability, the frequency range of emission and the pronounced circular polarization of the bursts (up to 80$- righthand polarized above 20 MHz, changing to lefthand below) is used to argue that the observed frequency is related to the electron gyrofrequency in the the source region. Magnetic field models based on the Pioneer 10 and 11 measurements of the Jovian magnetic field are consistent with these 97 deductions : the surface field in the northern hemisphere where the field line through lo intersects the ionosphere is ~ lU gauss giving a gyrofrequency of ~ U0 MHz and the polarity of the magnetic dipole (opposite to that of the Earth) can explain the polarization sense. Radiointerferometer measurements of the apparent source size indicate an extremely small source region: ~ UOO km if spatially incoherent and ~ U000 km if spatially coherent (like an antenna) compared to the Jovian radius of 70,000 km. Whatever the emission mechanism, it must be a collective process. Most effort has been expended in interpreting these observational characteristics for the most intense lo-related bursts. It is generally

1053 accepted that the lo-related emission is generated by lo's motion through the Jovian magnetosphere at a frequency related to the electron gyrofrequency (perhaps a harmonic). This radiation is generated as bursts in small source regions along the field lines connecting lo to Jupiter and is beamed by the source or by the allowed propagation paths (which may include wave-wave coupling) so that in :only certain lo-Jupiter-Earth geometries are the bursts observable at earth. Smith-'" has recently reviewed the many models suggested for the Jovian deca- metric emission in light of the Pioneer 10 and ll measurements and the possible analogy to the terrestrial kilometric radiation. These models for the coupling of energy from lo's orbital motion to plasma waves include mechanisms by which lo generates large amplitude Alfve*n waves or whistlers, accelerates particles along the lo flux tube or sweeps up the existing energetic trapped particles. Smith concludes that at present no comprehensive theory exists but that acceleration of particles by plasma sheaths near lo"*1 appears to be a promising coupling mechanism and that most emission properties could be explained by an 71 79 electron cyclotron instability or parametric instability due to the highly anisotropic distribution of these streaming electrons. R? Also Scarf has suggested that the emissions could be analogous to the 3/2 f emissions seen in the Earth's magnetosphere. Many of the o fine scale features may be attributable to the propagation paths deter- mined by the structure of the multipole magnetic field and the plasma distribution in the magnetosphere. A pictorial representation of the basic lo sheath-acceleration 9U model is shown in Figure lo. Because of rapid rotation of Jupiter's

1054 magnetosphere (~ 10 hour period past lo) and the proximity of lo to Jupiter (6 Rj) it may be that a motional emf of up to 570 kV is developed across lo's ionosphere. (An ionosphere on lo was detected with the Pioneer 10 radio occultation experiment). This emf is thought to drive a current down the field lines connecting lo to the Jovian ionosphere and back up to lo for closure in the lo ionosphere. Plasma sheaths may form at or near lo (perhaps similar to the double Q layers described by Block and Falthammar ) to separate the plasma moving with lo from that moving with the Jovian magnetic field. A significant fraction of the motional emf may be dropped across these sheaths leading to the acceleration of electrons and ions to ~ 1DO keV energies. The streaming electrons carry up to 10 watts of power Q compared to 10 watts for a large decametric burst. This model seems plausible based on the Pioneer 10 and 11 observations of order of magni- tude increases in the flux of 100-500 keV electrons only on the field lines associated with lo. On comparing the models depicted in Figure 18 and Figure 13 and the observed characteristics of the Jovian decametric and terrestrial kilometric emissions, the similarities are striking. These similarities are the basis for suggesting that the mechanisms may be the same and that perhaps this process is common in plasma systems. ' 5.1.3 Trapped Plasma Waves No direct measurements of trapped plasma waves have yet been made in the Jovian magnetosphere. However, the measurements of Pioneer 10 and ll revealed sources of free energy in the plasma and energetic

1055 particles which must lead to the generation of a variety of plasma waves similar to those at the Earth. The frequency range for these waves would be scaled by the relative plasma densities and magnetic field magnitude. Some examples of likely types of plasma waves are mentioned here. Greenstadt and Fredricks (in this report) have discussed the bow shock structure expected for Jupiter and the associated bow shock waves. If a current system of high energy electrons is driven by lo as suggested in Figure 18 then it is reasonable to expect whistler mode noise similar to VLF hiss and saucers from instabilities or ampli- 90 fication along the field lines in analogy to Figure 13- Sentman et al. report significant pitch angle anisotropies of the relativisitic elec- trons (> 20 MeV) which are apprently recirculating through the Jovian magnetosphere. They have assumed that whistler mode turbulence main- tains this anisotropy in the range of 3 to 5 RT anc* deduced the required u cold plasma density in good agreement with the values derived from 110 5 Pioneer 10 plasma measurements. Barbosa and Coroniti considered this same instability and arrived at upper limits to the stably trapped flux in agreement with observed values. Also they estimated the lower limit amplitude for this whistler mode noise (probably like ELF hiss) -1/2 of ~ 0.5 mv L ' where L ~ U RT. Another estimate for this noise u amplitude based on particle data from Pioneers 10 and 11 gives ~ 2 my for a typical amplitude [Sentman and Goertz, private communication, 1977]• Maximum values at the Earth reach 30 irry. Outside of lo's orbit where the cold plasma density decreases below 10 cm~^ it is very likely that electrostatic waves are generated at the upper hybrid resonance

1056 frequency and near odd-half harmonics of the electron gyrofrequency (3/2 f~, 5/2 f~, ...) according to Ashour-Abdalla and Kennel. At the o g Earth, the corresponding waves may cause particle precipitation leading to diffuse aurora. Also, the non-thermal continuum radiation may re- sult from coupling of this electrostatic noise to an electromagnetic mode. The escaping component of such a process is a likely explanation for the non-Io-associated decametric radiation. Results from the plasma wave instrument on MJS are anxiously awaited to test these ideas and to identify other significant plasma wave processes in the Jovian mag- netosphere. 5.2 Other FLanets Direct spacecraft measurements indicate that Venus and Mars do not have a magnetic dipole moment large enough to produce a magneto- sphere. Siscoe (in this report) considers the nature of the inter- action of the solar wind with these bodies. Although plasma waves may be generated as part of the interaction processes, measurements have not yet been made and the problem has been given little attention theoret- ically. Arguments that Saturn, Uranus and Neptune may have a magnetic moment significant enough to produce an extensive magnetosphere are reviewed by Siscoe (in this report) and by Van Allen. These argu- ments are based primarily on similarity of the planets to Jupiter in terms of rapid rotation and size and on the detection of hectometric bursts from Saturn and possibly Uranus. So far synchrotron emission has not been detected from these outer planets. Both magnetic field and energetic particle measurements at Saturn are forthcoming from Pioneer 11

1057 on 1 September 1979 and MJS and possibly at Uranus with MJS. At present descriptions of plasma wave processes at these major planets are entirely based on analogies to the Earth assuming an extensive magnetosphere with trapped plasma and energetic particle populations. 5.2.1 Synchrotron Radiation In the case of Jupiter, detection and mapping of the decimetric synchrotron radiation provided definitive evidence for an extensive magnetosphere with a trapped energetic electron (> 10 MeV) population. Detections of synchrotron radiation from Saturn, Uranus or Neptune have not been reported. These measurements are difficult because the expected lower energy electrons and weaker magnetic fields imply that the emission frequencies would fall in the 10 to 10^ MHz range where the galactic background noise level is high and the radiotelescope resolution is poor. Models for synchrotron emission from Saturn have 68 58 been studied by Luthey and for Uranus and Neptune by Kavanagh . An example of the situation for Uranus is shown in Figure 19« Measure- ments above 1000 MHz trace out the l80°K black-body atmospheric emission curve whereas the synchrotron emission is expected to be significant below 1000 MHz. The 3U MHz presently achievable upper limit point is almost an order of magnitude above the model synchrotron flux levels. Both Saturn and Uranus are uniquely different from Jupiter in character- istics which may affect the emission of detectable synchrotron radiation: Saturn has a system of particulate rings which may preclude the possi- bility of intense radiation belts close to the planet and Uranus may have a magnetic axis nearly aligned with its rotational axis which makes

1058 an angle of 98° with the ecliptic plane normal. Since the spin axis of Uranus is currently pointing at ~ U0° to the direction of the sun and the synchrotron emission would be beamed in the equatorial plane (perpendicular to the magnetic axis), if emitted, it may not be beamed toward the Earth. Improved radio-astronomical techniques or measure- ments closer to the planets themselves are required to possibly detect this radiation. 5.2.2 Hectometric Radiation Using the spinning antenna on IMP 6, sporadic bursts of hecto- 57 11 metric radiation attributable to Saturn and possibly to Uranus have been reported. Typical spectra for these bursts are shown in Figure 16 along with those of the terrestrial kilometric and the Jovian•decametric emissions. By assuming that the Saturn and Uranus peak emission fre- quencies (l.l and 0.5 MHz respectively) scale as the relative magnetic field strengths (as they do for the Earth and Jupiter) an estimate of the planetary surface magnetic field can be obtained. These values are compared in Table 5A to values predicted from a magnetic Bode's law ' with reasonable agreement. Kennel and Maggs have also used the observed power flux at Earth to compute the total power emitted assuming that it is beamed into a hemisphere (2ir steradians). This power is then compared to an estimate of the power intercepted from the solar wind by the planetary magnetosphere and in Table 5B the ratio is calculated. Included, also, are the recent data for Uranus. Calculated in this manner, the power ratios (conversion efficiency) increase from 0.1$ at the Earth to 25% at Uranus! Following this trend, Neptune would

1059 be predicted to emit nearly 100% of the power received from the solar -20 -2 -1 wind which would produce a power flux of 10 Wm Hz at the Earth. However, the radiation may not be beamed into 2rr steradians as assumed by Kennel and Maggs. For the Earth the emission solid angle is ~ 2 P steradians and for Jupiter it is ~ 10 steradians leading to lower estimates of the emitted power as indicated in Table 5B. Using the Jovian efficiency of .02%, Neptune would produce a power flux of 2 X -2k -2 -1 10 Wm Hz at Earth. Of course, it may be that the power source comes from the rotational energy loss of the planets and not from the solar wind. Detection of these hectometric bursts is significant in indicating the presence of magnetic fields nearly in agreement with the predicted values scaled from Jupiter and the existence of significant free energy from the plasma or particle populations. Because of the nature of the bursts it is tempting to conclude that the emission process is the same as for the Earth and Jupiter but the mechanism for coupling energy into this process is unresolved — is it Earth-like coming from the solar wind flow past the magnetosphere or Jupiter-like coming from the planetary rotational energy via a close-in moon? Both Saturn and Uranus do have moons of appreciable size relatively close-in. Hopefully, measurements with MJS will resolve this question. 5.3 Cosmic Systems Radio emissions from the solar chromosphere and corona and from cosmic plasma systems beyond the solar system are the result of plasma wave processes with the characteristic plasma frequencies scaled up

1060 for the plasma number density and the magnetic field magnitude. Inter- pretation of these processes must depend on the properties of processes studied in space and the laboratory with computer simulations and theories taking into account the different parameter regimes and rela- tivistic effects. Three plasma processes, for example, have escaping plasma waves with characteristics that are somewhat similar to the es- caping waves from the planetary magnetospheres. These wave phenomena are solar radio bursts, flare star radio outbursts and pulsar radio emissions. 5«3«1 Solar Radio Bursts The energy for most solar radio emissions that are non-thermal (not due to thermal Bremsstrahlung) is thought to come from energetic electrons associated with solar flares. The most intense bursts and noise storms occur after some large optical flares in which electrons are found to be accelerated to > 10 MeV energies. One suggested accel- lA eration mechanism by Carlqvist is that of a double layer which is formed on interruption of the current flowing along the magnetic field lines linking sunspot regions on the solar surface. Stored magnetic energy is released across this potential region which accelerates particles to high energies causing electromagnetic emissions from x-ray to radio wavelengths, and particle as well as shock wave injecticns into the solar corona. In this model the double layer is thought to be similar to that possibly existing in the terrestrial auroral regions and in the vicinity of lo. Radio noise is produced by a variety of processes ranging from gyrosynchrotron emission from the highest energy

1061 electrons in the chromosphere for the microwave bursts to plasma oscil- lations driven by the outward streaming electrons in the corona for 6^ type III bursts. 5«3«2 Flare Star Radio Outbursts A class of relatively close stars (dMe stars, dwarf, M-type with emission spectra) have been observed to include members which exhibit increases in optical brightness, especially in the UV, lasting for minutes. A few of these optical flare stars also have detectable radio outbursts which are associated with the optical flare within +5 minutes. An example from Wolf k2k is shown in Figure 20. These outbursts last from tens of seconds to tens of minutes and are highly polarized (60$) with both circular and linear components. In morphology, these out- bursts are similar to solar microwave and type III bursts but the total energy released in an outburst is over two orders of magnitude greater than for a solar radio burst from a star that is about half the solar diameter. Some features of these flare stars may help explain the radio outburst characteristics: they are rapid rotators (several day period), they have large surface magnetic fields of ~ 20,000 gauss and many are part of binary star systems. A coherent synchrotron emission mechanism is likely. The short duration of the emission is suggestive of a rapid electron acceleration process similar to that for solar flares (with 10^ more energy released) and/or to that for planetary particle acceleration associated with the radio bursts. A double layer acceleration process could result from a current driven in the stellar magnetosphere by the rapid rotation or by the rotation with respect to the companion star.

1062 5*3.3 Pulsar Emission Pulsars are generally thought to be rapidly rotating (periods •03 to 10 seconds) neutron stars (10 km radius) with surface magnetic fields as high as 10^ gauss. In a model for pulsar emissions due 10U to Sturrock field-aligned currents are driven along polar field lines due to the interaction of the rapidly rotating pulsar magneto- sphere with the surrounding nebula. This current is carried by beams of electrons which have been accelerated to 10 eV energies by a plasma sheath at the star surface. These electrons emit Y-ra(iiation which annihilates to produce an electron-positron plasma. The resulting high energy plasma may lead to a two-stream instability which breaks the current into current sheets. These current sheets may then radiate in the radio spectral range like single particles, with a large effective charge, moving along curved magnetic field lines. Observa- tion of the radiation in the form of complex pulses depends on the relative orientation of the rotation and magnetic axes with respect to the line of sight. This model, shown in Figure 21 (from Kennel-^) is also capable of explaining the \- and x-ray bursts observed from the Crab pulsar (CP 1919). Of interest for this pulsar emission model are the elements of sheath particle acceleration and current driven plasma wave instabilities which may be common to solar system pro- cesses as well.

1063 6. RECOMMENDATIONS FOR CONTINUED PLASMA WAVE RESEARCH In the previous sections the characteristics of observed plasma wave processes and the current interpretations regarding these processes have been reviewed. The wide variety of plasma wave types in the Earth's magnetosphere was emphasized since our information is most complete on these, but, known characteristics and suggested mechanisms for plane- tary and cosmic escaping plasma waves were also discussed. In this final section we summarize the major advances in plasma wave research during the past several years, identify the most significant problem areas to be emphasized and make recommendations for the future direction of this research. 6.1 Recent Major Advances During the past several years, major advances in plasma wave re- search included the following: 6.1.1 Development of a theory which describes the dynamics of the outer electron radiation zone in terms of diffusion rates, amplitude of ELF hiss, cold plasma density and pre- cipitation fluxes. 6.1.2 Observations of electrostatic noise at the bow shock, mag- netosheath, cusp, tail, plasmapause, auroral field lines and the ionosphere — all regions of significant wave-particle interactions related to many of the fundamental plasma pro- cesses listed in the next section.

1064 6.1.3 Discovery of the terrestrial kilometric radiation and de- tailed measurements on location, beaming, and association with bright auroral arcs. 6.1.U Description of VLF hiss and saucers as coherent beam ampli- fication processes driven by the intense auroral field- aligned currents. 6.1.5 Discovery that power system harmonics of 60 Hz can leak into the magnetosphere and cause electron precipitation and that the active study of similar wave-particle and wave-wave interaction using the Siple VLF transmitter and electron beams injected from rockets can be carried oat. 6.1.6 Detection of hectometric radio bursts from Saturn and Uranus and realization that these bursts may result from processes similar to the Jovian decametric and terrestrial kilometric bursts which may also be similar to solar, flare star and pulsar emission mechanisms. 6.2 Significant Research Problems In my opinion research concerning the following plasma processes should be emphasized during the next decade. Examples of unanswered questions on the involvement of plasma waves in these processes are given: 6.2.1 Energy transfer from the solar wind to the magnetosphere and heating at the bow shock. Greenstadt and Fredricks (this report) emphasize the role of plasma waves in the collisionless bow shock structure. It seems that

1065 ion-acoustic waves are responsible for thermalizing the directed solar wind electron flow at the shock but the waves responsible for the ion heating have not been identified. It is suggested that electro- t static waves near the lower hybrid resonance frequency produced by a counterstreaming ion distribution may be responsible. Measurements of waves and particles in the bow shock region must be done by mother- daughter spacecraft pairs (like ISEE) to account for the rapid spatial variations. 6.2.2 Precipitation of electrons and ions from the trapped radiation regions to cause diffuse aurora. Lyons (this report) thoroughly discusses the dynamics of the radiation belts. Electron diffusion and precipitation can be understood in terms of ELF hiss and to some extent chorus. Electrostatic electron cyclotron noise (3/2, 5/2 f~) is thought to be important also but its o spatial distribution and conditions for generation are not yet well enough documented, along with adequate particle distribution function measurements, to assess its significance. Ion cyclotron noise has long been thought to control ion precipitation but as yet it has not been conclusively detected in the interaction region. More specialized instru- mentation may be required for future missions. 6.2.3 Acceleration of electrons along auroral field lines causing bright auroral arcs and intense kilometric radio emissions. The evidence for field-aligned currents and field-aligned elec- tric field regions is discussed by Haerendel (this report). Many mech- anisms have been proposed to account for the intense beams of electrons

1066 which carry most of this current at low altitudes and which cause bright auroral arcs. However, so far it has not been possible to identify the responsible mechanism because observations are required in the magnetosphere and because the various possibilities need more study through theoretical analysis, computer plasma simulation and laboratory plasma experiments. If anomalous resistivity is the cause then it is thought to be electrostatic ion cyclotron or ion acoustic plasma waves which cause the resistivity. If it is double layers then waves may be important to the formation but not the persistence of these layers and if it is a magnetic mirror effect then plasma waves may play only a secondary role. A good case has been made that the VLF hiss is Cerenkov emission amplified by the auroral electron beam and that the saucer emissions are due to the same process for a return current lower energy electron beam which has not yet been observed. Several completely different mechanisms (amplified electron cyclotron and parametric wave decay for example) have been suggested as the cause for the high power kilometric radiation. Measurements of the wave characteristics (especially polari- zation), plasma parameters and energetic particle distribution function in or near the source region are not yet available with which to identi- fy the specific plasma wave process. 6.2.U Energy dissipation in the magnetotail allowing field line merging which determines the magnetosphere shape and the plasma convection patterns. For the process of field line merging between the solar wind and magnetosphere magnetic fields at the bow shock and along the magnetosheath

1067 and for the process of field line reconnection in the tail region (see Sonnerup, this report), an energy dissipation process is required. One likely suggestion is that energy is dissipated through the creation of electrostatic waves. Both broadband electrostatic noise and electro- static electron cyclotron emissions are observed in the regions where field line merging should be taking place. The detailed associations between field-aligned currents, changes in the magnetic field vectors, particle energization and the amplitudes of these waves have not yet been made to determine if the waves can provide the required localized resistivity. ISEE should provide some of these measurements. 6.2.5 Energy transfer between the magnetosphere and atmosphere and heating in the ionosphere. Energy is transferred directly from the magnetosphere through the ionosphere into the atmosphere through precipitating particles, field-aligned and transverse current systems and by electric fields. Banks (this report) discusses a number of these coupling mechanisms. As with many plasma wave processes it is not known whether waves are instrumental in the particle acceleration, current resistivity or electric field penetration mechanisms or whether they are just by- products. Many of these questions should be answered by the continued Atmospheric Explorer and the Etynamics Explorer Missions. Sturrock (this volume) has mentioned several processes by which this energy coupling could affect the atmospheric weather systems.

1068 6.2.6 Energization, diffusion and precipitation of energetic particles in the magnetospheres of other planets • Some evidence for wave-particle interactions in the Jovian mag- netosphere was presented in Sections 5-1.1 and 5*1.3 and other characteristics are discussed by Lyons, Haerendel and by Kennel and Coroniti (this report). There is no doubt that trapped plasma waves exist in the Jovian magnetosphere with much the same variety as for the Earth. However, because of differences in the plasma distribution, density and tempera- ture (~ 100 eV) and because the particles are mildly relativisitic, different wave processes may dominate. MJS'77 will give the first clues on plasma waves and the wave-particle interactions at Jupiter, Saturn and possibly Uranus. However, missions such as the Jupiter Orbiter are required to obtain any detailed understanding of the plasma wave processes. 6.2.7 Emissions of the intense radio bursts from Jupiter, Saturn and Uranus• As indicated in Section 5, it is tempting to conclude that the mechanisms for planetary radio emissions are similar because they seem to scale with the supposed magnetic field in frequency and with the solar wind energy input in intensity. But, in fact, the details for the emission process of terrestrial kilometric radiation are unknown. Direct measurements with MJS(U) of beaming, polarization and intensity are definitely required along with measurements of any associated trapped plasma waves similar to saucers and VLF hiss, broadband electro- static noise and magnetic noise bursts as well as evidence for electron

1069 acceleration, electron beams, and field-aligned currents. It is desirable to determine if the current systems are driven by solar wind convection or by planetary rotation (with respect to a moon?). For example, if one argues that the Jupiter-Io situation is unique and that each planet should have a solar wind current system which causes radiation in addition, as at the Earth, then Jupiter may have a night- time decametric source of 10 -10 3 watts which could explain the ob- served non-Io associated emissions by leakage to the dayside. 6.2.8 Cascading and transport of energy within the solar wind. Observations of solar wind density, velocity and temperature parameters are reported by Feldman (this report). Observations of waves within the solar wind have been reported from the early Pioneers and IMF's near the Earth (l AU). Pioneer 10 and 11 have extended measurements in the magnetometer frequency range to beyond 7 AU and Helios 1 and 2 to 0.3 AU for a wide frequency range. Observed waves cover the ex- pected spectrum (as in Figure 2) from long-period Alfve'n waves (like some micropulsations and ion cyclotron waves) to waves at twice the electron plasma frequency generated by electron beams from solar flares. However, it remains to identify the role of these waves in the trans- port of energy from the solar atmosphere through the chromosphere and corona and into the distant solar wind. For the solar wind it must be determined if the Alfven waves, for example, carry most of the energy from the photosphere and are dissipated to heat the corona and if the fluctuations in the plasma density (which lead to interplanetary scin- tillations of radio sources) are due to a wave-wave interaction process. Missions such as Helios and the proposed Out-of-Ecliptic are required

1070 to determine the solar-solar wind energy balance and the significance of plasma wave processes to this balance. 6.2.9 Emission of plasma waves from cosmic plasma systems such as the solar atmosphere, flare stars, pulsars, galaxies and quasars. Examples given in Section $.k suggested that some cosmic plasma wave processes may be interpreted in terms of processes that can be well studied by passive and active experiments in the magneto- spheres of the Earth and, in the future, Jupiter. Once the local pro- cess is explained in terms of concrete physical principles, extensions in the theory, in laboratory experiments and in computer simulations can be made to better describe the particular cosmic plasma system. 6-3 Future Direction I feel that future plasma wave research should emphasize the following: 6.3«1 Completion of survey for plasma and plasma wave processes throughout the solar system Within the Earth's magnetosphere, the auroral and polar regions between 3000 km and k PL, have not been surveyed. Auroral particle El acceleration, kilometric wave emission and plasma convection pro- cesses occur in these regions. The proposed Dynamics Explorer Mission supplemented by ISEE and GEOS would provide much-needed measurements. Pioneer 11, Pioneer Venus, MJS(u) will provide the necessary survey data for Venus, Jupiter, Saturn and possibly Uranus. However, a flyby of Neptune and Pluto, additional investigations at Mercury and orbiters

1071 of Jupiter (as proposed), Saturn and Uranus are necessary to provide the detailed data on magnetospheric processes and their response to solar and solar wind variations. Data on plasma and plasma wave processes in the solar wind into 0.3 AU in the ecliptic are being collected by Helios 1 and 2. However, the solar heliosphere processes both closer to the sun and out of the ecliptic plane must be at least surveyed in order to develop theories for the dynamics of stellar magnetosphere c. 6.3.2 More detailed measurements of electromagnetic and electro- static wave characteristics using multiple receivers and corre- lation of wave measurements with more detailed energetic particle and plasma measurements in the Earth's magnetosphere. A new level of plasma wave instrumentation and analysis technique needs to be developed to obtain more definitive plasma wave measurements. 103 Storey and Lefeuvre are developing the mathematical and data pro- cessing techniques to derive wave distribution functions f(k, J3, cu) from measurements of the six electromagnetic wave components. Appro- priate plasma wave instrumentation is to be flown on GBOS but these same measurements are also required elsewhere in the magnetosphere. Similar instrumentation and analysis methods need to be developed for determining the complete wave characteristics of electrostatic waves. GEOS will also carry an on-board correlator to produce spectrograms of a selectable frequency-time segment. This processor provides broadband data at a much lower data rate than would be required to transmit the analog data directly -- thus conserving telemetry as is required for

1072 obtaining detailed spectral information from the solar wind and from plane- tary magnetospheres. Satellite-to-satellite interferometry on plasma waves will be attempted for the first time between ISEE-A and ISEE-B. This technique may be particularly valuable for determining the spatial correlations of trapped plasma waves and the source locations (and source sizes) for escaping plasma waves such as the kilometric radiation. This more sophisticated instrumentation must be accompanied by simi- larly sophisticated instruments for measuring the plasma parameters and particle distribution functions -- generally on pairs of spacecraft to be able to separate spatial from temporal variations. 6.3-3 Development of active plasma experiments for further probing the magnetosphere under controlled conditions and for carrying out basic wave-particle experiments in the extensive magneto- spheric plasma. hQ Helliwell reviews the techniques and results of VLF wave in- jection into the magnetosphere in order to study wave stimulation and particle precipitation processes (see also Figure 7)« The results to date suggest that more sophisticated experiments should be carried out in conjunction with _in situ spacecraft that are capable of measuring the local wavefields -- original and stimulated -- and the detailed evolu- tion of the particle distribution functions. Such experiments are planned in conjunction with ISEE and with the proposed DE (which is more suitable) but better experiments would include the VLF transmitter also on a spacecraft. The injected electron and ion beam experiments with the Echo rockets are examples of using the magnetosphere as a

1073 laboratory plasma in which to produce controlled wave-particle processes 20 (see Cartwright and Kellogg ) as well as to study the magnetosphere itself. A concentrated effort should be made to take advantage of the Shuttle launch capability to expand these laboratory experiments along Q/' the lines reviewed by Scarf for the Plasma Physics and Environmental Perturbation Laboratory (PPEPL) and the proposals for a series of Atmospheric, Magnetospheric and Plasma Physics (AMPS) payloads. Already it seems that Spacelab I will carry an electron gun with diag- nostic instrumentation for waves, particles and optical emissions. However, it must be remembered that a laboratory carried by Shuttle can examine a limited plasma parameter range so that active experiments to the outer magnetosphere are also required. These can be launched from Shuttle or by conventional rocketry from Earth. 6.3.U Performance of laboratory plasma experiments, computer simu- lations, and theoretical analyses that treat specific processes identified in the magnetosphere. More emphasis must be placed on research efforts that comple- ment the direct space measurements especially since the space experi- ments are being designed to obtain more quantitative results on some 26 well identified processes. Falthammar emphasizes the increasing need for laboratory plasma experiments. He suggests that these fall into two categories: (l) configuration simulation in which the geo- metrical properties of a large system such as the magnetosphere or moon (terrella and lunella experiments) are modeled and (2) process simulation in which the local behavior of a real plasma exhibiting a

1074 particular process of interest is studied. Some experiments relating to current flow and plasma convection in a dipole field (Birkeland currents); energy dissipation at a magnetic neutral line or point (neutral sheets and solar flares); anomalous resistivity, electric potentials in magnetic mirrors and electric double layers (particle acceleration in neutral sheet and on auroral field lines); and the pene- tration of plasma into a magnetic field region (bow shock) have been carried out. A few experiments related specifically to space plasma wave processes have been performed: An experiment by Bernstein, et al., with a large scale electron beam produced emissions at 3/2, 5/2, .... harmonics of the electron gyrofrequency and at the electron plasma rj frequency: Other experiments, for example, designed to study large magnetic-field-aligned electric potentials and particle acceleration, produce ion acoustic turbulence and emissions below the electron gyrofrequency and above the electron plasma frequency which are similar to the plasma waves associated with the auroral acceleration regions. Further research is required to ascertain, quantitatively, the role that these waves play [s. Torven, personal communication]. Computer plasma simulations provide a powerful technique to study specific plasma phenomena in comparison with space measurements, laboratory results and theoretical predictions. Such computer plasmas can develop smoothly into non-linear states. Since one has information on the position of each particle and on the applied microscopic fields, diagnostic quantities can be computed to give the evoluation of the

1075 plasma parameters, wave modes and particle distribution functions. Very few applications of this developing technique have been made 35 specifically to space plasma problems. Goertz and Joyce did investi- gate the formation of an electrostatic double layer in one dimension, for example, which confirmed a number of the theoretical precictions but the diagnostics to identify the associated electrostatic plasma waves has not yet been carried out. The computer simulation technique is an important key to interpreting observed processes and to extra- polating these processes into other plasma regimes. The significant research problems identified in Section 6.2 provide a working list of problems to be attacked by theorists. "Solutions" to these problems require that the theorist be provided with suitable results from space measurements, laboratory experiments and computer simulations. In order to obtain suitable data, the theorist needs to be involved in defining the space missions, in identifying the specific problems to be attacked, and in specifying the parameters to be measured. They also need to identify the computer and laboratory experiments to be carried out and the required parameters to be determined. 6.3.5 Identification and interpretation of plasma wave processes from cosmic plasma systems seeking analogies and contrasts to solar system plasma processes. Plasma and plasma wave processes in other cosmic systems may operate from different sources of free energy and under different plasma conditions. Examination of these processes (from the escaping plasma

1076 wave inform tion) is important in order to test the interpretation of solar system processes by extrapolation and to identify new processes which occur within the Universe. Such research provides the necessary cross-fertilization between solar system plasma physicists and astro- physicists.

1077 7- CONCLUSION It is only in the neighborhood of the Earth that one can hope to ever thoroughly investigate a large number of plasma phenomena, within reasonable financial resources, to the point where the physical mechanisms are really understood. Once that is done other planetary and cosmic plasmas can be understood much more easily (by analogy with the terrestrial magnetosphere) with limited information by remote sensing and perhaps a few in situ probe measurements.

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1079 V i ft) ci CI i ,, g ft) r-l CI u to ft) T.l i u o 4i f: 4) ft) td v| q w * 4i -H d -P •I H -H 3 -H H O Ot M ^ oj pI O H > •J C) M o > a -S3 N B M n •H '4 Oi O 3 0 4i p > o w .-l t* L« O r : o H r^ kt o) o -P -' O Ii O Ci O IJ ft) O, [i M l. t. o p O O' tl h -P •H Q 4i C * H O n ft) a i o 4 . 4) H ,O -' U 3k W ft 0 0 o w ti w b* C ft) *H £ 8'S •H ft) rH 0 ft) •rH C M ** V w H o o g 3|fl •H ir\ fiS -P0-H r-i (i ,-,: H ril % Q i v u tH O &fi •H 4i C n d o ti «i >, 4i 0 § >i s-2 £S •H l-l CI X) 0i O •O O tO C) O h rH -H o •-1 W O) S"0 Tl 0i 0I ft) « rH Is Cd £ R S 0 4) 4i t* *H ID 0i *J 4J C 33 a . ' p *O w 5 2 -P V; '. , a c. u •rH M O c a -f o u * * .0 -H n. .0 -H &£ •O CJ -H *) 4i O O f> ti IH 4J ic o c .8 d p o 9 S ft) C O H M C u ir\ O H -H IH i 4' > cJ 5 s. c° 43 > 0 IH Q 5 4i -C C O 1O 2> •H > -P 0 -P 0 O M h ft) O -H O 0 O oi ft) ir\ oj Ci X * o* BM >! rH r-l -P-^QH H -P Oi C Ul 4i > 4i C « C IH O -H O 4i IH O O O 4J C c *> J3 0 0 C 4i 45 IQ f w d ft) 0 ~" fi w IH T) p 3 >. d IH d O IH CH Pi O .X 4i P..C -H S£ 85 ft) M 0^| IH •»J 0) il- <• i., ft) C Li > -H S-H -H > 0 ft) ft) x: W O 0 X » rH i i • i i | g) h Si<m 4i C i 8 tO T) 4) o) 311 i ' « i to b• "* Tj ft) r : O) ft) r-f U h 3 &S5 &.S 4i § >i WAVE PROPERTIES -aode turbulence at all Fills plasmasphere. Ca recipitation from outer e. f overlapping rising or s. Quasi-periodi0. Pr ayside equator. 0I noise bands on frequen ten superimposed. Down 2500 km. Asso0iated wi tati0 turbulence. noise bands in frequen ten superimposed. Upgo 1000 tan. Falling or mixed tones, or quasi-periodi0. Ra h spa0ecraft. C noise band above LHR fr ly ele0trostati0 with k s: below ion 0y0lotron :tron 0y0lotron frequenc idband noise trapped be' ase and magnetosheath. n day side 01*00- 1UOO LT. h a, a O ft) -O TJ ^H W •O CH 0 -P T3 u O ol O 4i O c oi 0 V 0 4i d * " ^ p, , \ - :: tl 4J M O n o >i P, In S?«l * VI ft) 1i 4J H a - u •" e5 -. M C C •H R *i d '. M C H B £ aI •s § £ risg W 0 iJ •H I5, O) C >~ H 0 5§ S-1' X w -P oi d d £1 -H O ~l 5 4J 4i 4J o c m i .i a) .H £ ft «J tO -H C: > 4) :• o CO 8 0 ON co 0 O) S H H vO to ir\ § •k ro ro S Ir\ M ^ «* ccT o? irv t\J & w" - co* co* j* H* lf\\O -* W H fn 1"1 •* C\J CO m a H r_i ).( 1i d oj i 60 i to i to i 10 i I lo J o '-i o o r-f Ul S K V y i ,p V V V Vl tl r : 5 A. i1" <H A V> H N M a K tt N •c ^ i HP M M a a 2 S w . •. o o l> w o tM CO J t- OJ tf, K m ir\ H r-J w c/ i i a i .rl ^ £ 10 <•1 SIX N ^N !„ u: 8 •^ 3 rH o o o o O O H rH -i ir\ H ^•/ . t\j LT\ ti M o sSfi d' •o •a 0i 4i fl Vt fll rH & w .ri 4i c u O "i •O S a. hi • O M fl x t: rf jg n ,n r>. 3 N rt c^ O -r ' •5 Pi *O H 7! PI fl g sill •U ft) r-l Mi >l P a) O ;•: p t/i IH ft) Oi L M h w 'U w oi ,c a ii 4: j) g O li t] O v n c< •H -~1 *O 4i id 4 -o w h 1• Ol w nl -H o5 4 i fii w at fiH ^ *;' c- 3 S <n 1:1 51 .'« C.4 P. O V) ^^^ i t5 IU VI o fl •d 0) •o i i i ol f; i, r: c! .Q I' (! *rl r: n <•• 15 i'i n ii) P. dl r-l Q) C V) ft' Ij ll : • I i oi VJ ffl VI a] •"- - W •*-i o w •Ii^i { : -.i n :' -. • l , " « h it H p 1i r i ;- r! n l;j X M di ii i!' !i i'; in c 'i ;i i: >• i• fl d tl H CI bj , I/! -! •'• i.K- fc' v. - w cO n r-i ; i i . ti : -.i •. i " Ifl '•' :< ^ r-) ' * *) **

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1082 rt -H H d OH C 'H t) «O I •3 ft) u S d 0 fl 3 i . ^ » e 3 H n H H H rt i3 -H i£) 4) -P *J O o ft w o3 6) ()i -f i *• BOH " ^i p; TJ -* ^ 000. O ^ i • ' c-l O 4i t, *i t. — -o L, y C O ol d m .. i. a- c' *i -O O -^ c „ o n IB tti c t ) M ^ i •H C V 4) > O O O 4) I o o u P IX « C lH >, c fili,^ -rH O O* t) 4) C s C d O) P J3 0 . 0 ^- «) tj O -P -P JJ H"^P' o w c *i 4) O O ,O w > 4i P O ^^ * 4) S i CS •O > Oi H ' "^ 3 ft) i h 4' W ^^i 4J TJ H ^' O-t * •) 4) H T1 °- *-i 3 O O O) C -H H H4» « . *H P 0 ft) ^ . *J tl t+ 4J A n •] •) O 4) T5 ^H i -» ?' B-H 0 W *O O 3 35 a-g A- s « *s N . , 4) -P •l c 4) -HO 0 -H ^ ft) O O TJ P W O o c: H •H -H O -P q wo OJ c r 4J c/j r^ CM IH ^-j -H -ri OJ t O) R x; oj C d o Tl C [TJ P« •5 l« • ft) 4) [4-1 T) C M .0 4t C 0 J«* O 05 t* f^ O 3 I T* C O -H Bl d •*H O tt) Cl O -P 0 O C a) ni ai r\i ,t: -P i ft C -P O) flS C O i -rl O Si «H H .D v m • Pp. MO J« C ft) O Oi VH I-, fl) •iH »-' Bl 4' M P -P Oi C EH M TJ nj Tl H C) t-3 t. d oi ri o 4» 0/ O 3 g O) J^ >' 4) ti r : P, rH r- H ft "° ^ -H -H U . CU i 4) « rH O -P iU H W O I/I •H OJ Tl w -H -H / O Oi O {0 O rH t. flj O R a ig H C O 4) «t " C T3 4J Ul C w o. 4J -H o t: o M d -a 0J E C -P rH bD d «J •act* ^ u d 4i UJ C xi X3 T3 oJ fll -r^ C «H ft) •D HJ l-iO> ^ 4-i o} ^; -H c x: w rt H 0 O w w -P O in *-• i-H M O Ll ft) ;5 -M (H Pi d CX j. -P -H ^ o E "S C w ,0 o5 '; )^ P -H tO ^ u ITv & i 4S O\ B OJ O? s ON i-4 l> rn f* u «* f-> 0 i a EN V] m H j'l tM 0 o o o• i Pcl i W) P A > •H A N S i q ft 8 fcS w o• rH OJ t i M [|J H 01 til ^ se o 0 OJ OJ 3 £ H IB H .C 0 P; •& 64 H N 4) O H S S «J *' r. . 0 « t-l W t' 1^ " *i :i m « O O. fl ^-N^x B i "iJ n n n :l o •HI1 -H H »" O .-^ ti a h 3 •*i fi ^ S1 ? r 0) C 4J H .i:-i o) 4 i 4 i ..4 lIi '' ''

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1084 Table 5A. PLANETARY RADIO BURSTS: COMPARISON OF PEAK EMISSION FREQUENCIES* PLANET PEAK FREQUENCY, MHz OBSERVED PREDICTED MAGNETIC OBSERVED FIELD, GAUSS BODE LAW FREQ/FIELD, MHz/GAUSS EARTH .25 — 0.62 — o.Uo JUPITER SATURN 8 1.1 ll+.UN 10.8S 0.56 (0.5) 2.U U.8 URANUS 0.5 0.8 — 1.6 (0.5) NEPTUNE — 0.6 — 1.3 (0.5) *Adapted from Kennel and Maggs with Uranus observations added. The predicted peak frequencies are obtained by multiplying the Bode Law pre- dicted magnetic field value by the average ratio of the observed peak fre- quency to polar field value for the Earth and Jupiter. Table 5B. PLANETARY RADIO BURSTS: COMPARISON OF PEAK POWER FLUXES* PLANET POWER FLUX AT EMITTED EARTH, Vm~2 Hz~1 2n STR, POWER IN WATTS POWER FROM SOLAR WIND, WATTS RATIO EMIT"' ' TO SOLAR W1UL» EARTH .- 5 x 108 5 x 10 11 1 X 10 -3 JUPITER 2 X 10~ 19 1. 5 x 1012 [1010] 5 x 10 13 3 x io~j; [2 X 10" SATURN 5 x 10-20 L 5 X ion 5 X 10 12 1* x 10 _p URANUS 1.5 X I0~20 1. 0 X 1011 U X 10 11 2. 5 x Kf1 NEPTUNE (2 X 10~ 2l* to 1 x 10~20)(1 • 5 x 1011) 1-5 X 10 11 (2 x 10~1* to 1) *Adapted from Kennel and Maffts with Ilranu:: observations added. [ ] Total omitted power est1mate including beaming i'actor. ( ) Scaling to Neptuuo I'rom other planuts. The farther planets :;et;m to have 1ncr-..-u.j I; effic1encies if the beaming is into 2n storad1anc. Howr-ver, the conversion effi- c1ency for Jupiter and the Karth ic much loss when accouutirIR for tIn- measur : beaming', factorr..

1085 AKR IMAONETOSHEATHI TRAPPED CONTINUUM (AURORAL) KILOMETRIC RADIATION, AK TAIL ELECTROSTATIC NOISE• AURORAL FIELD LINE TURBULENCE MAGNETIC NOISE BURSTS VLF HISS ICROPULSATIONS ION CYCLOTRON WAVES ELF HISS DISCRETE EMISSIONS fELECTROSTATIC ELECTRON NONTHERMAL CONTINUUM CYCLOTRON EMISSIONS [NEUTRAL SHEETt= ION CYCLOTRON WHISTLERS ELF HISS HR NOISE ELECTRON WHISTLERS UHR NOISE IPLE EMISSIONS DISCRETE EMISSIONS// FARLEY MICROPULSATIONS//INSTABILITIE ELF HISS FIELD ALIGNED .CURRENTS I BOUNDARY LAYERI MICROPULSATIONS \JION CYCLOTRON WAVES AURORAL FIELD LINE TURBULENCE AURORAL KILOMETRIC RADIATION BOW SHOCK UPSTREAM WAVE! PLASMA WAVES TURBULENCE PLASMA OSCILLATI = PLASMAPAUSE FIGURE 1 Regions of plasma wave occurrence located in a noon-midnight meridian cross-section of the Earth's magnetosphere.

1086 KILOMETRIC RADIATION NONTHERMAL CONTINUUM DISCRETE EMISSIONS UPSTREAM WHISTLER MODE WAVES CHORUS ELF HISS ELECTRON WHISTLERS MAGNETIC NOISE BURSTS ION CYCLOTRON WHISTLERS UPSTREAM WHISTLER MODE WAVES ION CYCLOTRON WAVES MICROPULSATIONS Im4)fg" (ELECTROSTATIC ELECTRON CYCLOTRON NOISE) UHR NOISE (BOW SHOCK PLASMA OSCILLATIONS) (TAIL ELECTROSTATIC NOISE) V (BOW SHOCK TURBULENCE) (AURORAL FIELD LINE TURBULENCE) LIONS ROAR LHR NOISE SAUCERS VLF HISS BOW SHOCK PLASMA WAVES (FARLEY INSTABILITIES) WAVE NORMAL-00 WAVE FIGURE 2 Association of plasma wave types with the characteristic frequencies of a plasma for wave nor- mal directions nearly along the geomagnetic field direction (0°) and nearly transverse (90°). The curves rep- resent the index of refraction, n, for the left- and right-hand (L and R) and the extraordinary and ordinary (X and 0) wave modes. Wave types in parentheses ( ) are electrostatic (adapted from Jones and Grard53).

1087 FIGURE 3a Polar magnetospheric model showing the variation of the electron plasma frequency f ~ gyrofrequency f ~ and the cutoff frequency for the right-hand mode fR=0. Also indicated is the fre- quency range for the kilometric radiation which is consistent with the frequency range for which the 3/2 f ~ electrostatic emission would be expected to exceed the right-hand mode cutoff (from Gur- nett41). 10 *0 iO R, EARTH RAOII FIGURE 3b Dayside equatorial magne- tospheric model showing the radial vari- ation of the gyro, plasma, upper hybrid and R-mode cutoff frequencies. Non- thermal continuum noise is found to be trapped in the low density region be- tween the plasmapause and the magne- tosheath (after Gurnett and Shaw39).

1088 -,- ,71-69-1 LEPEDEA 'A' ELECTRONS, REV 1403, DEC 2, 1968 0412 U.T.(HM) I 4.0 3.0 2.0 0404 0405 0406 0407 U.T.(HM) FIGURE 4 Frequency-time spectrogram of VLF hiss, saucer and ELF noise band (similar to lion's roar) associated with an intense auroral electron precipitation event evident on the energy-time spectrogram at ~ 0406 UT observed by Injun 5 at ~ 2500 km altitude (from Gurnett and Frank38).

1089 f- (o) (b) FIGURE 5a Examples <>f non-ducted electron whistler ray paths in the magnetosphere that make up a magnetospherically reflected (MR) whistler train. Note that the whistler energy can propagate across magnetic field lines (from Smith and Angerami100). FIGURE 5b Examples of ray paths for nonther- mal continuum radiation which is trapped between the magnetopause and plasmapause boundaries (from Jones and Grard55). POSITIVE FMOUCNCT KMOO MOON-MOM OKI MERIDIAN .THERMAL CONDUCTION HEAT FLUX PHOTON PRECIPITATION FIGURE 6 Pictorial representation of the hot ring current plasma mixing with the cold plasma- sphere plasma to produce ion cyclotron waves at the plasmapause. These waves cause ion precipi- tation and damp as they propagate leading to a downward heat flux which produces stable au- roral red (SAR) arcs in the ionosphere (from Electrodynamics Explorer Final Study Team Report, NASA/GSFC, September 1976).

1090 LOSS CONE ELECTRON rHf.ASUREMENTS_ SIPIE TRANSMITTER EMITTED WIVES FIGURE 7 Schematic drawing of VLF wave in- jection from the Siple, Antarctica, transmitter along the L« 4 field line toward the Roberval, Canada, conjugate point. These injected waves organize the energetic electrons to emit and am- plify waves at adjacent but variable frequencies in the equatorial region. These large amplitude waves cause electron precipitation (from Electrody- namics Explorer Final Study Team Report, NASA/ GSFC, September 1976). II JULY i963 0400 UT FIGURE 8 The location of the electron density knee (plasmapause boundary) as a function of local time and radial distance as deduced from the dispersion of electron whistlers for three days in July 1963 (from Carpenteri5). FIGURE 9 Values for the total electron density outside the plasmapause derived from the non- thermal continuum spectral cutoff at the plasma frequency as observed with IMP 6 compared to the supra-thermal proton densities determined simultaneously (from Gurnett and Frank40). IMP.6. ORBIT 95 APRIL 6. i972 ELECTRON DENSITY FROM THE PLASMA FREQUENCY CUTOFF, fp LEPEDEA 66 tv i E 4 38.00O «v PROTON DENSITY I I _J i100 52 49 50i 2200 UT IHR Mil M.I RfRtl U LTIHRi MS )»,tOEol

1091 SPECTROGRAM CF OBSERVED EMISSION FIGURE 1 Oa Idealized frequency-time (distance) spectrogram of saucer emission extending in fre- quency from the local lower hybrid resonance frequency at the satellite to the lower of the elec- tron plasma or gyrofrequency (from Mosier and Gurnett73). HORIZONTAL DISTANCE FIGURE lOb Representative ray paths for the saucer energy from a source below the satellite. Higher frequency components make a large angle to the source field lines (from Mosier and Gurnett73). 2500- itOOO— 024 NWBER OF FUSSES WITH SAUCERS -. 1 i f—l 1 1 1 ' 1 O 20 SO 40 50 TOTAL NUMBER OF VLF PASSES PROCESSED FIGURE lOc Distribution of saucer occurrence with height of the observing satellite. Generally the source region was deduced to be within 100 km of the satellite by ray tracing downward (from James").

1092 FIGURE 11 The amplitude of kilometric radiation at 178 kHz for a 24 hour period (magnetically active) on 25 January 1973 and a sequence of DAPP auroral zone photographs taken during the same period. The kilometric radiation appears to be more closely related to the discrete auroral arcs than to the diffuse au- rora. On frames 1094 and 1096 both discrete and diffuse aurora are evident along with intense AKR. How- ever on frames 1095 and 1097 diffuse aurora is occurring but the AKR is absent (from Gurnett41).

1093 SOUTH 18 FEB. 1975 17.5 MLT • ~ 0915 U.T. • - 1255 ffl ~ 1635 D - 2020 80* 90* FIGURE 12 Locations of the kilometric radiation source by lunar occultations with RAE 2 at several times during 18 February 1975. Note that the source regions trace out nightside auroral field lines (from Alexander and Kaiser1).

1094 ELECTROSTATIC NOISE-100 Hz- AURORAL lTERRESTRIAL) KILOMETRIC RADIATION ~ 200 KHz ACCELERATION/GENERATION REGION ? > 3000 KM V-SHAPED VLF HISS I -30KHz INVERTED-V PRECIPITATION ELECTRONS 1-IOkeV IOOO KM FIELD ALIGNED CURRENTS EMF FROM SOLAR WIND? UPWARD DRIFT ELECTRONS? 5eV VLF SAUCERS -30 KHz ACCELERATION/GENERATION ? IOOO- 3000 KM EARTH FIGURE 13 Conceptual view of auroral zone plasma wave phenomena and the associated parallel elec- tric field regions, precipitated and accelerated electrons and bright auroral arcs. Kilometric radiation (~ 200 kHz) is an escaping plasma wave from 0.5-6 RE in altitude whereas the VLF hiss (~ 3 kHz) is a downward propagating whistler-mode wave from nearly the same altitude range. Saucers (~ 3 kHz) are upgoing whistler-mode waves from regions adjacent to the hiss.

1095 FIGURE 14 Decimetric brightness distribution of Jupiter at 21.3 cm for a central meridian longitude of 15°. Note that this synchrotron radiation peaks on either side of the dark circle representing the optical disk (from Berge and Guilds'). FIGURE 15 A comparison of Jupiter's escaping radiation at two wavelengths (~ 12 cm and 21 cm) with (a) the 10.7 cm solar radio flux (jagged curve) and (b) the square of Jupiter's distance from the sun (sine curve with adjusted phase) as a function of date. The large intensity variations are not correlated with solar or solar wind parameters directly but may be due to changes in the trapped electron distribution function and plasma parameters of the inner Jovian magnetosphere (adapted from Klein62). (0) 0.1 1.0 l0 Frequency (Hhz) l000 r XX) FIGURE 16 Relative intensities of the ob- served escaping decametric/hectometric/kilo- metric radiation as a function of frequency for Jupiter, Saturn and Uranus and the Earth. A predicted range for Neptune (see Table 5) is also indicated (adopted from Kaiser and Stone57 and Brown11).

1096 SW I80- Z70* . /IQCCI 360' I80' 90' FIGURE 17 Occurrence probability for Jovian decametric emission as a function of the lo or- bital position ('f>lo, 'f> = 180° is toward Earth) and the central median longitude of Jupiter Xn, for two different time periods. Note par- ticularly the lo-associated regions near .f =* 90° and 240° and the lo-independent emission for X~ 240°-270° (from Boyzan and Douglasi0). COLLECTED THERMAL ELECTRON FLUX JOVIAN MAGNETIC FIELD LINES WAVE EMISSIONS -£? ACCELERATED PHOTOELECTRON FLUX JOVIAN IONOSPHERE NEGATIVE CONDUCTIVITY SHEATH I0 IONOSPHERE CONDUCTIVITY FIGURE 18 The lo-sheath-acceleration model. lo's rapid motion through the Jovian magnetic field sets up a motional potential of several hundred kilovolts. This potential is dropped across plasma sheaths in which electrons and ions can be accelerated to 100 keV energies. The potential drives a beam of these high energy electrons (carrying up to 10i3 watts) toward the Jovian atmosphere. By a plasma instability along the field lines decametric radiation is stimulated and amplified (adapted from Shaw run etal.9*).

1097 FLUX JV K)i I0° K> -2 K) Mf* KT THERMAL DISK I80* K I0I 10° 10" FREQUENCY, MHl .0° .O6 FIGURE 19 Possibility of detecting synchrotron radiation from Uranus. Most observations are above 1000 MHz and are consistent with the blackbody emission from a 180°K disk. Model calculations (shaded region) indicate that the emission should fall below 1000 MHz where radiotelescopes are less sensitive. An upper limit measurement for 34 MHz (from one of the largest low frequency radio- telescopes) is indicated. Larger radiotelescopes or measurements closer to the planet are required (from Shawhan and Cronyn96 ). FIGURE 20 Simultaneous detection of a radio (196 MHz and 318 MHz) and optical (ultraviolet) outburst from the flarestar Wolf 424 AB on 30 January 1974. These outbursts contain orders of magnitude more energy than for a solar flare but the time scale (10's seconds) and high degree of radio polarization (60%) suggest that the basic mechanisms may be similar (from Spangler and Moffetti0i). jU tx) I5 316 MHI .25 FU 1 U-FILTER WOLF 424 AB 665 866 .667 HJD( 2442077.+)

1098 J,,in Et overcomes binditlg of magnelized eleclrons " ti Iron Nuclei For Crob. E, = 10" eV Era I0"'" «V FIGURE 21 Sturrock's model of pulsars. Shown here is the pattern of currents flowing in and out of the polar cap of rotating magnetized neutron star whose dipole and spin axes are aligned. In this model the cur- rent is carried by beams of superrelativistic electrons, which emit curvature photons. When these photons propagate at a small angle to a superstrong magnetic field they can produce pairs which also radiate pho- tons, thereby breaking down the vacuum and populating the outer magnetosphere with plasma. The elec- tron-positron plasma is subjected to a two stream instability which produces emission in the radio spec- trum (from Kennel59).

1099 ACKNOWLEDGMENTS I thank Drs. E. Ungstrup, C.-G. Falthammar, L. Lanzerotti, R. Fredricks, D. Gurnett, C. Goertz, N. D'Angelo, and J. A. Van Allen for suggestions on the content of this review and I appreciate the detailed comments of Drs. T. Bell, R. Thome and L. Block on the manuscript. Work on this review was initiated while I was a visiting scientist at the Danish Space Research Institute, Lyngby, Denmark, and at the Department of Plasma Physics, the Royal Institute of Technology, Stockholm, Sweden. Support at The University of Iowa is provided in part by NASA Grant NGL 16-001-002 and NSF Grant ATM76-82739.

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The space age began exactly 20 years ago with the launch of Sputnik I and Explorer I. The Explorer spacecraft discovered regions of trapped radiation around the earth—the van Allen belts. This was the beginning of the study of particles and fields in space, or space plasma physics. A large part of the effort in the early years of the space program was devoted to the mapping of the magnetosphere, the measurements of time variations in particles and fields, and the exploration of the solar wind.

From these studies a sophisticated empirical knowledge of phenomena in space plasma physics has emerged. with the attainment of this observational maturity in the field, NASA funding for space plasma physics has declined as priorities have shifted to other exploratory ventures. The present study of space plasma physics was requested by NASA to obtain guidance for future directions in the subject.

The Committee on Space Physics of the Space Science Board was charged with the responsibility for soliciting technical review papers on a large number of topics in space plasma physics. These reviews are Volume 2 of the report; they constitute a most valuable resource for those working in the field.

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