Space Plasma Physics The Study of Solar-System Plasmas (1978) / Chapter Skim
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Magnetospheric Plasma Waves
Magnetospheric Plasma Waves
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MAGNETOSPHERIC PLASMA. WAVES Stanley D
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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 planetary magnetospheres.
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Plasma waves are generated in many natural plasma systems: planetary ionospheres and magnetospheres, the solar wind, the solar atmosphere, 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.
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From page 264... ...
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.
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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. 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.
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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 groundbased instrumentation since the late l800's.
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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 (electrons and ions) interacting with the plasma and plasma waves in the magnetosphere.
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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)
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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.
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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.
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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
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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.
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3.1 Characteristic Plasma Wave Frequencies A plasma having electrons, ions and neutrals of finite temperatures and permeated by a magnetic field can support a variety of electromagnetic, 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.
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From page 274... ...
With a magnetic field, the plasma is anisotropic 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.
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fTTt_ . upper hybrid resonance frequency UrlH Representative curves for the variations of some of these characteristic 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.
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These whistler waves may be amplified or damped through electron and ion cyclotron resonances with the energetic particle populations. The energy for plasma waves generated internal to the magnetosphere comes ultimately from the solar wind flow past the Earth and the Earth's rotation.
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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)
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' 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.
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. At the plasmapause boundary and beyond, electrostatic plasma waves with frequencies near 3/2 f~, 5/2 fg, etc.
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. In general, plasma waves are created with phase velocities near v*
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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 intimately 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.
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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.
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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 gyrofrequency is identified with the reflected electrons as is the electrostatic oscillations at the electron plasma frequency.
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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.
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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 presented here to illustrate the variety and significance of wave-particle interactions: 1.
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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.
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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.
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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.
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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.
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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
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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 micropulsations 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.
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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 characteristic as seen in Figure k and illustrated in Figure lOa.
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Also, the apparent source position of the kilometric radiation as observed 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.
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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.
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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.
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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.
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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.
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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)
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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.
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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.
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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.
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From page 302... ...
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 ampli90 fication along the field lines in analogy to Figure 13- Sentman et al.
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considers the nature of the interaction 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 theoretically.
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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.
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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.
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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.
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Three plasma processes, for example, have escaping plasma waves with characteristics that are somewhat similar to the escaping waves from the planetary magnetospheres. These wave phenomena are solar radio bursts, flare star radio outbursts and pulsar radio emissions.
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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.
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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. Observation 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.
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6.1 Recent Major Advances During the past several years, major advances in plasma wave research 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 precipitation fluxes. 6.1.2 Observations of electrostatic noise at the bow shock, magnetosheath, cusp, tail, plasmapause, auroral field lines and the ionosphere -- all regions of significant wave-particle interactions related to many of the fundamental plasma processes listed in the next section.
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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.
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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 electric field regions is discussed by Haerendel (this report)
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For the process of field line merging between the solar wind and magnetosphere magnetic fields at the bow shock and along the magnetosheath
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One likely suggestion is that energy is dissipated through the creation of electrostatic waves. Both broadband electrostatic noise and electrostatic electron cyclotron emissions are observed in the regions where field line merging should be taking place.
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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.
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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 transport of energy from the solar atmosphere through the chromosphere and corona and into the distant solar wind.
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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 magnetospheres of the Earth and, in the future, Jupiter. Once the local process 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.
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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.
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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.
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6.3.U Performance of laboratory plasma experiments, computer simulations, and theoretical analyses that treat specific processes identified in the magnetosphere. More emphasis must be placed on research efforts that complement the direct space measurements especially since the space experiments are being designed to obtain more quantitative results on some 26 well identified processes.
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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.
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Goertz and Joyce did investigate 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 extrapolating these processes into other plasma regimes.
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From page 323... ...
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.
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with limited information by remote sensing and perhaps a few in situ probe measurements.
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0I noise bands on frequen ten superimposed.
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1080 8 § 4i C H ii U)
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1081 W O ft)
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1082 rt -H H d OH C 'H t)
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From page 331... ...
The predicted peak frequencies are obtained by multiplying the Bode Law predicted magnetic field value by the average ratio of the observed peak frequency to polar field value for the Earth and Jupiter. Table 5B.
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From page 332... ...
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.
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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 normal directions nearly along the geomagnetic field direction (0°)
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10 * 0 iO R, EARTH RAOII FIGURE 3b Dayside equatorial magnetospheric model showing the radial variation of the gyro, plasma, upper hybrid and R-mode cutoff frequencies.
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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)
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whistler train. Note that the whistler energy can propagate across magnetic field lines (from Smith and Angerami100)
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. FIGURE 9 Values for the total electron density outside the plasmapause derived from the nonthermal 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)
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Higher frequency components make a large angle to the source field lines (from Mosier and Gurnett73)
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The kilometric radiation appears to be more closely related to the discrete auroral arcs than to the diffuse aurora. On frames 1094 and 1096 both discrete and diffuse aurora are evident along with intense AKR.
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Note that the source regions trace out nightside auroral field lines (from Alexander and Kaiser1)
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From page 341... ...
> 3000 KM V-SHAPED VLF HISS I -30KHz INVERTED-V PRECIPITATION ELECTRONS 1-IOkeV IOOO KM FIELD ALIGNED CURRENTS EMF FROM SOLAR WIND?
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From page 342... ...
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)
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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.
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KT THERMAL DISK I80* K I0I 10° 10" FREQUENCY, MHl .0° .O6 FIGURE 19 Possibility of detecting synchrotron radiation from Uranus.
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From page 345... ...
When these photons propagate at a small angle to a superstrong magnetic field they can produce pairs which also radiate photons, thereby breaking down the vacuum and populating the outer magnetosphere with plasma. The electron-positron plasma is subjected to a two stream instability which produces emission in the radio spectrum (from Kennel59)
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From page 346... ...
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.
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From page 347... ...
L., Terrestrial kilometric radiation: 2-Emission from the magnetospheric cusp and dayside magnetosheath, J Geophys.
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P., and Falthammar, C.-G., Mechanisms that may support magnetic-field-aligned electric fields in the magnetosphere, Ann. Geophys., 32, 161-17U, 1976.
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From page 349... ...
J., Observations of radiation from an electron beam artificially injected into the ionosphere, J Geophys.
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From page 350... ...
C., Propagation perpendiculaire au voisinage de la frequence de la resonance hybride basse, Plasma Waves in Space and in the Laboratory, Edinburgh University Press, 1970.
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From page 351... ...
T., Field-aligned currents, plasma waves and anomalous resistivity in the disturbed polar cusp, J Geophys.
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From page 352... ...
A., The earth as a radio source: terrestrial kilometric radiation, J Geophys.
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From page 353... ...
P., Plasma waves in the distant magnetotail, J Geophys.
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From page 354... ...
, Mode coupling of Cerenkov radiation as a source of noise above the plasma frequency, in K Khott and B
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From page 355... ...
R., Ion cyclotron growth calculated from satellite observations of the proton ring current during storm recovery, J Geophys.
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From page 356... ...
J., Jr., Fluctuating magnetic fields in the magnetosphere, II: VLF waves, Space Sci.
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From page 357... ...
G., Whistler mode plasma waves observed on Electron Echo 2, J Geophys.
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From page 358... ...
and Gurnett, D A., Correlation of bow shock plasma wave turbulence with solar wind parameters, J
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From page 359... ...
J , Jr., Flue tuating magnetic fields in the magnetosphere, 1.
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From page 360... ...
D., The use of multiple receivers to measure the wave characteristics of VLF noise in space, Space Sci.
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From page 361... ...
S., Dyal, P., and Sonnett, C P., Jupiter's magnetic field, magnetosphere and interaction with the solar wind: Pioneer ll, Science, 188, U51-U55, 197598.
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From page 362... ...
Stix, T H., The Theory of Plasma Waves, McGraw-Hill: New York, 1962.
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From page 363... ...
G , A comparison of ULF and VLF measurements of magnetospheric cold plasma densities, J
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