Showing papers on "Whistler published in 1993"

Upstream waves, shocklets, short large‐amplitude magnetic structures and the cyclic behavior of oblique quasi‐parallel collisionless shocks

Abstract: The re-formation process of more oblique quasi-parallel shocks is investigated using one-dimensional hybrid simulations. Several types of simulations have been performed. The simulation of a shock with a magnetic field-shock normal angle of 30° shows that a more oblique quasi-parallel shock exhibits reformation cycles with a larger length scale, that is of about 20 ion inertial lengths. This is considerably larger than the distance specularly reflected ions are able to propagate upstream before they are deflected so that their velocity in the shock normal direction is close to zero. These cycles are due to steepening and growth of upstream waves into pulsationlike structures when they are convected into the region of strongly increasing diffuse ion density immediately upstream of the shock. When the steepening wave packet crashes into the shock, the shock ramp dispersively radiates whistler waves into the region between the shock ramp and the approaching wave, while the steepening of the pulsation leads to phase standing whistler waves on the upstream side. Entropy production occurs either at the shock ramp or at the upstream edge of the pulsation when the steepening process has produced a large kink in the magnetic field and is due to nonadiabatic motion of the incident solar wind ions. In order to analyze the wave steepening, upstream waves have been isolated, and their subsequent interaction with a hot, tenuous ion beam representing the diffuse backstreaming ions has been studied. When an upstream wave is convected into or a region with increasing hot beam density, the wave steepens and becomes a pulsationlike wave packet. In order for the wave to grow to a pulsationlike structure the characteristic scale length of the density increase has to be of the same order as the wavelength of the original magnetosonic wave. Similar results are obtained when counterstreaming beam of hot ions is injected into a solar wind which does not initially contain a wave field. In this case the polarization of the pulsations depends on the hot beam temperature. The strong density increase of hot beam ions in these simulations is due to the steady injection of beam ions in a solar wind with an embedded field which is inclined relative to the solar wind direction. In the shock simulation the shock itself is the steady source of the hot backstreaming ions. These simulations suggest that upstream waves, shocklets, and short large-amplitude magnetic structures are all the same entity in different stages of their development and play a crucial role in re-forming oblique quasi-parallel shocks.

VLF signatures of lightning‐induced heating and ionization of the nighttime D‐region

Abstract: 48.5 kHz signals from a transmitter in Silver Creek, Nebraska, propagating to Huntsville (HU), Alabama over a approximately 1200 km Great Circle Path (GCP) exhibit characteristic amplitude changes which appear within 20 ms of cloud-to-ground (CG) flashes located within 50 km of the path. Data are consistent with the heating of ionospheric electrons by the electromagnetic (EM) pulse from lightning producing ionization changes in the D-region over the thunderstorm.

The role of ducted whistlers in the precipitation loss and equilibrium flux of radiation belt electrons

Abstract: Evidence is presented for a close association betwen individual whistler ducts and conjugate ionospheric disturbances sensed by the perturbation of subionospheric VLF, LF, and MF signals. It is found that even the weakest whistlers can be associated with ionospheric disturbances in both hemispheres. A case study has shown that slow-onset and 'overshoot' perturbation signatures to be consistent with multiple ionospheric disturbances that are associated with individual components of multipath-ducted whistlers.

Pitch angle diffusion of low-energy electrons by whistler mode waves

Abstract: The authors look at the question of precipitating electrons from the radiation belts. The common argument has been that pitch angle scattering of trapped electrons on whistler mode waves was the source of the precipitating electrons. As part of this theory there has been a threshold energy E[sub C] = B[sup 2]/2[mu][sub 0]N, the magnetic energy per particle which has been a threshold for this process. This energy is typically 10 keV. The observation of diffuse aurorae, as distinguished from distinct aurorae, was made in the 1970's. This is a rather weak and structureless ion and electron precipitation which extends around the auroral oval. The intensities are not inconsistent with pitch angle diffusion of plasma sheet electrons into the loss cone as being the source. The authors argue that there is no fundamental energy threshold for the whistler mode interactions, and therefore such scattering can account for the diffuse aurorae. They show theoretically that there is no reason why the characteristic energy E[sub C] should limit the energy of electrons participating in pitch angle diffusion from whistler waves. An additional consequence of this work is that experimental releases of lithium in the outer magnetosphere to stimulate increased precipitation or initiatemore » a magnetic substorm are not likely to succeed, consistent with CRRES lithium releases.« less

A survey of low frequency waves at Jupiter: The Ulysses encounter

Abstract: We report the results of a survey of low-frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 10(exp 2)-s and 5-s, as measured in the spacecraft frame. The 10(exp 2)-s waves are highly nonlinear propagate at large angles to vector-B(sub 0) (typically 50 deg), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 10(exp 2)-s waves had mixed right-and left-hand polarizations. We argue that these are all consistent with being right-hand magnetosonic waves in the solar wind frame. The 10(exp 2)-s waves with attached whistler are similar to cometary waves. The trailing portions are linearly polaraized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by approximately 2-keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5-s waves have delta vector-B/B(sub 0 approximately = 0.5, are compressive and are left-hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by-products of the steepened lower-frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the theta(sub kB) values were consistently in the range of 80 deg to 90 deg, exceptionally high values.

Pulsed currents carried by whistlers. Part I: Excitation by magnetic antennas

Abstract: Time-varying plasma currents associated with low-frequency whistlers have been investigated experimentally. Pulsed currents are induced in the uniform, boundary-free interior of a large laboratory plasma by means of insulated magnetic antennas. The time-varying magnetic field is measured in three dimensions, and the current density is calculated from del x B(r,t) = mu(0)J, where J includes the displacement current density. Typical fields B(r,t) and J(r,t) induced by a magnetic loop antenna show three-dimensional helices due to linked toroidal and solenoidal field topologies. Constant amplitude and phase surfaces assume conical shapes since the propagation speed along B0 is higher than oblique to B0. The electric field in the wave packet contains both inductive and space-charge contributions, the latter arising from the different dynamics of electrons and ions. The dominant electric field in a whistler packet is a radial space-charge field.

On the generation of plasma waves in Saturn's inner magnetosphere

Abstract: Voyager 1 plasma wave measurements of Saturn's inner magnetosphere are reviewed with regard to interpretative aspects of the wave spectrum. A comparison of the wave emission profile with the electron plasma frequency obtained from in situ measurements of the thermal ion density shows good agreement with various features in the wave data identified as electrostatic modes and electromagnetic radio waves. Theoretical calculations of the critical flux of superthermal electrons able to generate whistler-mode waves and electrostatic electron cyclotron harmonic waves through a loss-cone instability are presented. The comparison of model results with electron measurements shows excellent agreement, thereby lending support to the conclusion that a moderate perpendicular anisotropy in the hot electron distribution is present in the equatorial region of L = 5-8.

Interplanetary transport of solar electrons and protons: Effect of dissipative processes in the magnetic field power spectrum

Abstract: Quasi-linear theory describing charged particle transport in interplanetary space within the framework of a slab turbulence picture was for a long time confronted with the problem that the theoretically predicted mean free path for scattering at magnetic field fluctuations was too small when compared with the corresponding parameter derived from fits to observed intensity and anisotropy profiles of solar energetic particles. The theory up to now has neglected the dispersiveness of some of the scattering plasma waves and the effects resulting from the finite temperature of the plasma through which the particles propagate. We have therefore attempted to give a more realistic model of an observed interplanetary turbulence spectrum. The most simple picture we find requires the dissipation range to be constituted by parallel dispersive whistler, electron and ion cyclotron waves in a warm plasma (β ≈ 1). The dispersion relation of heavily thermally affected ion cyclotron waves needed in this model is calculated for the first time. The model is fitted to the turbulence spectrum measured on June 7, 1980, by ISEE 3 (ICE). The derived model parameters are used in a quasi-linear calculation of both proton and electron mean free paths. As a first step resonance broadening due to wave damping is neglected. The results are compared with typical empirical proton mean free paths and with electron mean free paths obtained from fits to electron intensity and anisotropy profiles observed simultaneously by the same satellite. In both cases the theoretical values seem to be systematically larger than the empirical ones. Thus scattering enhancements by nonlinear or thermal resonance broadening or oblique waves now bear the hope to remove the discrepancy between theory and observation.

Generation mechanism of plasmaspheric ELF/VLF hiss: A statistical study from GEOS 1 data

Abstract: ELF/VLF hiss is whistler mode waves that interact with energetic electrons in the magnetosphere. One of the consequences of these interactions is the precipitation of energetic electrons into the atmosphere due to pitch angle diffusion. One related question that is still controversial is how these waves can reach the intensities that are measured. A study of ELF/VLF hiss in the plasmasphere (Solomon et al., 1988) has shown that the observed plasmaspheric hiss intensities can be explained by the presence of a relatively high level of anisotropic energetic electron fluxes, with the wave growth rate γ being directly proportional to the flux level. Sonwalkar and Inan (1989) have argued, however, that this result was based on a limited number of cases only and that lightning-generated whistlers may be a more important source for plasmaspheric hiss. In order to clear up this problem we have performed a statistical study using 75 orbits of GEOS 1, providing 48 events. All of them occurred on the day side at 4 < L < 5, where L is the McIlwain parameter, in the vicinity of the geomagnetic equator during relatively quiet geomagnetic conditions. These events have the usual characteristics of plasmaspheric hiss (frequency range and intensity). Simultaneous measurements of hiss and electron fluxes show on a statistical basis that (1) there is some trend for the strongest wave events to correspond to the largest electron fluxes, (2) the observed fluxes for the 48 events are generally much above the “typical” values used in previous theoretical studies, and (3) in agreement with the suggestion of Solomon et al. (1988) the statistical results are consistent with wave generation in the equatorial region, the plasmaspheric hiss being amplified during a single crossing of the waves through this region.

Plasma waves in the magnetosphere

Abstract: I Propagation and Generation of Plasma Waves.- 1 Basic Equations.- 1.1 Introduction.- 1.2 Electromagnetic Equations.- 1.3 Fluid Equations.- 1.4 The Kinetic Equation.- 1.5 Poynting's Theorem.- 1.6 Harmonic Oscillations.- 1.7 The Wave Equation.- 1.8 Summary.- 2 Waves in a Uniform Cold Magnetoplasma - 1. Infinite Plane Waves.- 2.1 Introduction.- 2.2 Characteristic Frequencies and Speeds.- 2.3 Linearization of the Equation of Motion.- 2.4 Constitutive Relations.- 2.5 Plane Waves.- 2.6 Polarization.- 2.7 Properties of the Refractive Index.- 2.8 Energy Flux in a Plane Wave.- 2.9 Summary.- 3 Waves in a Uniform Cold Magnetoplasma - 2. Rays and Wave Packets.- 3.1 Introduction.- 3.2 Wave Packets and Rays.- 3.3 Classification of Waves in a Cold Plasma.- 3.4 Refractive Index and Dispersion Relation.- 3.5 Summary.- 4 Propagation of Electromagnetic Waves in a Non-Uniform Cold Magnetoplasma.- 4.1 Introduction.- 4.2 Plane Stratified Media.- 4.3 Ray Tracing in General Media.- 4.4 Summary.- 5 Waves in a Uniform Warm Magnetoplasma.- 5.1 Introduction.- 5.2 Characteristic Speeds.- 5.3 The Constitutive Relation.- 5.4 Dispersion Relations and Refractive Index.- 5.5 Polarization.- 5.6 Summary.- 6 Waves in a Hot Plasma - 1. General Features.- 6.1 Introduction.- 6.2 Unperturbed Particle Orbits.- 6.3 Electrostatic Approximation.- 6.4 Propagation Parallel to the Magnetic Field.- 6.5 Growth and Decay of Waves.- 6.6 The Equilibrium Distribution Function - The Maxwellian.- 6.7 Non-Equilibrium Distribution Functions.- 6.8 Summary.- 7 Waves in a Hot Plasma - 2. Equilibrium and Non-Equilibrium Distributions.- 7.1 Introduction.- 7.2 Waves in Plasmas in Thermal Equilibrium.- 7.3 Longitudinal Waves Excited by a Particle Beam.- 7.4 Electrostatic Waves Associated with Anisotropic Distributions.- 7.5 Summary.- 8 The Effect of Wave Fields on Energetic Particles.- 8.1 Introduction.- 8.2 Particle Resonance.- 8.3 Trajectories of Resonant Particles in Velocity Space.- 8.4 Diffusion in Velocity Space.- 8.5 Some Non-Linear Effects.- 8.6 Waves Resonant with a Test Particle.- 8.7 Summary.- II Applications of the Theory to Plasma Wave Observations.- 9 Magnetospheric Plasmas.- 9.1 Introduction.- 9.2 Structure of the Earth's Magnetosphere.- 9.3 Cold Plasma Populations in the Magnetosphere.- 9.4 Hot Plasma Populations.- 9.5 Waves in the Magnetosphere.- 9.6 Summary.- 10 Waves in the Plasmasphere - 1. Whistler Observations and Basic Theory.- 10.1 Introduction.- 10.2 Properties of the Whistler Mode.- 10.3 Observations of Whistlers.- 10.4 Elementary Theory of Whistlers.- 10.5 Use of Whistlers as a Magnetospheric Probe.- 10.6 Summary.- 11 Waves in the Plasmasphere - 2. Details of Whistler Propagation.- 11.1 Introduction.- 11.2 Lightning as a Source of Electromagnetic Radiation.- 11.3 Propagation in the Earth-Ionosphere Waveguide.- 11.4 Transmission of Whistlers Through the Ionosphere.- 11.5 Propagation of Unducted Whistlers.- 11.6 Propagation in Ducts.- 11.7 Summary.- 12 Waves in the Plasmasphere - 3. Ion Cyclotron Whistlers.- 12.1 Introduction.- 12.2 Observations.- 12.3 Ion Cyclotron Whistler Propagation in a Uniform Medium.- 12.4 Nature of Ion Cyclotron Whistler Generation.- 12.5 The Effect of Collisions on Coupling.- 12.6 Summary.- 13 Waves in the Plasmasphere - 4. Doppler Shifted Cyclotron Resonance of Electrons with Whistlers.- 13.1 Introduction.- 13.2 Some Relevant Observations.- 13.3 Whistler Wave-Particle Interaction in a Uniform Medium.- 13.4 Whistler-Mode Noise in a Non-Uniform Medium.- 13.5 Whistler-Mode Signals Generated by Energetic Particles.- 13.6 Summary.- 14 Waves in the Auroral Region.- 14.1 Introduction.- 14.2 Observations.- 14.3 Propagation of Whistler Mode Hiss.- 14.4 Z-Mode Radiation.- 14.5 Terrestrial Myriametric Radiation.- 14.6 Auroral Kilometric Radiation.- 14.7 Summary.- 15 Some Final Words.- 15.1 Introduction.- 15.2 Man-Made Plasma Waves.- 15.3 Ultra Low Frequency Pulsations.- 15.4 Waves in the Magnetotail, Magnetosheath and Solar Wind.- 15.5 Future Work.- A The Essence of Cartesian Tensors.- B Some Mathematical Results.- B.1 Properties of Bessel Functions and Related Results.- B.2 The Plasma Dispersion Function.- C Properties of the Earth's Dipole Field.- D Definition of Symbols.

The scattering of VLF waves by localized ionospheric disturbances produced by lightning‐induced electron precipitation

Abstract: A three-dimensional model of the scattering of VLF waves in the Earth-ionosphere waveguide by localized disturbances in the lower ionosphere is examined for typical disturbances expected to be produced by lightning-induced electron precipitation events. Results indicate that the scattering is generally independent of the conductivity and permittivity of the Earth`s surface immediately beneath the disturbed region except for extremely low conductivities such as that found over deep ice caps. Thus the scattered signal is principally a function of the ionospheric perturbation. For typical disturbances characterized by altitude profiles of enhanced ionization expected for 1.4 {le} L {le} 3, most of the measurable wave energy scatters within a fairly narrow angular region (15-dB beam width of {+-}7{degrees} for a disturbance radius of 100 km) centered on the forward scatter direction. Thus moderate- to large-scale disturbances (radius 50-200 km) must be located within <250 km of a moderate-length path (3000-16000 km) in order to scatter a measurable signal to the receiver. These two findings suggest that the scattered signals can be used with confidence as a diagnostic tool to determine the characteristics of the energetic electron precipitation. 28 refs., 9 figs.

Electron - whistler interaction at the Earth's bow shock: 2. Electron pitch angle diffusion

Abstract: The effect of whistler waves on the electron distribution function is considered for the November 7, 1977, bow shock crossing. Order of magnitude estimates of the diffusion times due to whistler waves and to electrostatic noise shows that whistler waves are also effective in shaping the electron distribution function fnof;e, causing pitch angle diffusion in the limit of low frequencies. A Monte Carlo simulation of the electron dynamics, which includes electrostatic as well as whistler random terms, is then set up. A study of the diffusion coefficients, together with the use of the experimental data on electromagnetic noise, allows us to assess spatial and velocity profiles for the random terms of the simulation. The moments of fnof;e, including the perpendicular temperature and the heat flux densities, are reproduced satisfactorily by the numerical results. The resulting scenario for electron heating in quasi-perpendicular shocks can be described as follows: (1) the shock steady electric field in the deHoffman-Teller frame energizes the electrons, but the parallel temperature is too high and the perpendicular temperature too low, and a hole in fnof;e is formed; (2) parallel diffusion due to the electrostatic noise fills the hole in fnof;e and creates a flat-topped distribution, thus cooling the electrons, but the perpendicular temperature remains too low; (3) pitch angle diffusion due to whistler waves transfers energy from parallel to perpendicular, increasing the perpendicular temperature up to the observed values.

Linear and nonlinear interactions of an electron beam with oblique whistler and electrostatic waves in the magnetosphere

Abstract: Both linear and nonlinear interactions between oblique whistler, electrostatic, quasi-upper hybrid mode waves and an electron beam are studied by linear analyses and electromagnetic particle simulations. In addition to a background cold plasma, we assumed a hot electron beam drifting along a static magnetic field (Bo). Growth rates of the oblique whistler, oblique electrostatic, and quasi-upper hybrid instabilities were first calculated. We found that there are four kinds of unstable mode waves for parallel and oblique propagations. They are the electromagnetic whistler mode wave (WW1), the electrostatic whistler mode wave (WW2), the electrostatic mode wave (ESW), and the quasi-upper hybrid mode wave (UHW). When the angle θ between the wave vector k and Bo is small enough (≤10°), the electrostatic instability is dominant compared with the whistler mode instability. When θ is around 30°, the growth rates of whistler (both electrostatic and electromagnetic) mode waves and electrostatic mode waves are of the same order. When θ increases to 60°, the WW2 mode will be the most unstable mode wave. For a very large θ, (∼ 80°), the WW2 instability still has positive growth rates, and the UHW instability begins to have positive growth rates. Electromagnetic particle simulations were performed for parallel and three oblique cases, θ = 0°, 30°, 60° and 80°. When θ = 0°, whistler mode waves can hardly grow from the thermal fluctuation level because the electron beam which is supposed to provide free energy to the whistler mode waves is quickly diffused in the velocity space by much faster growing ESW. The ESW can lead to a secondary electrostatic instability. With θ = 30°, both electrostatic and whistler mode waves grow simultaneously. Also, the electrons diffused by the whistler mode instability to higher υ∥ velocity regions can lead to a secondary electrostatic instability. When θ is 60°, diffusion of the electron beam is controlled mainly by the WW2 instability. For θ = 80°, both WW2 and UHW grow despite their small growth rates. The simulations agree with linear analyses on the ESW growth rate for θ = 0°, but from our simulation data, growth rates of oblique whistler mode, electrostatic mode and quasi-upper hybrid mode waves are usually smaller than those predicted by linear analyses. The electrostatic and whistler mode instabilities affect each other through their interactions with the electron beam. While the most intense ESW is generated for parallel propagation, the most intense whistler mode wave is observed at an oblique direction. Modulations between different electrostatic waves are found for θ = 0° after the electric field reaches its saturation level. Modulations between whistler waves (θ = 60°,80°) are also found while their magnetic fields increase nonlinearly and reach their saturation levels. A possible mechanism is proposed to explain the satellite observations of whistler mode chorus and accompanied electrostatic waves, whose amplitudes are sometimes modulated at the chorus frequency.

Electron - whistler interaction at the Earth's bow shock: 1. Whistler instability

Abstract: The interaction of whistler waves with the electrons in the Earth's quasi-perpendicular bow shock of November 7, 1977, is considered. A Monte Carlo simulation which includes the effect of electrostatic noise is used to obtain the electron distribution function fnof;e across the shock layer. The simulated fnof;e is rather anisotropic and in the shock foot has a marked loss cone structure which is evident in the data, too; therefore we study the instability of fnof;fe with respect to the emission of whistler modes. Strong whistler emission from the electrons is found, with frequency and spatial dependencies of the growth rate in good agreement with the observed magnetic noise. Most of the whistlers are generated in the shock foot, where the loss cone in the electron distribution function is found. This indicates that in quasi-perpendicular collisionless shock, besides the reflected ion beam and the field-aligned electron beam, an important source of whistler mode waves is the electron loss cone due to reflection from the magnetic ramp.

Whistlers and plasmaspheric hiss: Wave directions and three-dimensional propagation

Abstract: Wave propagation directions are determined on the basis of wave data from the DE 1 satellite showing simultaneously nonducted whistlers and hiss. Hiss wave normal angles are determined as about 70 and 77 deg for f = 3.5 and 2.5 kHz, respectively, with the wave vector being almost perpendicular to the meridional plane. A novel approximate analytical formulation of 3D propagation of whistler waves is developed and used to model the drift of magnetospherically reflected whistlers in azimuth. It is shown that depending on initial parameters, the time of arrival of whistler rays at a fixed observation point can differ by 10-20 s, with signals from different magnetospherically reflected whistlers overlapping to evolve into a hisslike signal. The total azimuthal drift of whistler rays is found to not exceed about 30 deg, so that plasmaspheric hiss may be produced by nonducted whistlers at longitudes correlated with the location of thunderstorm activity.

VLF wave growth from dispersive bursts of field-aligned electron fluxes

Abstract: Large-amplitude electrostatic whistler waves near the lower hybrid frequency were observed by an auroral sounding rocket during substorm breakup. The measured wavelengths indicate that the emissions were electrostatic and resonant with electrons that had parallel energies of a few hundred electron volts. We propose that the intense emissions drew their energy from dispersive bursts of low-energy, field-aligned electron fluxes. The dispersive bursts are known to cause a brief, but intense instability that results in large-amplitude Langmuir emissions. The high-frequency emissions can rapidly form a plateau in the one-dimensional electron distribution. We show, however, that these distributions remain unstable to electrostatic whistler waves near the lower hybrid frequency. The amplitude and wavelength of the observed emissions were sufficient to accelerate the hydrogen ions with energies between about 50 eV and about 200 eV.

The absorption mechanisms of whistler waves in cool, dense, cylindrically bounded plasmas

Abstract: It can be shown that whistler waves with low parallel phase velocity can be subject to strong cyclotron damping, even when the wave frequency is well below the cyclotron frequency. This resonance arises as a result of the Doppler effect which is proportional to the plasma density and increases the frequency experienced by electrons moving in the opposite direction to the wave. In this paper the dispersion relation of whistler waves in a cylindrically bounded plasma is obtained and then solved. This geometry is desirable for its relevance to experiment and also for comparison with theoretical results by other authors. In addition to cyclotron damping, Landau damping as well as electron–ion, electron–electron, and electron–neutral collisions are all included thus enabling the relative importance of these damping mechanisms to be evaluated. The dispersion relation that is obtained is used to explore the transition from Landau dominated damping to cyclotron dominated damping.

Generation of VLF saucer emissions observed by the Viking satellite

Abstract: Simultaneous observations by the Viking satellite of electric and magnetic fields as well as charged particles have been used to investigate V-shaped wave phenomena. The intensity of these VLF and ELF emissions is V-shaped when shown in a frequency versus time plot. Simultaneous observations of V-shaped so-called VLF saucer emissions, particles and field-aligned currents strongly suggest, for the first time, that upgoing electrons with energies less than a few hundred electron volts can generate these waves. Broadband waves observed inside the saucer generation region, from frequencies much less than the ion cyclotron frequency up to the plasma frequency, may also be generated by these electrons. Viking observations of VLF saucers at altitudes between 4000 km and 13,500 km show that these emissions occur at higher altitudes than discussed in previous reports. The generation regions seem to be more extended at these higher altitudes than what has been reported at lower altitudes by other observers.

An interpretation of the broadband VLF waves near the Io torus as observed by Ulysses

Abstract: The requirements for the Ulysses trajectory to attain high ecliptic latitudes using a Jovian gravitational assist resulted in a fortuitous passage through the Io torus region. Specifically, the spacecraft spent many hours at latitudes just above the torus. During this time the low-frequency cutoff of an ordinary mode (O mode) emission allowed a determination of the local electron plasma frequency (i.e., electron density) along the northern flank of the torus. Also, near a Jovian System III longitude of 100 deg, the spacecraft flew past a set of active field lines that have been previously identified to be associated with the hectometric generation region. During the passage, Ulysses observed a newly discovered O mode component and a whistler mode emission similar to that observed by Voyager 1 13 years previously. All of the broadband VLF emissions imply the presence of a particular population of electrons. We suggest that broadband VLF emissions can be used as a `particle detector' to qualitatively measure the electron plasma conditions in the torus region and identify active regions.

On the origin of short large-amplitude magnetic structures upstream of quasi-parallel collisionless shocks

Abstract: In order to model the development of the turbulence upstream of quasi-parallel shocks, one-dimensional hybrid simulations have been performed of the injection of a very hot ion population into a cold incident flow ULF electromagnetic waves are initially generated in the right-hand resonant ion beam mode As these waves are convected to the injection source and encounter larger beam densities, they grow in amplitude and scatter the beam ions Beam ion clumps characterized by distributions with a significant fraction of ions having velocities antiparallel to the beam bulk velocity are generated, and destabilize the left-hand resonant ion beam mode This process enables the formation of enhanced magnetic field structures which, like the short large-amplitude magnetic structures (SLAMS) observed upstream of the quasi-parallel Earth's bow shock, show scale lengths smaller than the ULF waves wavelength, have magnetic amplitudes up to a few times the ambient magnetic field, are left-hand polarized in the plasma rest frame, and often possess attached whistler waves 10 refs, 5 figs

A wave‐wave interaction in whistler frequency range in space plasma

Abstract: The quantitative theory is developed for the frequency spectrum broadening and sideband structure of VLF signals in the upper ionosphere. The theory is based on a common theory of wave-wave interaction in plasma and includes the transformation effects of a quasi-monochromatic whistler wave into quasi-electrostatic whistler mode waves (l waves) near the resonance cone with their interaction with field-aligned plasma density (electric field) inhomogeneities. Such inhomogeneities can be ion-acoustic or ion cyclotron electrostatic waves. We consider both cases of regular (deterministic plases) and irregular (random phases) inhomogeneities. The spectral characteristics and interaction length of l waves and some nonstationary effects are considered, which permit us to obtain the quantitative interpretation of main experimental data. The conditions of high coherence of interacting waves are investigated, which are compared with the results of bicoherence analysis of sideband structure of VLF signals. The estimations of bicoherence value are given for such type of wave-wave interaction.

Propagation characteristics of waves upstream and downstream of quasi-parallel shocks

Abstract: The propagation characteristics of waves upstream and downstream of quasi-parallel shocks are investigated by using 2D hybrid simulations. At low Alfven Mach numbers, M(A) below about 2, the shock is initially associated with upstream phase-standing whistlers. At later times, backstreaming ions excite longer-wavelength whistlers via the right-hand resonant ion/ion instability. These waves propagate along the magnetic field at a group velocity no smaller than the upstream flow speed, so that the waves remain in the upstream region. At higher MA (above about 3), these waves are convected back into the shock, causing its reformation and downstream perturbations. Shock transmitted waves mode-convert into Alfven/ion-cyclotron waves which have a wave vector along the shock normal (pointing upstream) and convect downstream. The 2D simulation results confirm our earlier suggestion that the upstream waves should be field aligned, and that their convection into the downstream is associated with linear mode conversion into the Alfven/ion-cyclotron branch.

ULF waves in the Io torus: Ulysses observations

Abstract: Throughout the Io torus, Ulysses has observed intense ultralow frequency (ULF) wave activity in both electric and magnetic components Such ULF waves have been previously suggested as the source of ion precipitation leading to Jovian aurorae The peaks of the wave spectra are closely related to the ion cyclotron frequencies, which is evidence of the waves being ion cyclotron waves (ICWs) Analysis of the dispersion relation using a multicomponent density model shows that at high latitudes (approximately 30 deg), peak frequencies of the waves fall into L mode branches of guided or unguided ICWs Near the equator, in addition to the ICWs below f(sub cO(2+)), there are strong signals at approximately 10 Hz which require an unexpectedly large energetic ion temperature anistropy to be explained by the excitation of either convective or nonconvective ion cyclotron instabilities Their generation mechanism remains open for the future study Evaluation of the Poynting vector and the dispersion relation analysis suggest that the waves near the equator had a small wave angle relative to the magnetic field, while those observed at high latitudes were more oblique The polarization of the waves below f(sub cH(+)) is more random than that of the whistler mode waves, but left-hand-polarized components of the waves can still be seen The intensity of the ICWs both near the equator and at high latitudes are strong enough to meet the requirement for producing strong pitch angle scattering of energetic ions

Low and high frequency waves generated by gyrophase bunched ions at oblique shocks

Abstract: The interaction of the solar wind with gyrophase bunched ions reflected at an oblique (ΘBn = 45°) shock ( MA=8) is simulated using a 1D hybrid code. These ions are unstable to low frequency hydromagnetic waves as well as whistler waves. At some point in the hydromagnetic wave growth ion trapping occurs which restricts the local distribution in gyrophase to a higher degree than the input distribution. Given the growth rate of the MHD waves, gyrophase mixing is ineffective in isotropising the ions, except at the leading edge of the injected distribution.

Effect of AC electric field on the whistler mode instability in the magnetosphere

Abstract: The dispersion relation for the whistler mode instability in an infinite bi-Maxwellian magnetoplasma in the presence of a parallel ac electric field has been derived using the method of characteristic solutions. The perturbed and unperturbed particle trajectories have been used to determine the perturbed distribution function. The growth rate has been determined. The earlier known results are recovered under suitable approximations, when the ac field amplitude and frequency are set to zero. It is found that the ac field parameters modify the growth rate and the resonance condition of the charged particles. The growth rate for various frequencies and magnitudes of the ac field, suitable to magnetospheric conditions, has been evaluated. It has been found that the growth rate depends significantly on the variation of the frequency rather than on the variation of the magnitude.

Waves in the whistler range directed by channels containing high-density plasma

Abstract: Waveguide propagation of waves in the whistler frequency range is studied in ducts with high plasma density. Attention is concentrated primarily on channels with width comparable to the whistler wavelengths. It is shown that under certain circumstances such channels can maintain weakly outgoing whistler modes. Results are presented from numerical calculations of the dispersion properties and field structures of the corresponding azimuthally symmetric modes directed by a cylindrical waveguide. 9 refs., 4 figs.

Observation of intense wave bursts at very low altitudes within the Venus nightside ionosphere

Abstract: Intense ELF (100 Hz) bursts were detected by the Pioneer Venus Orbiter plasma wave instrument during the final operations of the spacecraft prior to atmospheric entry. These bursts were detected at approx. 130 km altitude around 0400 local time. The wave activity lasted for several tens of seconds. Furthermore the bursts were not symmetric about periapsis, unlike instrument noise caused by neutral impacts on the spacecraft. The bursts had a vertical attenuation scale height of the order 1 km, consistent with that expected for whistler-mode waves propagating through a collisional ionosphere. Since the decay of the signals appears to be due to attenuation, the source must persist for several tens of seconds. The wave bursts could therefore be the signature of electromagnetic radiation entering the bottomside ionosphere from several distant sources, as would be expected if lightning were a relatively persistent phenomenon within the Venus atmosphere.