Showing papers on "Whistler published in 1990"

Television image of a large upward electrical discharge above a thunderstorm system.

Abstract: An image of an unusual luminous electrical discharge over a thunderstorm 250 kilometers from the observing site has been obtained with a low-light-level television camera. The discharge began at the cloud tops at 14 kilometers and extended into the clear air 20 kilometers higher. The image, which had a duration of less than 30 milliseconds,resembled two jets or fountains and was probably caused by two localizd electric charge concentrations at the cloud tops. Large upward discharges may create a hazard for aircraft and rocket launches and, by penetrating into the ionosphere, may initiate whistler waves and other effects on a magnetospheric scale. Such upward electrical discharges may account for unexplained photometric observations of distant lightning events that showed a low rise rate of the luminous pulse and no electromagnetic sferic pulse of the type that accompanies cloud-to-earth lightning strokes. An unusually high rate of such photometric events was recorded during the night of 22 to 23 September 1989 during a storm associated with hurricane Hugo.

Electrostatic lower hybrid waves excited by electromagnetic whistler mode waves scattering from planar magnetic‐field‐aligned plasma density irregularities

Abstract: This paper presents a theoretical model for electrostatic lower hybrid waves excited by electromagnetic whistler mode waves propagating in regions of the magnetosphere and the topside ionosphere, where small-scale magnetic-field-aligned plasma density irregularities are thought to exist. In this model, the electrostatic waves are excited by linear mode coupling as the incident electromagnetic whistler mode waves scatter from the magnetic-field-aligned plasma density irregularities. Results indicate that high-amplitude short-wavelength (5 to 100 m) quasi-electrostatic whistler mode waves can be excited when electromagnetic whistler mode waves scatter from small-scale planar magnetic-field-aligned plasma density irregularities in the topside ionosphere and magnetosphere.

Steepening of kinetic magnetosonic waves into shocklets: Simulations and consequences for planetary shocks and comets

Abstract: The generation and the nonlinear evolution of oblique low-frequency electromagnetic (kinetic magnetosonic) waves which were observed upstream of planetary bow shocks and at the Giacobini-Zinner comet, and referred to as shocklets, were investigated using an electromagnetic hybrid code. The observations show that the waves, which have a sinusoidal form when their amplitude is small, become steepened and linearly polarized as they grow in amplitude. The results of simulations show the original small-amplitude elliptically polarized wave grows and steepens, so that its polarization changes and becomes somewhat linear. The steepening process is associated with the coherent generation of a broad spectrum of waves on the magnetosonic whistler branch, which propagate at various phase and group velocities. It is shown that the presence of shocklets upstream of a planetary bow shock can modify its local structure by changing the solar wind Mach number and temperature, or by colliding with the shock.

Ion reflection and dissipation at quasi‐parallel collisionless shocks

Abstract: Large scale one-dimensional hybrid simulations have been performed of a quasi-parallel (ΘBn = 20°) high Mach number collisionless shock. It is found that backstreaming reflected ions, i.e., upstream ions with velocities exceeding the shock ram velocity, originate from the outer part (v≳ 1.7vth) of the velocity space of the incident distribution. The backstreaming ions produce very low-frequency magnetosonic waves which propagate upstream with about 1.3VA (Alfven speed). As the wave crests convect toward the shock, they steepen up and the shock reforms itself. During shock reformation a large part of the incident ions are reflected. This, in turn, slows the incident ions down. The slowed down incident particle distribution and the reflected particle distribution merge and constitute the new thermalized downstream distribution. In the interval of a relatively stationary shock low-frequency whistler waves stand at the shock front. During these time intervals the whistler waves are probably responsible for dissipation by nonadiabatic compression of the incident ions. The whistler waves are destroyed by the incoming large amplitude wave crest and reemerge at the new shock front. The reapparance seems to be due to the nonlinear steepening of the incoming wave crest at the upstream side.

Whistlers in Neptune's magnetosphere: Evidence of atmospheric lightning

Abstract: During the Voyager 2 flyby of Neptune, a series of 16 whistler-like events were detected by the plasma wave instrument near closest approach. These events were observed at radial distances from 1.30 to 1.99 R sub N and magnetic latitudes from -7 to 33 deg. The frequencies ranged from 6.1 to 12.0 kHz, and the dispersions fit the Eckersley law for lightning-generated whistlers. Lightning in the atmosphere of Neptune is the only known source of such signals. The frequency range of the whistlers (up to 12 kHz) indicates that the local electron densities are substantially higher (N sub e greater than 30 t0 100 per cu cm) than indicated by the in situ plasma measurements. The dispersion of the whistlers is very large, typically 26,000 sec Hz(exp 0.5). Based on existing plasma density models and measurements, the dispersions are too large to be accounted for by a single direct path from the lightning source to the spacecraft. Therefore, multiple bounces from one hemisphere to the other are required. The most likely propagation path probably involves a lightning source on the dayside of the planet, with repeated bounces through the dense dayside ionosphere at low L-values.

Whistlers in the solar corona and their relevance to fine structures of type IV radio emission

Abstract: This short report concerns a general consideration of whistler manifestations in fine structures, including possible trajectories of obliquely propagating whistlers, the role of quasilinear diffusion and an interpretation of new observations. A whistler ray tracing and kinetic whistler growth rates under arbitrary angles to the magnetic field in the solar corona were calculated. Different regimes of a whistler instability yield divers elements of fine structures: different stripes in emission and absorption or millisecond pulsations, moreover, zebra-stripes can convert into fiber bursts and inversely. A new explanation of low-frequency absorption in fibers is proposed: it is connected with an attenuation of plasma-wave instability due to the fast electron diffusion by whistlers. Rope-like chains of fiber bursts are explained by a periodic whistler instability in a magnetic reconnection region.

VLF heating of the lower ionosphere

Abstract: A controlled wave-injection experiment with a 28.5 kHz transmitter having a radiated power of 100 kW has revealed evidence of ionospheric heating by the VLF waves. Calculations indicate that the observed effect can be attributed to the absorption of wave energy in the lower ionosphere, which is estimated to result in a 30% enhancement in the collision frequency at 85 km. This process also represents a new means of direct coupling of lightning energy to the lower ionosphere.

Currents between tethered electrodes in a magnetized laboratory plasma

Abstract: Laboratory experiments on important plasma physics issues of electrodynamic tethers were performed. These included current propagation, formation of wave wings, limits of current collection, nonlinear effects and instabilities, charging phenomena, and characteristics of transmission lines in plasmas. The experiments were conducted in a large afterglow plasma. The current system was established with a small electron-emitting hot cathode tethered to an electron-collecting anode, both movable across the magnetic field and energized by potential difference up to V approx.=100 T(sub e). The total current density in space and time was obtained from complete measurements of the perturbed magnetic field. The fast spacecraft motion was reproduced in the laboratory by moving the tethered electrodes in small increments, applying delayed current pulses, and reconstructing the net field by a linear superposition of locally emitted wavelets. With this technique, the small-amplitude dc current pattern is shown to form whistler wings at each electrode instead of the generally accepted Alfven wings. For the beam electrode, the whistler wing separates from the field-aligned beam which carries no net current. Large amplitude return currents to a stationary anode generate current-driven microinstabilities, parallel electric fields, ion depletions, current disruptions and time-varying electrode charging. At appropriately high potentials and neutral densities, excess neutrals are ionized near the anode. The anode sheath emits high-frequency electron transit-time oscillations at the sheath-plasma resonance. The beam generates Langmuir turbulence, ion sound turbulence, electron heating, space charge fields, and Hall currents. An insulated, perfectly conducting transmission line embedded in the plasma becomes lossy due to excitation of whistler waves and magnetic field diffusion effects. The implications of the laboratory observations on electrodynamic tethers in space are discussed.

Electromagnetic instabilities attributed to a cross-field ion drift.

Abstract: Instabilities due to a cross-field ion flow are reexamined by including the electromagnetic response of the ions, which has been ignored in existing discussions. It is found that this effect can lead to significant enhancement of the growth rate. Among the new results, a purely growing, electromagnetic unstable mode with a wave vector k parallel to the ambient magnetic field is found. The plasma configuration under consideration is similar to that used in the discussion of the well-known modified-two-stream instability. This instability has a growth rate faster than the ion cyclotron frequency, and is not susceptible to high-plasma-beta stabilization.

Upstream waves at mercury, Venus and earth: Comparison of the properties of one Hertz waves

Abstract: Previous studies have shown that the Venus foreshock region contains low-frequency upstream waves similar to those in the terrestrial foreshock, but perhaps with different amplitudes than at earth. This paper compares the properties of a second class of upstream waves, analogous to the so-called 1 Hz waves at earth. The waves observed at Mercury, Venus, and earth have very similar properties, i.e., propagation angles less than 55 degrees to the magnetic field and less than 35 degrees to the solar wind flow direction. The waves occur exclusively on the field lines connected to the bow shock. They are most commonly left-hand elliptically polarized with similar fractional amplitudes, approximately 0.1 of the background field strength. Their amplitudes decrease with increasing distance from the shock. The observed frequencies are similar for Mercury, Venus, and earth when scaled by the interplanetary magnetic field. If, as generally assumed at earth, these waves arise in regions of backstreaming electrons, these results imply that similar electron foreshocks occur at earth, Venus and Mercury despite differences in bow shock size and the nature of the obstacle to the solar wind.

Intense ionospheric electric and magnetic field pulses generated by lightning

Abstract: Electric and magnetic field measurements have been made in the ionosphere over an active thunderstorm and an optical detector onboard the same rocket yielded an excellent time base for the study of waves radiated into space from the discharge. In addition to detection of intense, but generally well understood whistler mode waves, very unusual electric and magnetic field pulses preceded the 1-10 kHz component of the radiated signal. These pulses lasted several ms and had a significant electric field component parallel to the magnetic field. No known propagating wave mode has this polarization nor a signal propagation velocity as high as those measured here. This study investigated and rejected an explanation based on an anomalous skin depth effect. Although only a hypothesis at this time, a more promising explanation involving the generation of the pulse via a nonlinear decay of whistler mode waves in the frequency range 10-80 kHz is being investigated.

Density measurements in key regions of the Earth's magnetosphere: Cusp and auroral region

Abstract: Two active experiments, a relaxation sounder and a mutual impedance probe, have been implemented on board Viking. These active measurements, together with measurements from the electron spectrometers, are used to determine the plasma density in the cusp region, up to 13,500 km, the apogee of Viking, as well as in the sources of the auroral kilometric radiation (AKR). When active experiments are switched on in the cusp region, several plasma resonances are detected; they correspond to zero group velocity waves, indicative of a characteristic frequency of the plasma, namely the plasma frequency, and the fqn (n = 1, 2, 3,…n) series, above fuh, the upper hybrid frequency. The fqn series allow an independent estimate of the plasma density. Plasma density in the cusp proper is found to be much larger (> 100 cm−3 in some cases) than in the adjacent regions. An attempt is also made to estimate the respective densities and temperatures of the various components of the plasma. In contrast, auroral regions are low-density regions (fpe/fce ≪ 1). The active sounding of the plasma by a relaxation sounder gives a resonance at fuh, which allows an estimate of the plasma density. Low-frequency whistler mode emissions are commonly observed in the night sector. Their upper cutoff frequency has often been used for estimating the plasma frequency. Active experiments are used to test this method, which is shown to be valid most of the time in the high-altitude auroral region, yet also sometimes misleading in other regions. Once the estimate of the density through the upper cutoff frequency of the hiss is validated, it can be used to follow with a good time resolution the sharp density variations experienced as Viking crossed AKR sources. It is found that the density within AKR sources is of the order of even less than 1 cm−3.

Space-time evolution of whistler mode wave growth in the magnetosphere

Abstract: The modeling and simulation of the growth of ducted single-frequency whistler-mode waves through cyclotron resonance with radiation belt electrons are considered. The Siple transmitter and a geomagnetically conjugate receiving station are used for the present study. The results can be applied to slowly varying frequency signals and to broadband 'noiselike' signals.

The plasma wave signature of a ``magnetic hole'' in the vicinity of the magnetopause

Abstract: Wave spectral measurements in the region of the September 4, 1984 magnetic hole obtained with the plasma wave instrumentation aboard the AMPTE IRM spacecraft are presented. The instrument is briefly described. The full wave data is given and possible reasons for the typical form of the spectra inside the hole are discussed. Relevant observations are presented and different wave modes and their possible origins are discussed. A summary is given with a discussion of ideas about the origin and formation of holes.

Spaced direction finding of nighttime whistlers at low and equatorial latitudes and their propagation mechanism

Abstract: The propagation mechanism of low-latitude and equatorial latitude whistlers is investigated on the basis of spaced direction finding measurements in South China. Observations were made continuously in the local time (LT) interval from 0000h to 0400h during the period of January 5–11 1988 at the three stations (Zhanjiang (geomagnetic latitude 10.1°), Guilin (14.1°), and Wuchang (19.4°)) and two horizontal magnetic components and one vertical electric field component were simultaneously recorded over a wide frequency range to enable comprehensive direction finding. Two major occurrence peaks on January 5 and January 6 and two minor ones on January 9 and January 11 have been analyzed and the following experimental results have emerged. (1) The whistler occurrence at very low latitudes is generally very small compared with that at low latitude (geomagnetic latitude ≳20°), but once it occurs, the occurrence rate becomes comparable to that at low latitudes. (2) The whistler dispersion is single-valued at any particular local time. (3) The ionospheric exit region of whistlers is very much restricted to the geomagnetic latitude range of 10°–14°, and there are no observed whistlers whose path latitude is between 14° and 20°. (4) The extent of their ionospheric exit region is very stable on different days, and the radius of the distribution is less than 40–50 km. (5) Surprisingly high occurrence of echo train whistlers is observed. (6) The propagation of whistlers in the Earth-ionosphere waveguide after ionospheric transmission is more likely toward higher latitudes than toward the equator and the subionospheric propagation seems to exhibit a horizontal beaming around the magnetic meridian plane. We attempt to interpret these characteristics of the observations in terms of either nonducted or field-aligned propagation, but discussion indicates that especially findings 4 and 5 are strongly indicative of field-aligned propagation for very low latitude whistlers localized in geomagnetic latitudes of 10°–14°.

The magnetoionic modes and propagation properties of auroral radio emissions

Abstract: The nature of the magnetoionic wave modes which accompany the aurora is clarified here by a detailed analysis, using multiple techniques, of DE 1 auroral radio observations. All four of the possible magnetoionic wave modes are found to occur, apparently emitted from two different source regions on the same auroral field line. AKR originates primarily in the X mode near the electron cyclotron frequency, and is frequently also accompanied by a weaker O-mode component from the same location. The next most prominent auroral emission is the W-mode auroral hiss originating from altitudes always well below the DE 1 satellite at frequencies below the local cyclotron frequency. The previously reported Z-mode auroral radiation was also detected, but from sources also below the satellite at the poleward edge of the cavity, and not from the expected AKR source at the cyclotron frequency.

Observations and modeling of wave‐induced microburst electron precipitation

Abstract: Energy-time features of X ray microbursts are examined and compared with the predictions of a test particle simulation model of wave-induced electron precipitation resulting from gyroresonant wave-particle interactions in the magnetosphere. The observations were obtained on a balloon flight at Siple station, Antarctica (L ≈ 4.2) on December 30, 1980. The energy and time evolution of the microbursts were studied for X rays in the energy range from 25 to 175 keV. An algorithm designed to search the E > 25 keV counting rate data for single isolated microbursts identified 651 events in a 3-hr interval. The distribution of burst durations ranged from 0.2 to 1.2 s. Approximately two-thirds of the distribution were narrow bursts (0.2–0.6 s), the rest wide (0.6–1.2 s), with the average burst durations equal to ∼0.4 s and ∼0.7 s, respectively, for the two classes. The precipitation was characterized by exponential electron spectra with e-folding energies Eo of 25-50 keV. Individual and superposed microburst profiles show that the X ray energy spectrum is softest (equivalent to a reduction in electron Eo of ≤ 5 keV) near the peak of the energy influx. Computer simulations of the flux- and energy-time profiles of direct and mirrored electron precipitation induced by a whistler-mode wave pulse of 0.2-s duration and linear frequency increase from 2 to 4 kHz were performed for plasma, energetic particle and wave parameters appropriate for the location and geophysical conditions of the observations. The wave pulse is representative of the VLF chorus elements that were observed at Siple station and at the conjugate location of Roberval, Quebec during the microburst activity on this day. In both the direct and mirrored cases the combined effect of latitude dependence of the energy of maximum gyroresonant scattering and energy dispersion leads to energy spectrum variations qualitatively consistent with the microburst observations. The average burst durations in the two cases were ∼0.3 s (direct) and ∼0.7 s (mirrored). On this basis we identify the narrow microbursts with electrons which are precipitated directly after undergoing pitch-angle scattering interactions with the wave pulse; the wide microbursts are identified with electrons which, after interacting with the wave, mirror in the conjugate ionosphere before precipitating into the southern ionosphere. In general, the results provide further support for the gyroresonant test particle simulation model, and for the belief that the observed type of microbursts originates in the vicinity of the magnetic equator in a gyroresonant process involving discrete chorus emissions.

Force-free electromagnetic pulses in a laboratory plasma.

Abstract: A short, intense current pulse is drawn from an electrode immersed in a magnetized afterglow plasma. The induced magnetic field B(r,t) assumes the shape of a helical double vortex which propagates along B(0) through the uniform plasma as a whistler mode. The observations support a prediction of force-free (J x B + neE = 0) electromagnetic fields and solitary waves. Energy and helicity are approximately conserved.

Electron beam injection during active experiments: 1. Electromagnetic wave emissions

Abstract: The wave emissions produced in Echo 7 experiment by active injections of electron beams were investigated to determine the properties of the electromagnetic and electrostatic fields for both the field-aligned and cross-field injection in such experiments and to evaluate the sources of free energy and relative efficiencies for the generation of the VLF and HF emissions. It is shown that, for typical beam energies in active experiments, electromagnetic effects do not substantially change the bulk properties of the beam, spacecraft charging, and plasma particle acceleration. Through simulations, beam-generated whistlers; fundamental z-mode and harmonic x-mode radiation; and electrostatic electron-cyclotron, upper-hybrid, Langmuir, and lower-hybrid waves were identified. The characteristics of the observed wave spectra were found to be sensitive to both the ratio of the electron plasma frequency to the cyclotron frequency and the angle of injection relative to the magnetic field.

Path-dependent properties of subionospheric VLF amplitude and phase perturbations associated with lightning

Abstract: A comprehensive study of lightning-associated amplitude and phase perturbations on multiple VLF/LF signals (Trimpi events) observed at Stanford, California and at Palmer Station, Antarctica, has revealed a number of new properties that appear to be characteristic of the particular signal paths. (1) Signal amplitude changes are on the whole evenly distributed between enhancement and attenuation, but some individual signal paths have strong preferences for one or the other. (2) Phase changes on almost all paths show a strong preference for advancement, with phase retardation occurring rarely. (3) The range in size of amplitude and phase changes appears to be relatively constant for a given path, but it is found to vary between different paths. None of the existing models of the Trimpi effect are found to explain all of the observed new features. Instead, the new experimental findings provide an empirical framework to guide the evaluation of more sophisticated models. Analysis also indicates that the magnitudes of simultaneous amplitude and phase changes are only weakly correlated and that the recovery signatures of amplitude and phase events can be substantially different, with the signal amplitude generally recovering faster. This apparent independence of amplitude and phase perturbations is interpreted to result frommore » the altitude distributed nature of the ionospheric disturbances.« less

Simultaneous Disturbance of Conjugate Ionospheric Regions in Association With Individual Lightning Flashes

Abstract: Characteristic whistler-associated amplitude perturbations of subionospheric VLF or LF signals (“Trimpi events”) observed within one second of each other at Palmer Station, Antarctica and at Arecibo, Puerto Rico suggest that ionospheric regions in both northern and southern hemispheres are disturbed together in association with individual lightning flashes. During a one hour period on March 21, 1989, the onsets of 44 out of 47 perturbations measured on a 21.4 kHz signal from Maryland to Arecibo occurred within l s of perturbation onsets measured on a 23.4 kHz signal from Hawaii to Palmer Station. Similar activity occurred before and after this period, and on the preceding and following days. The observations are consistent with the disturbance of geomagnetically conjugate ionospheric regions by multiple bounces between hemispheres of bursts of radiation belt electrons, scattered in pitch angle by whistlers in the magnetosphere. Analysis of patterns of perturbations with corresponding whistler and lightning information from this period suggests that there were at least two distinct ionospheric disturbances in each hemisphere.

Whistler instability in an electron-cyclotron-resonance-heated, mirror-confined plasma

Abstract: The warm‐electron‐driven (2 keV) whistler electron microinstability [Phys. Rev. Lett. 59, 1821 (1987)] of the Constance B electron‐cyclotron‐resonance‐heated (ECRH), quadrupole mirror‐confined plasma experiment has been studied. Experiments show (i) that the instability comes in fairly regular bursts on axis and continuously in time off axis due to the minimum‐B geometry, (ii) a frequency spectrum that is insensitive to changes in the plasma parameters, and (iii) instability‐induced power losses which are not greater than 10% of the ECRH power input for the regimes studied. A linear perturbation analysis of the relativistic Vlasov equation together with Maxwell’s equations has been made. Using the ECRH distribution function, a new distribution function well suited for describing ECRH, mirror‐confined plasmas, the analysis shows the instability frequency spectrum to be insensitive to changes in cyclotron frequency, temperature, and density, in agreement with experimental results, and only sensitive to chan...

Observation of the Z mode with DE 1 and its analysis by three-dimensional ray tracing

Abstract: Certain Z-mode wave emissions in the earth's magnetosphere have been identified using the wave spectra and polarization measurements of the DE 1 satellite. Although such emissions accompany the aurora, and thus presumably originate from the evening-sector auroral zone, they are found to occur over much wider ranges of latitude and longitude. Since the predicted cyclotron maser emission at the cyclotron frequency could not have produced waves which travel such great distances, as shown by three-dimensional ray tracing, it is proposed instead that these emissions must originate from lower altitudes within the auroral zone and probably from near the plasma frequency inside the auroral plasma cavity.

Modeling of induced currents from electrodynamic tethers in a laboratory plasma

Abstract: The presently accepted picture of the current path for electrodynamic tethers envisions a quasi-dc current flow in a 'phantom loop' consisting of the tether, two field-aligned current channels into the ionosphere and a cross-field closing current in the E-layer. Predictions are made on the establishment and maintenance of a current loop in space based on observations of time-dependent currents between tethered electrodes in a large laboratory magnetoplasma. In addition to radiation from the contactors ('whistler wings'), the insulated tether is observed to emit waves (a 'whistler wedge'). The 'wedge' provides closure during loop formation by carrying cross-field polarization currents. Whistler spread within the ray cone leads to overlapping of the current wings not far from the tether hence minimizing the role of the ionospheric closure. Maintenance of the loop requires the continuous emission of whistler waves by the entire tether thereby providing severe radiation losses.

The ionospheric effects of the solar eclipse.

Abstract: In this paper, the observations and theoretical interpretations of the ionospheric effects of the previous 6 solar eclipses are summarized. The ionospheric observations of solar eclipse effects are made at 18 ionospheric stations of 14 provinces in China. The first one of the 6 solar eclipses occurred, on 20 June 1955, the last one on 18 March 1988. In addition to the ionospheric observations, the observations of Faraday rotation of satellite beacons and the whistlers were made some time during the solar eclipses. They have shown that:(1)The optical eclipse effects on the E and F1 region and the dynamic effect on the F2 region are all quite clear;(2)The values of f0F2 on the eclipse day, comparing with those in the control day, could decrease, increase or remain unchanged;(3)There are either concave or non-concave outline for the curves of the TEC, the values of the TEC on the eclipse day comparing with those in the control day could increase during the solar eclipse.The dynamic characteristics of the F2 region and outer ionosphere during the solar eclipses were also discussed briefly. It is found that striking decrease of the space plasma temperature and the space plasma diffusion along magnetic field lines are main factors of the solar eclipse effects on the F2 region and outer ionosphere, and the plasma diffusion along the magnetic field line over the geomagnetic equator, the fountain effect, the thermospheric wind and position of the zone of the total (annular) solar eclipse are important factors of the ionospheric effects of the solar eclipse on the magnetic equatorial anonaly.

An analysis of whistler mode radiation from a 100 mA electron beam

Abstract: Observations of whistler mode radiation generated by 2-, 4-, and 8-keV electron beams with a current of 100 mA, are analyzed. The electron accelerator was carried to ionospheric heights by a Nike Black Brant V rocket (National Research Council of Canada NVB-06). The instability causing the whistler mode radiation is investigated. Spectral measurements (0.1–13.0 MHz), from a sweeping receiver located on the ejected forward payload, are used to determine the nature of the instability. The sweeping receiver was connected alternatively to an electric or a magnetic dipole antenna. Most of the whistler mode radiation detected was consistent with Cerenkov radiation. The radiation fields observed were too large (cB ∼ 0.1 µV/m Hz1/2) to be explained by incoherent processes. If electrostatic bunching in the beam at the plasma frequency is responsible for the whistler radiation, there would be a correlation between the plasma frequency radiation, and the whistler mode radiation for electron beams that are fired toward the detector. The observed correlation is minimal. Hence no evidence was found to support the hypothesis that electrostatic bunching at the plasma frequency was responsible for the enhancement of the whistler mode radiation produced by the NVB-06 electron beam.

Associated ground‐based observations of optical aurorae and discrete whistler waves

Abstract: Discrete VLF wave emissions in association with auroral light intensity fluctuations have been recorded at L = 4 during simultaneous operation of a broadband VLF receiver and a low light level TV system. The correlated wave-particle interaction events are interpreted as being typical examples of the electron cyclotron resonance process responsible for the precipitation of energetic electrons in the vicinity of the plasmapause. A range of delay times between the correlated VLF wave and the optical signals, coupled with the frequency versus time characteristics of the VLF wave emissions, is consistent with the electron cyclotron source region being randomly located within 8,000 km of the equatorial plane. The events are shown to be triggered by ducted whistler mode waves that originate from the opposite hemisphere to that of the station at which the observations are made. Events characterized by VLF waves of rising tone are presumed to occur in a region of enhanced plasma density commonly observed within the plasma trough. It is proposed that this region is responsible for ducting the VLF waves from the source to ground.