Local Electron Plasma Research Articles

On the generation of solitary waves observed by Cluster in the near-Earth magnetosheath

Abstract. Through case studies involving Cluster waveform observations, solitary waves in the form of bipolar and tripolar pulses have recently been found to be quite abundant in the near-Earth dayside magnetosheath. We expand on the results of those previous studies by examining the distribution of solitary waves from the bow shock to the magnetopause using Cluster waveform data. Cluster's orbit allows for the measurement of solitary waves in the magnetosheath from about 10 RE to 19.5 RE. Our results clearly show that within the magnetosheath, solitary waves are likely to be observed at any distance from the bow shock and that this distance has no dependence on the time durations and amplitudes of the solitary waves. In addition we have found that these same two quantities show no dependence on either the ion velocity or the angle between the ion velocity and the local magnetic field direction. These results point to the conclusion that the solitary waves are probably created locally in the magnetosheath at multiple locations, and that the generation mechanism is most likely not solely related to ion dynamics, if at all. To gain insight into a possible local generation mechanism, we have examined the electron differential energy flux characteristics parallel and perpendicular to the magnetic field, as well as the local electron plasma and cyclotron frequencies and the type of bow shock that Cluster is behind, for several time intervals where solitary waves were observed in the magnetosheath. We have found that solitary waves are most likely to be observed when there are counterstreaming (~parallel and anti-parallel to the magnetic field) electrons at or below about 100eV. However, there are times when these counterstreaming electrons are present when solitary waves are not. During these times the background magnetic field strength is usually very low (<10nT), implying that the amplitudes of the solitary waves, if present, would be near or below those of other waves and electrostatic fluctuations in this region making it impossible to isolate or clearly distinguish them from these other emissions in the waveform data. Based on these results, we have concluded that some of the near-Earth magnetosheath solitary waves, perhaps in the form of electron phase-space holes, may be generated locally by a two-stream instability involving electrons based on the counterstreaming electrons that are often observed when solitary waves are present. We have not ruled out the possibility that the solitary waves could be generated as a result of the lower-hybrid Buneman instability in the presence of an electron beam, through the electron acoustic mode or through processes involving turbulence, which is almost always present in the magnetosheath, but these will be examined in a more comprehensive study in the future.

Electron-beam-generated plasmas for materials processing

The results of investigations aimed at characterizing pulsed, electron-beam-produced plasmas for use in materials processing applications are discussed. In situ diagnostics of the bulk plasma and at the plasma/surface interface are reported for plasmas produced in Ar, N 2, and mixtures thereof. Langmuir probes were employed to determine the local electron temperature, plasma density, and plasma potential within the plasma, while ion energy analysis and mass spectrometry were used to interrogate the ion flux at an electrode located adjacent to the plasma. The results illustrate the unique capabilities of electron-beam-produced plasmas and the various parameters available to optimize operating conditions for applications such as nitriding, etching, and thin film deposition.

Electrical Field Excitation in Non-Uniform Plasma by a Modulated Electron Beam

Excitation of electric fields due to a modulated electron beam in a warm non-uniform plasma is treated for weak beams in warm plasma. It is shown that the maximum electric field magnitude that is reached near the local plasma resonance point depends significantly on the direction of the electron stream motion. In collisional plasma the magnitude of the Langmuir wave that propagates to the subcritical plasma also depends on the direction of the electron stream motion. The motion of the modulated electron stream front results in beatings between oscillations on the modulation frequency and on the local electron plasma frequencies at the initial moment. Later these beatings damp in the supercritical plasma, whereas in the subcritical plasma they are transformed into spatial beatings between the field of the modulated electron stream and the excited Langmuir wave.

Experimental Study of Planar Langmuir Probe Characteristics in Electron Current‐Carrying Magnetized Plasma

AbstractExperimental study of planar Langmuir probe characteristics in a magnetized plasma with an electron current along the direction of the magnetic field shows that the usual procedure for determination of the electron temperature and plasma density, which is applicable in a current‐free magnetized plasma, gives erroneous results for these plasma parameters. When this procedure is applied on the characteristics measured at two opposite orientations of the probe collecting surface with respect to the direction of the electron drift, different values of the electron temperature are obtained. These virtual electron temperatures and corresponding plasma densities calculated from the measured ion saturation currents are higher and/or smaller than the exact local electron temperature and plasma density. Calculation of particular averages of these quantities is proposed as a possible way to obtain correct results for the local electron temperature and plasma density. These averages are used in the approximate evaluation of the electron drift velocity from the electron saturation currents measured at the two orientations of the probe collecting surface.

HF plasma emission detected in the cusp region at ionospheric altitude

HF waves measurements performed in the cusp area on board the altitude orbiting satellite Magion-3 reveal the existence of two types of emission: whistler mode waves and strong wide band emissions, reaching the local electron plasma frequency. The HF emissions restricted to the cusp region, provide a spatial picture of the ionospheric region and are good tools to describe the plasma properties both in situ and at remote distance from the satellite. Thermal measurements detected at the topside ionosphere are presented. This paper infers about the mechanism of broadband emission in the cusp region at low altitudes.

Local resonant excitation of plasma oscillations in a planarsurface-wave plasma device

The paper presents space-resolved microwave intensity measurements in a surface-wave plasma device clearly demonstrating the existence of a local resonance in a resonance layer where the local electron plasma frequency is equal to the surface-wave frequency. It has already been suggested in the literature that Landau damping of electron plasma waves excited in such a resonance layer and/or stochastic electron heating there might contribute to the surface-wave plasma energy balance. Since this does not involve collisions, it may become an important energy channel at low gas pressures. In order to avoid having the resonance layer too close to the plasma boundary, the measurements were performed not in the original surface-wave plasma, but using a weak non-ionizing 2.4 GHz microwave propagating in an inductively coupled plasma created by an internal loop antenna fed by a high-power 13.56 MHz generator. Still the original surface-wave plasma source geometry, including the microwave input port, was preserved. The resonance layer was identified by the microwave intensity peak, which was found to shift on changing the plasma density profile and/or the wave driving frequency in compliance with theoretical expectations. Accompanying space-resolved plasma density measurements confirmed this interpretation.

Properties of ELF emissions in the dayside magnetopause

We undertake a comprehensive study of the extremely low frequency emissions within the magnetopause current layer using measurements obtained with ISEE 1 spacecraft. Since the emissions vary greatly, both spatially and temporally, to derive meaningful and robust statistical results is challenging. We design an averaging procedure and choose the normalized wave amplitudes for a correlation analysis. Also, we separate the parameters for the correlation with local or global properties. We found that most of the wave power in the current layer is carried by the magnetic field fluctuations, while the electric field carries relatively little energy. These emissions are best correlated with the local magnetic field shear and electron plasma beta. The power increases as the shear and beta increase. The wave is correlated with the global interplanetary magnetic field (IMF) clock angle, although it is weaker than with the local shear. The wave is weakly correlated with the IMF Bz and is not correlated with the electron temperature anisotropy. The correlation of the electric wave field with our chosen parameters is weaker than and varying from that of the magnetic wave field. These ELF emissions have the characteristics of the whistler mode dispersion with an increasing phase velocity above the lower hybrid frequency and with a cutoff at the electron cyclotron frequency. The wavelength is ∼20 to 100 km. In a case study, the electric field is polarized nearly perpendicular to the background magnetic field.

The Direct Amplification of Electromagnetic Waves by Electron Beams: An Alternate Explanation for Solar Type III Bursts

The direct amplification of electromagnetic (EM) waves by the beam-plasma instability is proposed as an alternative for solar type III bursts in corona and planetary space, the main properties of type III bursts at meter bands are compared to this model, including the cutoff frequency, the timescale, the relative bandwidth, the polarization sense and degree, and the harmonic emission. This mechanism may explain the reason why the cutoff frequency or the low-frequency limit of type III bursts observed by the Ulysses spacecraft is much larger than the local electron plasma frequency, which may have resulted from the variation of the heliocentric distance of the spacecraft as well as the ratio between the local electron plasma and cyclotron frequencies.

Galileo plasma wave observations near Europa

In this paper we present results from the Galileo plasma wave instrument during the first two flybys of Europa, which occurred on December 19, 1996, and February 20, 1997. Strong whistler‐mode noise was observed in the vicinity of Europa during both flybys. Emission at the upper hybrid resonance frequency, fUH, and a propagation cutoff at the local electron plasma frequency, fpe, provided measurements of the local electron number density. The electron density measurements show a region of highly disturbed plasma in the vicinity of Europa with density enhancements ranging from about 30 to 100 cm−3 above the ambient Jovian magnetospheric background, which in both cases was about 80 cm−3.

Modulated electron plasma waves observed in the tail lobe: Geotail waveform observations

We present the results of the Geotail waveform observation on the electrostatic plasma waves with their frequency close to the local electron plasma frequency in the tail lobe. Those waves, which we call “electron plasma waves” in the present paper, are strongly modulated in their amplitudes and are very similar to the modulated Langmuir waves observed by previous spacecraft in the various regions. From the detailed analyses, we conclude that they include the Langmuir waves and 5 types of the Electron Cyclotron Harmonic (ECH) waves. Finally, we introduce an interesting event that the observed waves change from the ECH waves to the Langmuir waves in a short period within one second.

Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves

Abstract. Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 – 4 × 10–9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.

Density fluctuations measured by ISEE 1-2 in the Earth's magnetosheath and the resultant scattering of radio waves

Radio waves undergo angular scattering when they propagate through a plasma with fluctuating density. We show how the angular scattering coefficient can be calculated as a function of the frequency spectrum of the local density fluctuations. In the Earth's magnetosheath, the ISEE 1-2 propagation experiment measured the spectral power of the density fluctuations for periods in the range 300 to 1 s, which produce most of the scattering. The resultant local angular scattering coefficient can then be calculated for the first time with realistic density fluctuation spectra, which are neither Gaussian nor power laws. We present results on the variation of the local angular scattering coefficient during two crossings of the dayside magnetosheath, from the quasi-perpendicular bow shock to the magnetopause. For a radio wave at twice the local electron plasma frequency, the scattering coefficient in the major part of the magnetosheath is b(2fp) ≃ 0.5 – 4 × 10–9 rad2/m. The scattering coefficient is about ten times stronger in a thin sheet (0.1 to1RE) just downstream of the shock ramp, and close to the magnetopause.

Frequency‐Shifted Signature of the HF Pump in the Ionospheric Focused Heating Experiment

Ionospheric heating experiments have begun to examine interactions at altitudes where the heater HF wave frequency matches the local electron plasma frequency. We discuss the frequency‐shifted signature observed by radiowave receivers on a rocket launched into the HF beam and approaching the interaction altitude. The frequency shifts associated with the upgoing (transmitted) and the downgoing (reflected) waves arise from the Doppler effect and changes in the wave vector. The characteristic spectral signature provides a method to locate the reflection altitude accurately. Generally, the ratio of intensities of upgoing and downgoing waves decreases as the reflection altitude is approached.

CERA-V: Microwave plasma stream source with variable ion energy

A microwave plasma stream source with variable ion energy operated under low magnetic field electron cyclotron resonance conditions has been developed. A two mode resonant cavity (TE111, E010) was used. It was established that overdense plasma creation (TE111) and high energy in-phase space localized electron plasma oscillations (E010) in a decreased magnetic field lead to the potential for ion energy variation from 10 to 300 eV (up to 1 A of ion current, and a plasma cross section of 75 cm2, hydrogen) by varying the TE111, E010 power, the value of the magnetic field, and pressure. The threshold level of E010-mode power was also determined. An application of this CERA-V source to hydrogenation of semiconductor devices without deterioration of surface layers by ions and fast atoms is under investigation.

Spatial distribution of electron plasma oscillations in the Earth's foreshock at ISEE 3

Electric field oscillations recorded by the 10–56 kHz channels of TRW's plasma wave detector during parts of two of the ISEE 3 circumterrestrial orbits in 1983 have been used to make the first mapping of Earth's electron plasma wave foreshock. By combining data from the two trajectory segments, each of which provided relatively meager spatial sampling outside the bow shock, but high variation of interplanetary magnetic field (IMF) direction, a first‐order pattern of occurrence of electron plasma waves, hence also backstreaming electrons, has been determined. We depict the pattern with an adaptation of the mapping program previously used for the Venus electron foreshock. As at Venus, plasma wave activity was concentrated most densely along the IMF line tangent to the bow shock where energized electrons stream against the solar wind from the quasi‐perpendicular part of the shock. The size of the Earth's electron plasma wave foreshock, however, is vastly greater than that of Venus, implying that a foreshock's dimension scales with the size of its diamagnetic obstacle and associated bow shock. Our mappings with three additional ISEE 3 channels surrounding the local electron plasma frequency indicate a richer distribution of waves in the foreshock than the single electron frequency channel of Pioneer Venus Orbiter could detect around Venus.

Observation of fundamental magnetoplasma emissions excited in magnetosphere by modulated electron beams

There were several cases when modulated electron beam had been injected from APEX satellite into an otherwise unmodified ionospheric plasma. The beams were formed from 2 μs pulses repeated at a rate of 40kHz. Injections took place in the altitude range 400–1100km over Europe. The onboard receiver connected to the dipole antenna swept the frequency interval 1–10 MHz in 1 sec. Due to a relatively narrow receiver bandwidth (15 kHz), moderate frequency step (50 kHz), slow changes of f n and f c (local electron plasma and gyro frequencies) fine structure of excited emissions was detected. In many cases, a very prominent doublet could be convincingly identified as a ( f n,f u=sqrt(f n ∗f n+f c ∗f c) ) band, with f n practically equal to the local plasma frequency in the unperturbed ionosphere. Determination of plasma frequency is of prime importance in analysis of complex structures composed from various harmonics f n,f c,f u. Complexity is manifested as coalescing of various harmonics with maxima shifted from nominal frequencies or splitting them into components due to mixing of local and propagation effects. Despite the existence of strong emissions on frequencies characteristic for the unperturbed ionosphere, very strong broadcasting transmissions are frequently cut off, even when the beam is directed upward. Most typical spectra are discussed.

Fine structure of Langmuir waves observed upstream of the bow shock at Venus

Highly structured Langmuir waves, also known as electron plasma oscillations, have been observed in the foreshock of Venus using the plasma wave experiment on the Galileo spacecraft during the gravity assist flyby on February 10, 1990. The Galileo wideband sampling system provides digital electric field waveform measurements at sampling rates up to 201,600 samples per second, much higher than any previous instrument of this type. The main Langmuir wave emission band occurs near the local electron plasma frequency, which was approximately 43 kHz. The Langmuir waves are observed to shift above and below the plasma frequency, sometimes by as much as 20 kHz. The shifts in frequency are closely correlated with the downstream distance from the tangent field line, implying that the shifts are controlled by the electron beam velocity. Considerable fine structure is also evident, with timescales as short as 0.15 ms, corresponding to spatial scales of a few tens of Debye lengths. The frequency spectrum often consists of beat‐type waveforms, with beat frequencies ranging from 0.2 to 7 kHz, and in a few cases, isolated wave packets. The peak electric field strengths are approximately 1 mV/m. These field strengths are too small for strongly nonlinear processes to be important. The beat‐type waveforms are suggestive of a parametric decay process.

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

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°, 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.

Observations of electron plasma waves upstream of the Jovian bow shock

The Ulysses spacecraft encountered the planet Jupiter in February 1992, on its journey towards high heliospheric latitude. During the approach to the planet, as well as on the outbound pass, while receding from the Jovian bow shock, the Plasma Frequency Receiver that is part of the Unified Radio and Plasma Wave experiment (URAP) recorded bursts of plasma waves in the frequency range of a few kHz. These emissions, first observed by the PWS experiment onboard the Voyager spacecraft, have been identified as upstream electron plasma waves. In this paper, we present the first analysis of the characteristics of these emissions, which are very similar to those found in the Earth's electron foreshock, upstream of the Earth's bow shock. These bursty emissions, with a peak frequency very close to the local electron plasma frequency F pe, have a typical electric field amplitude in the range 0.01–0.1 mV m −1, with some bursts above 1 mV m −1. The frequency bandwidth over which significant power can be found above the instrument background noise ranges from below 0.2 F pc to about 2 F pc. On the basis of our present knowledge of similar emissions observed at Earth, we suggest that the broadband emissions are triggered by suprathermal (a few tens of eV) electrons, streaming back from Jupiter's bow shock.

Ulysses observations of auroral hiss at high Jovian latitudes

During the Ulysses flyby of Jupiter, a whistler‐mode emission was periodically detected by the Unified Radio and Plasma wave (URAP) experiment during intervals when the spacecraft extended to high magnetic latitudes. The signal was detected between the local electron plasma frequency and lower hybrid resonance and appears as a funnel‐shaped structure on frequency‐versus‐time spectrograms; these characteristics are very reminiscent of whistler‐mode auroral hiss observed at high latitudes at Earth. Ray tracing of the emission occurrences suggests the emission source is on magnetic field lines extending out to at least 65 RJ. This location associates the emission with the boundary between open and closed field lines — not the Io torus. The emission radiates about 107 W of power. Consequently, the auroral input power derived from the solar wind to drive the emission is believed to be 1010‐12 W (or about 1% of the energy associated with Io torus electrical processes).