Electric Field Emissions Research Articles

The inner magnetosphere of Saturn: Cassini RPWS cold plasma results from the first encounter

We present new results from the inner magnetosphere of Saturn obtained by the Radio and Plasma Wave Science (RPWS) investigation onboard Cassini around the period of the Saturn orbit injection (July 1, 2004). Plasma wave electric field emissions, voltage sweeps by the Langmuir probe (LP) and radio sounder data were used to infer the cold plasma (<100 eV) characteristics within 20 RS of Saturn. A dense (<150 cm−3) and cold (<7 eV) plasma torus was found just outside the visible F‐ring. This torus of partly dusty plasma does not perfectly co‐rotate with Saturn, which suggests the cold plasma is electro‐dynamically coupled to the charged ring‐dust particles. The spacecraft potential was a few volts negative above the E‐ and G‐rings, indicating the dust‐particles were likewise negatively charged. The cold ion characteristics changed near the magnetically conjugate position of Dione, indicating release of volatile material from this icy moon.

ＭＵＳＥＳ‐ＣイオンエンジンシステムのＥＭＩ計測

Radiated electric field emissions from the prototype model of the Ion Engine System (IES) of the MUSES-C mission were measured in accordance to MIL-STD-461 E. The average noise level exceeded the narrowband specification at frequencies less than 5 MHz. The microwave discharge neutralizer generates a broadband noise and narrowband oscillations which have a fundamental frequency of about 160 kHz and are accompanied by its harmonics up to the 5th. The leakage of the 4.25 GHz microwave for plasma production and its second harmonic were 65 dB and 35 dB above specification, respectively. The X-band receiver onboard the MUSES-C measured the noise from the IES at the up-link frequency of 7.2 GHz through a horn antenna. This susceptibility test proved that the microwave discharge ion thruster will never interfere the deep space microwave communication.

マイクロ波放電型中和器とホローカソードからの電磁雑音

Noise and oscillatory behavior of a plasma column produced in front of the microwave discharge neutralizer developed for MUSES-C mission were experimentally investigated. Radiated electric field emissions were measured following to MIL-STD-461 E. The average noise level exceeded the narrowband specification by 30 dBμV/m at frequencies less than 5 MHz. Noise in electron emission current was also measured by using a current probe and a spectrum analyzer, and was compared with the noise of a hollow cathode. The microwave discharge neutralizer generates a broadband noise and oscillations which have a fundamental frequency of about 160 kHz and are accompanied by its harmonics up to the 5th. Considering the dependence on the diameter of the plasma column, they are probably the radial oscillation modes of ion acoustic waves. Although the hollow cathode shows nearly the same noise level at frequencies less than 1 MHz, intense oscillation exists in the 1-10 MHz range, which is generated by the keeper plasma.

Temporal behaviour of lightning HF radiation at 3 MHz near the time of first return strokes

Simultaneous recordings of the electric field and high frequency emissions (HF) at 3 MHz at times close to the first return stroke are presented. It is shown that there is no time delay between the beginning of the return stroke electric field and the build up of the HF. The delay observed in previous studies may be due to propagation effects. The results in turn suggest that the first return stroke from the very beginning is a strong source of HF at 3 MHz.

Short‐wavelength electrostatic waves in the Earth's magnetosheath

Recent observations with the ISEE 1 spacecraft have found electric field emissions in the dayside magnetosheath whose frequency spectrum is modulated at twice the spacecraft spin period. The upper frequency cutoff in the frequency‐time spectrum of the emission has a characteristic parabola shape or “festoon” shape. The low‐frequency cutoff ranges from 100 to 400 Hz, while the high‐frequency limit ranges from about 1 to 4 kHz. The bandwidth is found to minimize for antenna orientations parallel to the wave vectors. The wave vector does not appear to be related to the local magnetic field, the plasma flow velocity, or the spacecraft‐sun directions. The spacecraft observed frequency spectrum results from the spacecraft antenna response to the Doppler‐shifted wave vector spectrum which exists in the plasma. Imposed constraints on the plasma rest frame wave vectors and frequencies indicate that emissions occur within the frequency range from about 150 Hz to 1 kHz, with wavelengths between about 40 and 600 m. These constraints strongly suggest that the festoon‐shaped emissions are ion‐acoustic waves. The small group velocity and k direction of the ion‐acoustic mode are consistent with wave generation upstream at the bow shock and convection downstream to locations within the outer dayside magnetosheath.

Three-component non-symmetric counter-streaming instabilities: oblique propagation case

The effects of the background plasma as well as of the non-resonant plasma-cyclotron and cyclotron–cyclotron interactions on the oblique unstable electrostatic waves generated by resonant plasma–cyclotron (P–C) and cyclotron–cyclotron (C–C) interactions in magnetized non-symmetric two-stream plus background plasma systems are theoretically investigated. Two main effects are reported: (a) preferential increase or decrease of the maximum growth rates of certain m−modes for appropriate plasma parameters and (b) an additional contribution to the frequency shift from the integer multiple values of half the cyclotron frequency. Closed analytical expressions for the maximum growth rates and frequency shifts of both plasma-cyclotron and cyclotron–cyclotron unstable interactions are presented and discussed. The theoretical results obtained in this paper are supported by recent experimental results on electron counter-streaming instabilities in the presence of significant background plasma concentrations in which dominant electric field emissions at 3Ωe/2 have been observed to occur.

Non-symmetric two-stream instability

The recent Plum Brook—NASA experiments on counterstreaming plasma instabilities in which electric field emissions at frequencies [(n + ½) ±α]ωe (n = 1, 2, 3,…, 9, α < 0·5) have been observed, raised the theoretical problem of the non-symmetric two-stream instability. This is the case in which the counterstreaming beams have different velocities in the directions parallel and perpendicular to the static magnetic fields as well as different particle density.We investigate theoretically this problem. The instability of quasi-electrostatic waves at shifted half odd integer multiples of the cyclotron frequency, due to two non-symmetric counterstreaming electron beams, is considered. The beam velocities parallel and perpendicular to the static magnetic field are represented by different double Dirac delta functions; no parameter limitation is imposed. A systematic investigation of (i) the coupling between plasma modes and cyclotron modes and (ii) the coupling between cyclotron modes (beam 1) and cyclotron modes (beam 2) resulting in shifted half odd integer multiples of the cyclotron frequency is carried out.The results include approximate but simple analytical expressions for maximum growth rates and for marginal stability as well as exact numerical solutions for the detailed unstable spectra (1 ≤ m ≤ 20) in both ωτ- and kr-space. The relative weakening or suppression of certain modes is also predicted.

Laboratory observations of RF emissions at ωpeand (n+ ½)ωcein Electron beam-plasma and beam-beam interactions

Electric field emissions at frequencies of (n + ½)fce have been generated in a large-scale electron beam experiment in the giant NASA vacuum facility in Ohio. These emissions arise when a contrastreaming beam configuration exists, the primary beam consisting of monoenergetic electrons (50 eV to 5 keV) and the other beam of lower-energy ‘backscattered’ secondary electrons. It is suggested that the same mechanism could also be the source of the observed emissions at auroral latitudes. In the absence of the beam-beam instability, weak oscillations were observed at the plasma frequency. In the latter case no significant modifications of the primary beam velocity distribution occurred, and the beam configuration is adequately described by single-particle motion in the ambient magnetic field.

Recent studies of magnetospheric electric field emissions above the electron gyrofrequency

In a previous paper Kennel et al [1970] presented a survey of VLF electric field observations in the magnetosphere made with the short electric dipoles aboard Ogo 5. Particular emphasis was placed on the geomagnetic equatorial region between L = 4 ,and L = 10. The preliminary findings in that paper suggested that the predominant emissions were those at odd half-integral multiples of the local electron gyrofrequency ƒce and that, among these multiples, emissions at ƒ ≈ 3ƒce/2 were by far the most common, although other multiples, including ƒce/2, were also observed at times. Kennel et al. [1970] noted that the waves have large electric field amplitudes (1–10 mv/m), and they suggested that 3ƒce/2 emissions could be important sources of turbulent energization and pitch angle diffusion for auroral zone electrons.

VLF electric field observations in the magnetosphere

Magnetospheric VLF electric field emissions above electron cyclotron frequency from OGO 5 observation at magnetic equator