High and low frequency instabilities driven by counter-streaming electron beams in space plasmas

Abstract

A four-component plasma composed of a drifting (parallel to ambient magnetic field) population of warm electrons, drifting (anti-parallel to ambient magnetic field) cool electrons, stationary hot electrons, and thermal ions is studied in an attempt to further our understanding of the excitation mechanisms of broadband electrostatic noise (BEN) in the Earth's magnetospheric regions such as the magnetosheath, plasmasphere, and plasma sheet boundary layer (PSBL). Using kinetic theory, beam-driven electrostatic instabilities such as the ion-acoustic, electron-acoustic instabilities are found to be supported in our multi-component model. The dependence of the instability growth rates and real frequencies on various plasma parameters such as beam speed, number density, temperature, and temperature anisotropy of the counter-streaming (relative to ambient magnetic field) cool electron beam are investigated. It is found that the number density of the anti-field aligned cool electron beam and drift speed play a central role in determining which instability is excited. Using plasma parameters which are closely correlated with the measurements made by the Cluster satellites in the PSBL region, we find that the electron-acoustic and ion-acoustic instabilities could account for the generation of BEN in this region.

The effect of a warm electron beam on slow electron-acoustic solitons

“…The general trend observed in Fig. 7(d) that the cool electron density at which the switch to positive polarity solitons occurs increases with the drift velocity is explained in terms of the large magnitude of the sum of the positive signed contributions of the warm and hot electrons in (11), which will require a large density of the cool electrons n ce0 [negative signed contribution to (11)] to reduce the (positive signed) contribution of the hot electrons, for balance. The (positive signed) contribution of the ions (fourth term) is negligible in that it can be ignored because of the large ion to electron mass ratio (l i ¼ 1836).…”

“…The value v dbw0 ¼ 1:084 at which a switch to positive polarity solitons occurs is close to the minimum threshold value v dbw0 ¼ 1:094 for which the slow electronacoustic mode becomes unstable. We recall that the sign of (11) gives the polarity of small amplitude solitons. The vanishing of the third derivative of the Sagdeev potential (d 3 VðUÞ=dU 3 Þ U¼0 ¼ 0) at v dbw0 ¼ 1:084 and the reduction (increase) in the width of the soliton M ranges for v dbw0 < 1:084 (v dbw0 > 1:084) near v dbw0 ¼ 1:084 are consistent with a very large number of studies 14,15,[18][19][20] in which polarity switches have been reported for electron-acoustic solitons.…”

“…The simulations show that the temperature of the ions plays a much more significant role than the ion flow speed in the formation of the ESWs, which is in agreement with the characteristics of BEN inferred from satellite observations. 2 Whilst kinetic studies [7][8][9][10][11] have shown that the linear electron-acoustic wave accounts for frequencies up to the local electron plasma frequency, solitons which form in the nonlinear stage of the electron-acoustic instability are likely to be responsible for frequencies which exceed the local electron plasma frequency. 12,13 The mentioned studies highlight the importance of the contributions of both linear and nonlinear electron-acoustic waves to high frequency BEN.…”

Linear theory of low frequency magnetosonic instabilities in counterstreaming bi-Maxwellian plasmas

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] However, magnetized plasmas often experience counterstreaming along the ambient magnetic field, modifying a plasma velocity distribution function to a counterstreaming bi-Maxwellian or a counterstreaming kappa distribution function. In this case, much less results are available, especially as the analytical studies of the phenomena [21][22][23][24][25][26][27][28][29][30] are concerned. Since there is not yet a complete picture of kinetic instabilities in plasmas with counterstreaming distribution functions, the present manuscript might make a certain contribution to this field of researches.…”