Simulation studies of plasma waves in the electron foreshock: The generation of downshifted oscillations

Abstract

The generation of waves with frequencies downshifted from the plasma frequency, as observed in the electron foreshock, is analyzed by particle simulation. Wave excitation differs fundamentally from the familiar excitation of the plasma eigenmodes by a gentle bump‐on‐tail electron distribution. Beam modes are destabilized by resonant interaction with bulk electrons, provided the beam velocity spread is very small. These modes are stabilized, starting with the higher frequencies, as the beam is broadened and slowed down by the interaction with the wave spectrum. Initially, a very cold beam is also capable of exciting frequencies considerably above the plasma frequency, but such oscillations are quickly stabilized. Low‐frequency modes persist for a long time, until the bump in the electron distribution is completely “ironed” out. This diffusion process also is quite different from the familiar case of well‐separated beam and bulk electrons. A quantitative analysis of these processes is carried out.

Langmuir Turbulence and Suprathermal Electrons

“…However, Dum (1990aDum ( , 1990bDum ( , 1990c carried out detailed particle-in-cell simulations to show that the dominant particle trapping behavior observed in early simulations were partly owing to the insufficient mode resolution and small system size. He proceeded to demonstrate with his refined simulations that quasilinear/weak turbulence theories are actually quite good for certain parameter regime.…”

Nonlinear development of weak beam–plasma instability

“…60,64,65 Almost a decade had passed when Liang and Diamond 66 carried out a correct renormalized kinetic analysis to show that the effects due to turbulent trapping on the velocityspace diffusion is insignificantly small, thus reestablishing the validity of quasilinear theory. Meanwhile, on the basis of carefully designed numerical simulations, Dum 51 demonstrated that early simulations which showed wave trapping as a dominant saturation mechanism were largely due to a small simulation size and insufficient mode resolution. Dum's simulations showed that the wave trapping is but a transient phenomenon, a finding in agreement with experimental findings by Dimonte and Malmberg, 59 for instance.…”

Generation of harmonic Langmuir mode by beam-plasma instability

“…1,2 Although turbulence theories based upon random-phase approximation cannot describe coherent nonlinear processes such as particle trapping, [3][4][5] which was observed in early numerical simulations [6][7][8] and experiments [9][10][11][12] and thought to be one of the primary nonlinear saturation mechanisms for the beam-plasma instability, it is now known largely in part due to more carefully designed simulations 8,[13][14][15][16][17][18][19][20][21][22][23] and experiments, 24 -28 that for a weak or sufficiently warm beam, the incoherent turbulence theories provide a rather good description of the beamplasma instability development. 17,29,30 Among these theories, a straightforward perturbative theory called the weak turbulence theory, [31][32][33][34][35][36][37][38][39][40][41][42][43][44] which uses the wave-field amplitude as an expansion parameter, and which generalizes the quasilinear theory, is most widely used.…”