Theory and simulation of electromagnetic beam modes and whistlers

Abstract

Using particle-in-cell simulations and analytical methods, a study of the nonlinear evolution of electromagnetic instabilities driven by an anisotropic electron beam (Tb⊥≫Tb∥) in an external magnetic field is performed. The unstable waves are either whistlerlike or beam-mode-like depending on the external field strength and beam velocity. The evolution of the particle distribution differs significantly in the two regimes. Even in the presence of a faster electrostatic instability, the electromagnetic waves grow to a significant amplitude. In certain cases, an energetic tail is formed, resulting in enhanced Čerenkov emission of electrostatic waves. The initial evolution of the particle distribution is explained in terms of the interaction of a given linearly unstable wave with the self-consistent perturbed distribution.

Simulations of electron/electron instabilities: Electromagnetic fluctuations

“…Previous electromagnetic simulations of the electron/electron configuration have not addressed this issue. Newman et al 11 and Winglee et al 12 considered initially anisotropic electron beams which excited the intrinsically electromagnetic electron anisotropy instability. Yin et al, 9 Newman et al, 11 and Nambu et al 13 considered only e Ͼ 1 where ͑as we shall demonstrate͒ electromagnetic effects of the electron/electron beam instability are relatively weak.…”

Particle-in-cell simulations of velocity scattering of an anisotropic electron beam by electrostatic and electromagnetic instabilities

“…Earlier studies, [21][22][23] which assume a weaker, faster, and more anisotropic beam, showed a stronger interaction between the electrostatic beam instability and the oblique beam-whistler instability that left the beam more nearly isotropic and less susceptible to the growth of parallel whistler waves (and thus less further pitch-angle scattering of the beam) at late times. Using a different set of parameters (stronger, slower and higher beta), Newman et al 16 found weaker growth of the electrostatic mode and thus larger residual beam temperature anisotropy that drive stronger parallel whistler waves and thus more pitch-angle scattering of the beam.…”

“…Newman et al 16 assumed a relatively dense beam ($10%) at low velocities where the electrostatic mode was suppressed or only weakly unstable. The excited whistler waves (parallel modes only in their 1-D simulation) modified the beam distribution function by pitchangle scattering so that the energetic tail of the beam could excite secondary electrostatic waves.…”

“…This kind of beam can result from an initially isotropic beam traveling along a magnetic field with a spatially increasing magnitude, due to conservation of magnetic moment and/or "time-offlight" effects. 16 In this system, both electrostatic beam modes and electromagnetic whistler modes may be excited. While the growth rate for the cold beam instability is well known, 1,7 in general for a hot beam, the dependence of the growth rate of the electrostatic instability on the beam density and velocity cannot be obtained analytically.…”

“…[17][18][19] In addition, oblique whistler waves can be excited by the beam in a finite beta plasma via the Landau resonance, even when the beam is isotropic. 16,20,21 These waves occur below the electron cyclotron frequency and have both electrostatic and electromagnetic components. Finally, electromagnetic whistler waves can also be driven by an anisotropic electron beam due to the cyclotron resonance.…”