Beam-generated instabilities in space plasmas

Abstract

Broadband electrostatic noise (BEN) in the Earth's magnetosphere constitute electrostatic wave emissions ranging in frequency from a few Hz to well above the electron plasma frequency (rvIO kHz). BEN has been found via satellite data to be associated with field-aligned electron or ion beams in different regions of the magnetosphere such as the plasma sheet boundary layer, polar cusp, magnetotail, bow shock and auroral regions. In an attempt to study the generation of BEN, beam-generated electrostatic instabilities are investigated using kinetic theory in a multi-component magnetized plasma consisting of electron and ion beams (magnetic field-aligned), and background species. All species are fully magnetized and considered to have Maxwellian velocity distributions. The initial model is varied to allow for counter-streaming electron beams as well as counter-streaming electron and ion beams, to seek explanations for satellite observations in different regions of the magnetosphere. A number of instabilities such as the electron-acoustic, ion-acoustic, electron beam resonant and electron-electron streaming instabilities are found to be supported. It is found that the magnitude of the beam speed is crucial in determining which instability is excited. The dependence of the instability growth rates and real frequencies on various plasma parameters such as speed of the beam(s), particle densities and temperatures, magnetic field strength, angle of wave propagation relative to the ambient magnetic field direction and temperature anisotropy of the beam(s) is examined. Where possible, numerical solutions of the full kinetic dispersion are compared with approximate analytical solutions.