Beam generated instabilities in space plasma.

Abstract

Electrostatic instabilities associated with a model applicable to the auroral acceleration region consisting of an ion beam, precipitating electrons and stationary background electrons are theoretically investigated. The kinetic dispersion relation is solved numerically without approximations. It is shown that two low-frequency plasma instabilities are present and these may generate the low-frequency electric field fluctuations (LEFs) that have been observed in the acceleration region. A parameter variation study is carried out in order to reveal the features of the instabilities. The model is adapted to suit two possible regions of study: (1) drifting cool ions and (2) drifting cool ions and counter-streaming hot electrons. The slow ion-acoustic instability which dominates at low ion beam drift velocities is studied by varying plasma parameters such as the propagation angle, ky/k, the wavenumber, k, the cold background electron density, nco, the hot electron temperature and the cool ion beam temperature. The second mode, the modified two-stream instability, which dominates at larger ion beam drift velocities and at oblique angles of propagation is investigated in a similar manner. To complete the study of these two instabilities, the effect of drifting hot electrons is examined briefly through a similar parameter variation study.