Computer simulations of electromagnetic instabilities in the plasma sheet boundary layer

Abstract

One‐dimensional hybrid computer simulations are used to study electromagnetic instabilities driven by hot, anisotropic, counterstreaming proton components similar to those observed by ISEE‐1 and ‐2 in the plasma sheet boundary layer of Earth's magnetotail. These simulations are used to study two types of growing modes: the ion cyclotron anisotropy instability driven by the temperature anisotropy of individual ion components, and two different ion/ion instabilities driven by the relative streaming of two ion components. As in previous such simulations, the ion cyclotron anisotropy instability exhibits a very low saturation amplitude and weak ion scattering. Simulations of the current‐driven kink‐like instability using more general particle‐particle codes yield similarly weak fluctuation amplitudes and very small ion scattering. If the relative streaming speed of the hot proton components is increased to greater than 2υA, the proton/proton nonresonant instability can grow to saturation at larger amplitudes and produce strong proton scattering which leads to significant heating of the two hot proton distributions. Furthermore, if a relatively large density of cool ionospheric oxygen ions is introduced in the simulations, the ion/ion right‐hand resonant instability becomes the dominant mode, and pitch angle scatters the hot proton components to produce distributions which resemble those observed from ISEE. Thus electromagnetic ion/ion instabilities are a possible mechanism by which the hot streaming proton components of the plasma sheet boundary layer become the single very hot isotropic proton component of the central plasma sheet.