Electron Temperature Anisotropy and Electron Beam Constraints from Electron Kinetic Instabilities in the Solar Wind

Abstract

Electron temperature anisotropies and electron beams are nonthermal features of the observed nonequilibrium electron velocity distributions in the solar wind. In collision-poor plasmas these nonequilibrium distributions are expected to be regulated by kinetic instabilities through wave-particle interactions. This study considers electron instabilities driven by the interplay of core electron temperature anisotropies and the electron beam, and first gives a comprehensive analysis of instabilities in arbitrary directions to the background magnetic field. It clarifies the dominant parameter regime (e.g., parallel core electron plasma beta ${\beta }_{\mathrm{ec}\parallel }$ , core electron temperature anisotropy ${A}_{\mathrm{ec}}\equiv {T}_{\mathrm{ec}\perp }/{T}_{\mathrm{ec}\parallel }$ , and electron beam velocity V_eb) for each kind of electron instability (e.g., the electron beam-driven electron acoustic/magnetoacoustic instability, the electron beam-driven whistler instability, the electromagnetic electron cyclotron instability, the electron mirror instability, the electron firehose instability, and the ordinary-mode instability). It finds that the electron beam can destabilize electron acoustic/magnetoacoustic waves in the low- ${\beta }_{\mathrm{ec}\parallel }$ regime, and whistler waves in the medium- and large- ${\beta }_{\mathrm{ec}\parallel }$ regime. It also finds that a new oblique fast-magnetosonic/whistler instability is driven by the electron beam with ${V}_{\mathrm{eb}}\gtrsim 7{V}_{A}$ in a regime where ${\beta }_{\mathrm{ec}\parallel }\sim 0.1\mbox{--}2$ and A_ec < 1. Moreover, this study presents electromagnetic responses of each kind of electron instability. These results provide a comprehensive overview for electron instability constraints on core electron temperature anisotropies and electron beams in the solar wind.