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 firstly 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 β ec , core electron temperature anisotropy A ec ≡ T ec⊥ /T ec , 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-β ec regime, and whistler waves in the medium-and large-β ec regime. It also finds that a new oblique fast-magnetosonic/whistler instability is driven by the electron beam with V eb 7V A in a regime where β ec ∼ 0.1 − 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.

Electromagnetic Proton Beam Instabilities in the Inner Heliosphere: Energy Transfer Rate, Radial Distribution and Effective Excitation

“…Comparing expressions (A4)−(A6), we can see that (A6) is valid under the condition γ ω r . This implies (A6) cannot describe the energy transfer associating with plasma waves with zero real frequency, for example, unstable waves driven by the oblique firehose instability and ion/electron mirror instability (e.g., Sun et al 2019Sun et al , 2020. Although our expressions (e.g., 6, 7, A4, A5) and Quataert's expression (A6) have slightly different physical meanings, they are all helpful in quantifying the energy transfer between waves and particles.…”

“…The plasma wave eigenmodes correspond to solutions of Equation (1). Recently, a numerical solver (BO/PDRK) for Equation ( 1) is developed by Xie & Xiao (2016) and Xie (2019), and this solver is useful to perform a comprehensive study for ion and electron kinetic instabilities (Sun et al 2019(Sun et al , 2020. In this paper we use BO/PDRK to give the wave dispersion relation in the proton beam plasma.…”

Growth of Outward Propagating Fast-magnetosonic/Whistler Waves in the Inner Heliosphere Observed by Parker Solar Probe

“…This suggests that the inner heliosphere exhibits complex wave-particle coupling processes, involving the velocity distributions of various plasma species and the time-varying evolution of different wave modes. The free energy responsible for the fast-magnetosonic/whistler waves may come from the drift ion population, electron heat flux, and electron thermal anisotropy (Verscharen et al 2013;Narita et al 2016;Stansby et al 2016;Tong et al 2019;Sun et al 2020). In the future, we require a combination of both the electromagnetic field information and the particle phase space density to explore the mystery of kinetic waves and their wave-particle interactions in the inner heliosphere in a comprehensive way.…”

Growth of Outward Propagating Fast-Magnetosonic/Whistler Waves in the Inner Heliosphere Observed by Parker Solar Probe

“…This suggests that the inner heliosphere exhibits complex wave-particle coupling processes, involving the velocity distributions of various plasma species and the time-varying evolution of different wave modes. The free energy responsible for the fast-magnetosonic/whistler waves may come from the drift ion population, electron heat flux, and electron thermal anisotropy (Verscharen et al 2013;Stansby et al 2016;Narita et al 2016;Tong et al 2019;Sun et al 2020). In the future, we require a combination of both the electromagnetic field information and the particle phase space density to explore the mystery of kinetic waves and their wave-particle interactions in the inner heliosphere in a comprehensive way.…”