This Letter investigates the electron heat flux instability using the radial models of the magnetic field and plasma parameters in the inner heliosphere. Our results show that both the electron acoustic wave and the oblique whistler wave are unstable in the regime with large relative drift speed (ΔV e ) between electron beam and core populations. Landau-resonant interactions of electron acoustic waves increase the electron parallel temperature that would lead to suppressing the electron acoustic instability and amplifying the growth of oblique whistler waves. Therefore, we propose that the electron heat flux can effectively drive oblique whistler waves in an anisotropic electron velocity distribution function. This study also finds that lower-hybrid waves and oblique Alfvén waves can be triggered in the solar atmosphere, and that the former instability is much stronger than the latter. Moreover, we clarify that the excitation of lower-hybrid waves mainly results from the transit-time interaction of beaming electrons with resonant velocities v ∥ ∼ ω/k ∥, where ω and k ∥ are the wave frequency and parallel wavenumber, respectively. In addition, this study shows that the instability of quasi-parallel whistler waves can dominate the regime with medium ΔV e at the heliocentric distance nearly larger than 10 times of the solar radius.
The ion temperature anisotropy instability is widely thought of as a constraint on the distribution of the ion perpendicular and parallel temperatures in the solar wind. Besides the ion temperature anisotropy, proton and alpha particle beams are permeating in the solar wind. Therefore, this paper presents a comprehensive investigation on unstable waves resulting from both ion temperature anisotropy and ion beams. It finds that the strongest electromagnetic cyclotron instability triggers the left-hand circularly polarized Alfvén/proton-cyclotron wave propagating along the background magnetic field. The strongest fast-magnetosonic/whistler firehose instability generates the right-hand circularly polarized fast-magnetosonic/whistler wave propagating reversely to the background magnetic field. The mirror instability preferably drives oblique mirror mode waves with two anticorrelated perpendicular magnetic components. The Alfvén firehose instability is prior to generating oblique Alfvén waves with two unbalanced perpendicular magnetic components that are nearly positive-correlated. Due to the effects of streaming proton and alpha particles, both the mirror and Alfvén firehose instabilities produce slowly propagating unstable waves in comparison to nonpropagating waves in motionless plasmas. The differential proton and alpha particle flows result in the ion/ion beam instability, destabilizing obliquely propagating Alfvén/proton-cyclotron waves. The ion/ion beam instability can provide a constraint on electromagnetic fluctuations in the low-beta region. Moreover, this paper clearly explores the dependence of the frequency and electromagnetic polarization on the normal angle for each kind of instability, which could be useful for distinguishing the instability mechanism in the solar wind.
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.
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