On Kinetic Instabilities Driven By Ion Temperature Anisotropy and Differential Flow in the Solar Wind

Abstract

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.