Kinetic numerical scaling of Alfven cyclotron instability in non-thermal solar wind plasmas

Abstract

Linear plasma kinetic theory for a non-thermal, magnetized, homogeneous, and collisionless plasma is incorporated to study Alfvén cyclotron instability (ACI) driven by the ion/proton perpendicular temperature anisotropy (T⊥p/T∥p>1) (⊥, ∥ symbols designate directions perpendicular and parallel to ambient magnetic field, respectively), and the wave propagation is considered in the direction of the ambient magnetic field (k→∥B→0) with left hand circular polarization. We consider that electron–proton plasma with kappa distributed electrons and protons is taken to be Vasyliunas–Cairns distributed. We, further, validated our model distributions for the electrons and ions against the observations of solar wind at various heliocentric distances. The transverse dielectric response function of ACI is calculated and numerically solved to study its dispersion and growth characteristics under the influence of pertinent parameters, i.e., non-thermal parameters of protons and electrons αp, κp, and κe, proton and electron temperature anisotropy ratios τp,e=T⊥p,e/T∥p,e, and plasma beta of protons (β∥p(VC) and β∥p(M)). A concept of the non-thermality dependent effective temperature model is invoked, which updates plasma beta and makes it a non-thermality dependent quantity. The dispersion and growth rates of ACI are found appreciably and significantly augmented in the case of non-thermal protons as compared to Maxwellian protons previously presented by [Gary et al., J. Geophys. Res. 117, A08201 (2012); 122, 464–474 (2017)]. The increase in the magnitude of proton parameters enhances the growth rate of the instability, whereas the increment in electron parameters inhibits the growth rate. This study is advantageous to understand the plasma dynamics of natural environments, such as magnetosphere and solar wind, where the excessive non-thermal populations are present that cannot be modeled by the Maxwellian distribution.