MHD Kelvin-Helmholtz instability in the anisotropic solar wind plasma

Abstract

We investigated a shear instability of the Kelvin-Helmholtz (KH) type in a plasma with temperature anisotropy under the magnetohydodynamics (MHD) approximation. To solve the problem, a system of 16-moment MHD transport equations are used in a collisionless bi-Maxwellian plasma, including the various components of the heat fluxes along the magnetic field. We consider supersonic flows of two semi-infinite anisotropic and homogeneous plasma layers with different physical parameters and velocities. For the general case, i.e., when the interface between these two flows is a transition layer with a finite thickness, we derived a general linear differential equation framework for determining the eigenmodes in the system. Furthermore, we considered thoroughly the limiting case of a zero thickness transition zone (contact discontinuity). The analysis enabled applying appropriate boundary conditions to derive the dispersion equation for interface waves. The obtained equation analyzes in detail for the case when heat fluxes are absent along the discontinuity in the background state. It is shown that the shear flow excites the KH instability and “couples” the various branches of the free-plasma oscillations to each other. It is found that the region of mode interaction is determined by the resonance regions when the longitudinal phase velocities of the waves match. In the resonance flows with an average speed, the KH instability occurs. The growth rates of the KH instability are calculated as a function of the parameters, including the degree of plasma anisotropy. It is found that in most cases the KH instability is dominant in the considered configuration. The obtained results are applied to the plasma conditions in the bimodal solar wind in the vicinity of the contact discontinuity between different flow patterns (fast and slow wind).