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 MHD approximation. To solve the problem, a system of 16-moment MHD transport equations is 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 semiinfinite 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 is analyzed 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).

Possible Evidence for Shear-driven Kelvin–Helmholtz Instability along the Boundary of Fast and Slow Solar Wind in the Corona

“…It follows that even the sub-Alfvénic velocity shear along the fast/slow boundary in the solar corona can lead to KH instability growth, provided that, as expected, the streamer belt region does not have a nicely ordered magnetic field and instead possesses locally a very complicated structure with a nontangential magnetic field configuration. Although theoretically predicted to play an important role in the plasma dynamics between high-and low-speed coronal streams flowing alongside each other (Ismayilli et al 2018), the KH instability has not yet been detected at the edges of coronal holes adjacent to equatorial streamers, where shear due to the outflow of fluids with different velocities can initiate KH vortices. This lack of evidence motivates the present work, which indeed aims at providing, for the first time, possible observational evidence for manifestations of the KH instability in the solar corona in the interaction region between fast and slow solar wind.…”

Fire-hose instability of inhomogeneous plasma flows with heat fluxes

“…Indeed, the 16moment formalism has already proven to be successful in analyzing waves and instabilities in weakly collisional media. [32][33][34][35][36] In this paper, we present the stability analysis of the low frequency incompressible perturbations to the anisotropic MHD shear flows with heat fluxes. Effects of the velocity shear and heat fluxes on the dynamics of linear perturbations are characterized by lower frequency compared to the compressibility effects.…”

Kinetic features for the identification of Kelvin-Helmholtz vortices in \textit{in situ} observations

“…For instance, an unstable configuration is found when the velocity jump ∆u is larger than the component of the Alfvén velocity parallel to the bulk flow (Chandrasekhar 1961). Shear flows have been observed, for instance, at the interaction region between fast and slow solar wind (Bruno & Carbone 2013) and both teories and simulations have shown that such sites are good candidates for the KH instability to grow (Korzhov et al 1985;Ismayilli et al 2018). In addition, in such regions, the wave-particle interaction with the non-uniform velocity field can produce smallscale fluctuations leading to the dissipation of the waves (Pezzi et al 2017;Valentini et al 2012Valentini et al , 2017.…”