The Kelvin-Helmholtz instability in weakly ionized plasmas: ambipolar-dominated and Hall-dominated flows

Abstract

The Kelvin-Helmholtz instability is well known to be capable of converting well-ordered flows into more disordered, even turbulent, flows. As such it could represent a path by which the energy in, for example, bow shocks from stellar jets could be converted into turbulent energy thereby driving molecular cloud turbulence. We present the results of a suite of fully multifluid magnetohydrodynamic simulations of this instability using the HYDRA code. We investigate the behaviour of the instability in a Hall-dominated and an ambipolar diffusion dominated plasma as might be expected in certain regions of accretion discs and molecular clouds, respectively.We find that, while the linear growth rates of the instability are unaffected by multifluid effects, the non-linear behaviour is remarkably different with ambipolar diffusion removing large quantities of magnetic energy while the Hall effect, if strong enough, introduces a dynamo effect which leads to continuing strong growth of the magnetic field well into the non-linear regime and a lack of true saturation of the instability.

Kelvin-Helmholtz Instability in Partially Ionized Compressible Plasmas

“…However, nonlinear studies are needed to assess the real impact of the instability on the evolution of the plasma parameters. In a recent work Jones & Downes (2011) investigated the nonlinear evolution of the KHI in a partially ionized plasma in the ambipolar-dominated regime. Despite the differences in setup and formalism, it is instructive to compare the results of Jones & Downes (2011) with our present findings.…”

“…In a recent work Jones & Downes (2011) investigated the nonlinear evolution of the KHI in a partially ionized plasma in the ambipolar-dominated regime. Despite the differences in setup and formalism, it is instructive to compare the results of Jones & Downes (2011) with our present findings. They used a two-dimensional configuration in the xy-plane, with the flow and the magnetic field parallel to their y-direction, and the interface parallel to the x = 0 plane.…”

“…No dependence of the perturbations in their z-direction was considered. Therefore, the equilibrium of Jones & Downes (2011) is equivalent to our model with our k y = 0 and our k z set equal to their k y . Jones & Downes (2011) concluded that the linear growth rates in the ambipolar-dominated regime are the same as in the ideal case, i.e., in the absence of ambipolar diffusion.…”

“…Therefore, the equilibrium of Jones & Downes (2011) is equivalent to our model with our k y = 0 and our k z set equal to their k y . Jones & Downes (2011) concluded that the linear growth rates in the ambipolar-dominated regime are the same as in the ideal case, i.e., in the absence of ambipolar diffusion. In our formalism the ambipolar-dominated regime in which ions and neutrals are strongly coupled is equivalent to our case ν in /ω k ≫ 1.…”

“…The KHI in partially ionized incompressible plasmas has been studied under different contexts (e.g., Chhajlani & Vyas 1990;Birk & Wiechen 2002;Watson et al 2004;Shadmehri & Downes 2007Kunz 2008;Jones & Downes 2011). The paper that we take as a reference for the present investigation is that by Watson et al (2004).…”