The Kelvin-Helmholtz instability in weakly ionised plasmas II: multifluid effects in molecular clouds

TL;DR

This study investigates how multifluid magnetohydrodynamic effects, specifically ambipolar diffusion and the Hall effect, influence the nonlinear evolution of the Kelvin-Helmholtz instability in weakly ionised molecular clouds, revealing significant deviations from ideal MHD behavior.

Contribution

It extends previous work by analyzing the combined effects of ambipolar diffusion and the Hall effect on the Kelvin-Helmholtz instability in molecular clouds, highlighting nonlinear behavior changes.

Findings

Multifluid effects do not alter linear growth rates.

Ambipolar diffusion decouples magnetic fields from flow.

Hall effect's re-orientation is suppressed by ambipolar diffusion.

Abstract

We present a study of the Kelvin-Helmholtz instability in a weakly ionised, multifluid MHD plasma with parameters matching those of a typical molecular cloud. The instability is capable of transforming well-ordered flows into disordered flows. As a result, it may be able to convert the energy found in, for example, bowshocks from stellar jets into the turbulent energy found in molecular clouds. As these clouds are weakly ionised, the ideal magnetohydrodynamic approximation does not apply at scales of around a tenth of a parsec or less. This paper extends the work of Jones & Downes (2011) on the evolution of the Kelvin-Helmholtz instability in the presence of multifluid magnetohydrodynamic effects. These effects of ambipolar diffusion and the Hall effect are here studied together under physical parameters applicable to molecular clouds. We restrict our attention to the case of a single shear layer with a transonic, but super-Alfvenic, velocity jump and the computational domain is chosen to match the wavelength of the linearly fastest growing mode of the instability. We find that while the introduction of multifluid effects does not affect the linear growth rates of the instability, the non-linear behaviour undergoes considerable change. The magnetic field is decoupled from the bulk flow as a result of the ambipolar diffusion, which leads to a significant difference in the evolution of the field. The Hall effect would be expected to lead to a noticeable re-orientation of the magnetic field lines perpendicular to the plane. However, the results reveal that the combination with ambipolar diffusion leads to a surprisingly effective suppression of this effect.