Ion-neutral decoupling in the nonlinear Kelvin–Helmholtz instability: Case of field-aligned flow

Abstract

Nonlinear magnetic Kelvin-Helmholtz instability (KHI), and the turbulence it creates appear in many astrophysical systems. This includes those systems where the local plasma conditions are such that the plasma is not fully ionized, for example in the lower solar atmosphere and molecular clouds. In a partially ionized system, the fluids couple via collisions which occur at characteristic frequencies, therefore neutral and plasma species become decoupled for sufficiently high-frequency dynamics. Here, we present high-resolution 2D two-fluid simulations of the nonlinear KHI for a system that traverses the dynamic scales between decoupled fluids and coupled dynamics. We discover some interesting phenomena, including the presence of a density coupling that is independent of the velocity coupling. Using these simulations, we analyze the heating rate, and two regimes appear. The first is a regime where the neutral flow is decoupled from the magnetic field that is characterized by a constant heating rate, then at larger scales, the strong coupling approximation holds the heating rate with the KHI layer width to the power of –2. There is an energy cascade in the simulation, but the nature of the frictional heating means the heating rate is determined by the largest scale of turbulent motions, a fact that has consequences for understanding turbulent dissipation in multifluid systems.