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

The nonlinear magnetic Kelvin-Helmholtz instability (KHi), and the turbulence it creates, appears in many astrophysical systems. This includes those systems where the local plasma conditions are such that the plasma is not fully ionised, for example in the lower solar atmosphere and molecular clouds. In a partially ionised 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 analyse the heating rate, and two regimes appear. The first is a regime where the neutral flow is decoupled from the magnetic field that is characterised with a constant heating rate, then at larger scales the strong coupling approximation holds and the heating rate. At large scales with the KHi layer width to the $-2$ power. 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 the turbulent motions, a fact that has consequences for understanding turbulent dissipation in multi-fluid systems.