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.
