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