The Kelvin-Helmholtz instability in weakly ionized plasmas - II. Multifluid effects in molecular clouds

Abstract

We present a study of the Kelvin–Helmholtz instability in a weakly ionized, multifluid magnetohydrodynamic (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 ionized, the ideal MHD approximation does not apply at scales of around a tenth of a parsec or less. This paper extends the work of Jones & Downes on the evolution of the Kelvin–Helmholtz instability in the presence of multifluid MHD 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‐Alfvénic, 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.

Driven multifluid magnetohydrodynamic molecular cloud turbulence

“…At very short length-scales (k 100) we can see an upturn in the power spectrum for the magnetic field and the charged species (Figure 7) and this is almost certainly the influence of the Hall effect where, as expected, we see a decoupling at short length scales between the neutrals and the charged species, as well as the magnetic field. The phenomenon of the Hall effect having somewhat less impact than would be expected was also discussed in Jones & Downes (2012) who deduced that the presence of ambipolar diffusion can effectively short circuit the Hall current, thereby inhibiting the Hall effect even in systems with significant Hall resistivity.…”

“…Since molecular cloud material has very low kinematic viscosity (e.g. Jones & Downes ) flows, these systems have a high Prandtl number and thus we can consider them to be effectively inviscid. As with all higher order schemes, our simulations incorporate artificial viscosity which smears discontinuities over around 3–5 grid zones.…”

Vertical-probe-induced asymmetric dust oscillation in complex plasma

“…When macroscopic dust is introduced into a plasma, the resulting complex plasma system offers the opportunity to simulate, at its most fundamental level, interesting physics pertaining to a host of research areas, including protoplanetary [1] or protostellar [2] development, contamination in plasma enhanced semiconductor atomic layer deposition and etching systems [3], and wall erosion within a fusion device [4]. At low temperature and power, it also provides a basis for the study of entirely new plasma physics.…”

Effects of strong magnetic fields on photoionised clouds

“…The high density of molecular pillars means that we are in the fully collisional MHD regime but the ion/electron fraction is typically very low, so imperfect ion-neutral coupling may be an important consideration on small scales. Multi-fluid simulations of weakly ionised plasmas including non-ideal MHD effects [43] are now beginning to test the limits of applicability of ideal MHD on small scales in molecular clouds.…”