The role of shocks and the velocity gradient in the relative orientation of the magnetic field and dense gas clouds

TL;DR

This paper explores how shocks and velocity gradients influence the alignment of magnetic fields with dense gas clouds in the interstellar medium, highlighting the roles of MHD shocks, shear flows, and instabilities.

Contribution

It introduces a detailed mechanism showing how shocks and velocity gradients cause magnetic fields to align with dense gas structures, incorporating effects of MHD shocks and non-linear thin-shell instability.

Findings

Magnetic fields become more parallel to dense layers after shock interactions.

Velocity gradients from infalling gas can straighten magnetic field lines.

Transition from superalfvénic to subalfvénic regimes observed across structures.

Abstract

We investigate the alignment mechanism between the magnetic field and interstellar clouds formed by the collision of warm atomic gas. We find that the magnetic field, initially oriented parallel to the flow, is perturbed by a fast MHD shock, which amplifies magnetic field fluctuations parallel to the shock front. Behind the shock, the compressive downstream velocity field further amplifies the magnetic field component parallel to the shock front. This mechanism causes the magnetic field to become increasingly parallel to the dense layer, and the development of a shear flow around the latter. Furthermore, the bending-mode perturbations on the dense layer are amplified by the non-linear thin-shell instability (NTSI), stretching the density structures formed by the thermal instability, and rendering them parallel to the bent field lines. By extension, we suggest that a tidal stretching velocity gradient such as that produced in gas infalling into a self-gravitating structure must straighten the field lines along the accretion flow, orienting them perpendicular to the density structures. We also find that the upstream superalfv\'enic regime transitions to a transalfv\'enic regime between the shock and the condensation front, and then to a subalfv\'enic regime inside the condensations.