The role of shocks and the velocity gradient in the relative orientation of the magnetic field and dense gas clouds
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 superalfvénic regime transitions to a transalfvénic regime between the shock and the condensation front, and then to a subalfvénic regime inside the condensations.