Fantastic Striations and Where to Find Them: Origin of Magnetically
  Aligned Striations in Interstellar Clouds
  Thin, magnetically aligned striations of relatively moderate contrast with
the background are commonly observed in both atomic and molecular clouds. They
are also prominent in MHD simulations with turbulent converging shocks. The
simulated striations develop within a dense, stagnated sheet in the mid plane
of the post-shock region where magnetically-induced converging flows collide.
We show analytically that the secondary flows are an inevitable consequence of
the jump conditions of oblique MHD shocks. They produce the stagnated,
sheet-like sub-layer through a secondary shock when, roughly speaking, the
Alfv\'enic speed in the primary converging flows is supersonic, a condition
that is relatively easy to satisfy in interstellar clouds. The dense sub-layer
is naturally threaded by a strong magnetic field that lies close to the plane
of the sub-layer. The substantial magnetic field makes the sheet highly
anisotropic, which is the key to the striation formation. Specifically,
perturbations of the primary inflow that vary spatially perpendicular to the
magnetic field can easily roll up the sheet around the field lines without
bending them, creating corrugations that appear as magnetically-aligned
striations in column density maps. On the other hand, perturbations that vary
spatially along the field lines curve the sub-layer and alter its orientation
relative to the magnetic field locally, seeding special locations that become
slanted overdense filaments and prestellar cores through enhanced mass
accumulation along field lines. In our scenario, the dense sub-layer unique to
magnetized oblique shocks is the birthplace for both magnetically-aligned
diffuse striations and massive star-forming structures.
