INSTABILITY IN OBLIQUE C-TYPE SHOCKS

A sequence of steady, oblique, C-type shock waves differing in the angle between the ambient magnetic field and the direction of shock propagation, theta-s, are examined for stability to small perturbations in the plane containing the fluid velocities and the magnetic field. As theta-s, is reduced the steady shocks become stronger and the growth rate and wavenumber of the fastest growing mode increase. The growing perturbations propagate across the face of the shock front at a speed of order the Alfven speed in the pre-shock gas (typically 2 km s-1 in molecular clouds). The strength of the density perturbations in the neutral gas decreases relative to those in the ions as theta-s is reduced, and the perturbations in the neutral gas resemble ripples rather than the clumps found in the perpendicular case. It is noted that a similar instability may arise in other situations in which ambipolar diffusion plays a major role.