Shock waves in molecular clouds should evolve into continuous or C-type
structures due to the magnetic field and ion-neutral friction. We here
determine whether and how this is achieved.
The evolution away from jump shocks toward the numerous steady C-shock sub-types is investigated. The evolution passes through four stages, which possess distinctive observational properties. The time scales and length scales cover broad ranges. Specific results are included for shock types including switch, absorber, neutralised, oblique, transverse and intermediate. Only intermediate Type II shocks and `slow shocks', including switch-off shocks, remain as J-type under the low ion levels assumed. Other shocks transform via a steadily growing neutral precursor to a diminishing jump.
We compute the nonlinear development of the instabilities in C-shocks first described by Wardle, using a version of the ZEUS code modified to include a semi-implicit treatment of ambipolar diffusion. We find that, in three dimensions, thin sheets parallel to the shock velocity and perpendicular to the magnetic field lines form. High resolution, two-dimensional models show that the sheets are confined by the Brandenburg & Zweibel ambipolar diffusion singularity, forcing them to numerically unresolvable thinness.
We compare steady-state to unstable C-shocks, showing excitation diagrams, line ratios, and line profiles for molecular hydrogen lines visible in the K-band, with the Infrared Space Observatory, and with NICMOS on the Hubble Space Telescope.