  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. Hot and cold
regions form around these filaments, disrupting the uniform temperature
structure characteristic of a steady-state C-shock. This filamentary region
steadily grows as the shock progresses. 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.
