THE WARDLE INSTABILITY IN INTERSTELLAR SHOCKS .1. NONLINEAR DYNAMICALEVOLUTION

The nonlinear evolution of unstable C-type shocks in weakly ionized pl asmas is studied by means of time-dependent, magnetohydrodynamic simul ations. This study is limited to shocks in magnetically dominated plas mas (in which the Alfven speed in the neutrals greatly exceeds the sou nd speed), and microphysical processes such as ionization and recombin ation are not followed. Both the two-dimensional simulations of initia lly planar perpendicular and oblique C-type shocks and the fully three -dimensional simulation of a perpendicular shock are presented. For th e cases studied here, the instability results in the formation of dens e sheets of gas elongated in the direction of shock propagation and or iented perpendicular to the magnetic field. The formation of a weak J- type front is associated with the growth of the instability from an eq uilibrium shock structure. After saturation the magnetic held structur e consists of arches that bow outward in the direction of shock propag ation and are anchored by the enhanced ion-neutral drag in the dense s heets. Analogous to the magnetic buoyancy (Parker) instability, satura tion occurs when the magnetic tension in the distorted held lines is b alanced by drag in the sheets. For the magnetically dominated shocks s tudied here, the distortions in the magnetic held that produce saturat ion are very small. Nonetheless, the enhancements of the ion and neutr al densities in the sheets are very large, between 2 and 3 orders of m agnitude compared with the preshock values. At these high densities, r ecombination processes may be important. The sheets evolve slowly in t ime, so that shocks propagating in a homogeneous medium may leave behi nd a network of intersecting filaments and sheets of dense gas elongat ed in the direction of shock propagation and perpendicular to the mean field. The temperature structure and emission properties of unstable C-type shocks in the nonlinear regime are presented in a companion pap er.