SHOCK INTERACTIONS WITH MAGNETIZED INTERSTELLAR CLOUDS .1. STEADY SHOCKS HITTING NONRADIATIVE CLOUDS

We study the interaction of a steady, planar shock with a nonradiative , spherical, interstellar cloud threaded by a uniform magnetic field. For strong shocks, the sonic Mach number scales out, so two parameters determine the evolution: the ratio of cloud to intercloud density, an d the Alfven Mach number. We focus on the case with initial field para llel to the shock velocity, though we also present one model with fiel d perpendicular to the velocity. Even with 100 zones per cloud radius, we find that the magnetic field structure converges only at early tim es. However, we can draw three conclusions from our work. First, our r esults suggest that the inclusion of a field in equipartition with the preshock medium can prevent the complete destruction of the cloud fou nd in the field-free case recently considered by Klein, McKee, & Colel la. Second, the interaction of the shock with the cloud can amplify th e magnetic field in some regions up to equipartition with the postshoc k thermal pressure. In the parallel-field case, the shock preferential ly amplifies the parallel component of the field, creating a ''flux ro pe,'' a linear structure of concentrated magnetic field. The flux rope dominates the volume of amplified field, so that laminar, rather than turbulent, amplification is dominant in this case. Third, the presenc e of the cloud enhances the production of X-ray and synchrotron emissi on. The X-ray emission peaks early, during the initial passage of the shock over the cloud, while the synchrotron emission peaks later, when the flow sweeps magnetic field onto the axis between the cloud and th e main shock.