Draping of cluster magnetic fields over bullets and bubbles - Morphology and dynamic effects

High-resolution X-ray observations have revealed cavities and cold fronts'' with sharp edges in temperature and density within galaxy clusters. Their presence poses a puzzle, since these features are not expected to be hydrodynamically stable or to remain sharp in the presence of diffusion. However, a moving core or bubble in even a very weakly magnetized plasma necessarily sweeps up enough magnetic field to build up a dynamically important sheath; the layer's strength is set by a competition between plowing up'' and slipping around of field lines, and depends primarily on the ram pressure seen by the moving object. In this inherently three-dimensional problem, our analytic arguments and numerical experiments show that this layer modifies the dynamics of a plunging core, greatly modifying the hydrodynamic instabilities and mixing, changing the geometry of stripped material, and slowing the core through magnetic tension. We derive an expression for the maximum magnetic field strength and thickness of the layer, as well as for the opening angle of the magnetic wake. The morphology of the magnetic draping layer implies the suppression of thermal conduction across the layer, thus conserving strong temperature gradients. The intermittent amplification of the magnetic field as well as the injection of magnetohydrodynamic turbulence in the wake of the core is identified to be due to vorticity generation within the magnetic draping layer. These results have important consequences for understanding the complex gas-dynamical processes of the intracluster medium and apply quite generally to motions through other magnetized environments, e. g., the interstellar medium.