The collisions of high-velocity clouds with a magnetized gaseous galactic disk

We present two-dimensional MHD numerical simulations for the interaction of high-velocity clouds with both magnetic and nonmagnetic Galactic thick gas eous disks. For the magnetic models, the initial magnetic field is oriented parallel to the disk, and we consider two different field topologies (with and without tension effects): parallel and perpendicular to the plane of m otion of the clouds. The impinging clouds move in oblique trajectories and fall toward the central disk with different initial velocities. The B-field lines are distorted and compressed during the collision, increasing the fi eld pressure and tension. This prevents the cloud material from penetrating into the disk and can even transform a high-velocity inflow into an outflo w, moving away from the disk. The perturbation creates a complex, turbulent , pattern of MHD waves that are able to traverse the disk of the Galaxy and induce oscillations on both sides of the plane. Thus, the magnetic field e fficiently transmits the perturbation over a large volume but also acts lik e a shield that inhibits the mass exchange between the halo and the disk. F or the nonmagnetized cases, we also uncover some novel features: the evolut ion of the shocked layer generates a tail that oscillates, creating vortici ty and turbulent Bows along its trajectory.