INTERSTELLAR MASS OUTFLOW TO GALACTIC HALOS BY THE SUPERNOVA-DRIVEN PARKER INSTABILITY

We here demonstrate by means of two-dimensional magnetohydrodynamic si mulations that even a single supernova (SN) explosion can easily trigg er a Parker instability. When SNs occur in the galactic disk with hori zontal magnetic fields, an outgoing blast wave will lift up the fields , thus forming an Omega-shaped field structure. Hence, the interstella r medium (ISM) can effectively be confined inside the Omega-shaped, be nt fields because of their decelerating J x B force. It is thus usuall y thought that unless unrealistically large numbers of SNs occur, it i s difficult for the ISM to blow out into the halo. We find, however, t hat this is the case only before the Parker instability is excited; na mely, for the first several megayears after the explosion. The gas fro zen into the Omega-shaped magnetic fields will eventually slide down a long the fields to the galactic disk by gravity, so the magnetic field s can move upward by an enhanced magnetic buoyancy. In roughly tens of megayears after the explosions the Parker instability will be trigger ed, giving rise to a blowout of the disk material into the halo. Impor tantly, a nonlinear Parker instability can be triggered quite generall y; i.e., even when the underlying gas layer is stable against infinite simal perturbations.