Global three-dimensional simulations of magnetic field evolution in a galactic disk. II. Gas rich galaxies

A fully three-dimensional computation of induction processes in a disk is performed to investigate the evolution of a large-scale magnetic field in gas-rich barred and spiral galaxies in the presence of field diffusion. As input parameters we use time dependent velocity fields obtained from self-consistent 3D N-body calculations of galactic dynamics. Our present work primarily analyzes the influence of the gas flows in non-axisymmetric gravitational disturbances (like spiral arms and bars) on the behavior of the magnetic field. The magnetic field is found to evolve towards structures resembling dynamical spiral arms and bars, however its distribution is often much more complicated than the velocity field itself. We show that a random configuration of seed magnetic fields quickly dissipates, and is therefore unable to explain the magnetic field strength observed in nearby galaxies. The detailed comparison between the simulated velocity and magnetic fields allows us to establish that the reversals of magnetic vectors appear in the vicinity of the corotation radius of non-axisymmetric disk structures, where the velocity field shows rather abrupt changes. We argue that this explains the observed field reversals in our Galaxy.