  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.
