Global three-dimensional simulations of magnetic field evolution in a galactic disk I. Barred galaxies

The evolution of three-dimensional, large-scale, magnetic fields, in a galactic disk is investigated numerically. The N-body simulations of galactic dynamics are incorporated into the kinematic calculations of induction equations to study the influence of non-axisymmetric gas flows on the galactic magnetic field. The time-dependent gas velocity fields are introduced as input parameters for the MHD-simulations. The effects of interstellar turbulence are given as the turbulent diffusion of the magnetic field. Our principal concern is to check how dynamical evolution of the galactic gas affects the global magnetic field structure and intensity. We have found that the magnetic field responds sensitively to changes in the gas velocity field, and even slight variations of the dynamical parameters, such as the gas mass/total mass ratio results in nonuniform intensity structures, i.e. magnetic arms. The magnetic lines of force are well aligned with spiral arms and bar due to compressional flows in these features. In the inter-arm regions the areas with magnetic vectors going opposite to the main magnetic spirals are present. Moreover we could identify Parker-like intensity features perpendicular to the disk plane. Obviously even weak perturbations of the velocity field already produce almost instantaneous changes in the local magnetic field geometry. This proves the importance of our approach to use the velocity fields resulting from global dynamical instabilities for magnetohydrodynamical simulations.