EVOLUTION OF MAGNETIC-FIELDS IN GALAXIES WITH SPIRAL STRUCTURE

We present simulations of the 3D nonlinear induction equation in order to investigate the temporal evolution of large-scale magnetic fields in spiral galaxies. Our model includes differential rotation, ambipola r diffusion and, based on small-scale turbulence, eddy diffusivity and the tensorial alpha-effect with magnetic feedback. The nonaxisymmetri c spiral pattern and - if considered - the vertical stratification of the galaxy are represented in its density and turbulence profile. Negl ecting vertical stratification the lifetime and geometry of an initial magnetic field depend on the correlation time of interstellar turbule nce T-corr. Short correlation times increase the lifetime of the initi al magnetic field, but the field is rapidly wound pip. Its pitch-angle s develop to zero. The magnetic field has disappeared after at most 1 to 1.5 Gyr. A resonance like phenomenon is found by tuning the pattern velocity of the galactic spiral. The simulations then show an excepti onal amplification of the magnetic field in the case that the pattern speed and a magnetic drift velocity have similar values. Considering a vertical stratification we achieve sufficiently long living grand-des igned magnetic fields excited by dynamo action. The behaviour and geom etry of the resulting field is again significantly influenced by the c orrelation time T-corr. Small values of T-corr lead to axisymmetric fi elds with small pitch-angles and field-concentration between the spira l arms. Increasing the correlation time the solutions show larger pitc h-angles; and depending on very large correlation times the galactic d ynamo rather generates fields clearly within the spiral arms and havin g a bisymmetric structure.