Evolution of Magnetic Fields in Galaxies with Spiral Structure

3 S u m m a r y : We present simulations of the 3D nonlinear induction equation in order to investigate the temporal evolution of large-scale magnetic elds in spiral galaxies. Our model includes diierential rotation, ambipolar diiusion and, based on small-scale turbulence, eddy diiusivity and the tensorial-eeect with magnetic feedback. The nonax-isymmetric spiral pattern and { if considered { the vertical stratiication of the galaxy are represented in its density and turbulence proole. Neglecting vertical stratiication the lifetime and geometry of an initial magnetic eld depend on the correlation time of interstellar turbulence corr. Short correlation times increase the lifetime of the initial magnetic eld, but the eld is rapidly wound up. Its pitch-angles develop to zero. The magnetic eld 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 exceptional ampliication of the magnetic eld in the case that the pattern speed and a magnetic drift velocity have similar values. Considering a vertical stratiication we achieve suuciently long living grand-designed magnetic elds excited by dynamo action. The behaviour and geometry of the resulting eld is again signiicantly innuenced by the correlation time corr. Small values of corr lead to axisymmetric elds with small pitch-angles and eld-concentration between the spiral arms. Increasing the correlation time the solutions show larger pitch-angles; and depending on very large correlation times the galactic dynamo rather generates elds clearly within the spiral arms and having a bisymmetric structure. 1. INTRODUCTION Spiral galaxies have large-scale magnetic elds of diierent, but in the most cases spiral structure (Beck et al. 1996). In the last years several attempts were made in order to investigate the evolution of galactic magnetic elds with 3D numerical So far applied simulations were mostly concerned with mechanisms leading