NUMERICAL-SIMULATION OF LARGE-SCALE MAGNETIC-FIELD EVOLUTION IN SPIRAL GALAXIES

The evolution of large-scale magnetic fields in disk galaxies is inves tigated numerically. The gasdynamical simulations in a disk perturbed by spiral or bar potential are incorporated into the kinematic calcula tions of induction equations to elucidate the effects of non-axisymmet ric disk structure on magnetic fields. The effects of interstellar tur bulence are given as the turbulent diffusion of magnetic fields. The u sually adopted dynamo mechanism of alpha-effect is not considered in o ur computations, because it is not obvious about the actual existence of the effect in a galaxy. Our principal concern is to clear how obser vationally and theoretically well-established gas flow affects the mag netic-field structure and evolution, without putting a lot of artifici al parameters in the model. We have found that the density-wave stream ing motion of gas has a significant influence on the distribution of m agnetic fields: the lines of force are well aligned with spiral arms d ue to the compressional and additional shearing flow of gas in these r egions. In the inter-arm regions, the field lines have the finite angl es with respect to the imposed arms, because the gaseous streamlines i nduced by spiral arms deviate from the orientation of spiral arms them selves. These properties of magnetic-field orientation across the arm are well in agreement with the results of radio continuum observations . All simulation models have resulted in the eventual decay of magneti c energy due to the strong turbulent diffusion. We have also explored the azimuthally periodic function for the coefficient of turbulent dif fusion with the maximum in spiral-arm regions. This is anticipated fro m the enhancement of turbulence by young OB stars and supernovae. This effect of non-uniform dissipation gives rise to the modulation of mag netic-field structure and its time evolution. In particular, the dissi pation rate of magnetic fields can be much smaller than the usually as sumed rate of similar to 1/10(8) yr(-1).