THE ROLE OF SUPERNOVAE FOR THE GALACTIC DYNAMO .1. THE FULL ALPHA-TENSOR FOR UNCORRELATED EXPLOSIONS

We calculate all components of the dynamo alpha-tensor produced by sup ernova explosions in the galactic disk. In our turbulence model, we nu merically simulate only isolated explosions, taking into account galac tic differential rotation but neglecting Lorentz forces. From the time -dependent 3-dimensional magnetohydrodynamic (MHD) simulations, the tu rbulent electromotive force (EMF) is calculated and then averaged over an empirical supernova distribution function to derive the alpha-tens or for a field of explosions. We simulate explosions occurring in the solar neighborhood (8.5 kpc) and at a radius of 3 kpc from the galacti c center. According to the specific physical conditions of the interst ellar medium (ISM) at these galactocentric radii, we found components of the alpha-tensor, alpha(ij), which differ in their spatial profiles perpendicular to the disk and their amplitudes. Our results, based up on numerical experiments, generally confirm the analytic results given by Ferriere (1992a,b). The exact values of the amplitudes strongly de pend on the scale height of the supernova distribution. Generally, we find alpha-values of only a few meters per second. The component alpha (yx) is shown to represent an effective vertical velocity at which gas and the confined magnetic flux escape from the galactic plane. The am plitude of alpha(yx) is about two orders of magnitude larger than the other non-negligible alpha-components. This is a serious problem since horizontal magnetic flux is advected away from the galactic disk with much higher rate than can be regenerated through alpha-action.