MAGNETIC-FIELD STRUCTURE IN DIFFERENTIALLY ROTATING-DISKS

In order to gain a better understanding of the processes that may give rise to nonaxisymmetric magnetic fields in galaxies, we have calculated field decay rates for models with a realistic galactic rotation curve and including the effects of a locally enhanced turbulent magnetic diffusivity within the disc. In all cases we have studied, the differential rotation increases the decay rate of nonaxisymmetric modes, whereas axisymmetric ones are unaffected. A stronger magnetic diffusivity inside the disc does not lead to a significant preference for nonaxisymmetric modes. Although Elsasser's antidynamo theorem has not yet been proved for the present case of a non-spherical distribution of the magnetic diffusivity, we do not find any evidence for the theorem not to be valid in general. © 1990, Taylor & Francis Group, LLC. All rights reserved.