1996A&A...315...63M

The effects on a nonlinear galactic mean field dynamo of a large-scale nonaxisymmetric velocity field in the disc plane is investigated. The simplified dynamo model retains full radial and azimuthal dependence, but has no explicit structure perpendicular to the disc plane. The study is motivated by the M 81-NGC 3077 encounter, and the velocity fields are taken from a detailed numerical model of the interaction. First, a rather artificial problem is considered, using a time-invariant velocity field. This demonstrates that steep azimuthal gradients of magnetic field can arise, similar to the `contrast' structures previously discussed. Then the fully time-dependent velocity field is included from the interaction model. In mid-interaction, significant nonaxisymmetric magnetic field components are present, with the bisymmetric part contributing about 30 per cent of the total field energy at a time corresponding to the present day. Azimuthal field gradients do steepen significantly, but not as much as in the preliminary, more idealized calculations. The model illustrates the generation of bisymmetric structure by galactic encounters, but does not yet give a good representation of the large-scale field of M 81.