Generation of bisymmetric magnetic fields in galaxies with tidal interaction

Tidal interactions between neighbouring galaxies are expected to induce sig nificant nonaxisymmetric velocities in their disks. It has been suggested t hat these velocities play an important role in the generation of bisymmetri c magnetic fields observed in interacting galaxies. We investigate the effe ct of a nonaxisymmetric radial out ow on a three-dimensional linear mean fi eld dynamo. We find that the usually dominant axisymmetric quadrupole is ef fectively damped by the out ow. For sufficiently high velocities a bisymmet ric magnetic mode is then preferentially excited. The resulting field has a spiral-arm structure extending well into the differentially rotating outsk irts of the disk. The influence of velocity-induced mode-coupling effects o n bisymmetric field generation is found to be negligible. While being highl y idealized, the model seems to give reasonable representations of the larg e scale fields of the interacting galaxies M 81 and M 51.