The formation of regular interarm magnetic fields in spiral galaxies

Context. Observations of regular magnetic fields in several nearby galaxies reveal magnetic arms situated between the material arms. The nature of these magnetic arms is a topic of active debate. Previously, we found a hint that taking into account the effects of injections of small-scale magnetic fields (e.g. generated by turbulent dynamo action) into the large-scale galactic dynamo can result in magnetic arm formation. Aims. We now investigate the joint roles of an arm/interarm turbulent diffusivity contrast and injections of small-scale magnetic field on the formation of large-scale magnetic field (magnetic arms) in the interarm region. Methods. We use the relatively simple no-z model for the galactic dynamo. This involves projection onto the galactic equatorial plane of the azimuthal and radial magnetic field components; the field component orthogonal to the galactic plane is estimated from the solenoidality condition. Results. We find that the addition of diffusivity gradients to the effects of magnetic field injections makes the magnetic arms much more pronounced. In particular, the regular magnetic field component becomes larger in the interarm space than that within the material arms. Conclusions. The joint action of the turbulent diffusivity contrast and small-scale magnetic field injections (with the possible participation of other effects previously suggested) appears to be a plausible explanation for the phenomenon of magnetic arms.