  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 generated,
e.g., by turbulent dynamo action, into the large-scale galactic dynamo can
result in magnetic arm formation. 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. We use the relatively simple "no-$z$" model for the
galactic dynamo. This involves projection on to 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.
We find that addition of diffusivity gradients to the effect of magnetic field
injections makes the magnetic arms much more pronounced. In particular, the
regular magnetic field component becomes larger in the interarm space compared
to that within the material arms.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.

Astrophysics of Galaxies