Galactic winds and the origin of large-scale magnetic fields
  Observations of dwarf galaxies suggest the presence of large-scale magnetic
fields. However the size and slow rotation of these galaxies appear
insufficient to support a mean-field dynamo action to excite such fields. Here
we suggest a new mechanism to explain large-scale magnetic fields in galaxies
that are too small to support mean-field dynamo action. The key idea is that we
do not identify large-scale and mean magnetic fields. In our scenario the the
magnetic structures originate from a small-scale dynamo which produces
small-scale magnetic field in the galactic disc and a galactic wind that
transports this field into the galactic halo where the large turbulent
diffusion increases the scale and order of the field. As a result, the magnetic
field becomes large-scale; however its mean value remains vanishing in a strict
sense. We verify the idea by numerical modelling of two distinct simplified
configurations, a thin disc model using the no-$z$ approximation, and an
axisymmetric model using cylindrical $r,z$ coordinates. Each of these allows
reduction of the problem to two spatial dimensions. Taken together, the models
support the proposition that the general trends will persist in a fully 3D
model. We demonstrate that a pronounced large-scale pattern can develop in the
galactic halo for a wide choice of the dynamo governing parameters. We believe
that our mechanism can be relevant to explaining the presence of the fields
observed in the halos of dwarf galaxies. We emphasize that detailed modelling
of the proposed scenario needs 3D simulations, and adjustment to the specific
dynamo governing parameters of dwarf galaxies.
