  The large-scale magnetic fields observed in spiral disc galaxies are often
thought to result from dynamo action in the disc plane. However, the increasing
importance of Faraday depolarization along any line of sight towards the
galactic plane suggests that the strongest polarization signal may come from
well above (~0.3-1 kpc) this plane, from the vicinity of the warm interstellar
medium (WIM)/halo interface. We propose (see also Henriksen & Irwin 2016) that
the observed spiral fields (polarization patterns) result from the action of
vertical shear on an initially poloidal field. We show that this simple model
accounts for the main observed properties of large-scale fields. We speculate
as to how current models of optical spiral structure may generate the observed
arm/interarm spiral polarization patterns.
