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 ( \sim 0.3–1 kpc ) this plane , from the vicinity of the warm interstellar medium ( WIM ) /halo interface .
We propose \citep [ see also ] [ ] Henriksen:2016aa 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 .