Rotation measures in the fourth Galactic quadrant

We reduced the influence of structure in electron density as much as possible by excluding objects for which H{alpha}-data indicate large fluctuations in n_e_ somewhere along the line of sight. We also excluded objects for which the RM may have been significantly "corrupted" by an intervening supernova remnant.

We modelled RM(l), the longitude dependence of RM of the unaffected extragalactic sources and pulsars. We assumed several geometries for the large-scale field. All but one of those are based on logarithmic spiral arms (with various pitch angles and widths), while one has circular symmetry.