Pulsar Rotation Measures and the Large‐Scale Structure of the Galactic Magnetic Field

The large-scale magnetic field of our Galaxy can be probed in threedimensions using Faraday rotation of pulsar signals. We report on thedetermination of 223 rotation measures from polarization observations ofrelatively distant southern pulsars made using the Parkes radio telescope.Combined with previously published observations these data give clear evidencefor large-scale counterclockwise fields (viewed from the north Galactic pole)in the spiral arms interior to the Sun and weaker evidence for acounterclockwise field in the Perseus arm. However, in interarm regions,including the Solar neighbourhood, we present evidence that suggests thatlarge-scale fields are clockwise. We propose that the large-scale Galacticmagnetic field has a bisymmetric structure with reversals on the boundaries ofthe spiral arms. Streaming motions associated with spiral density waves candirectly generate such a structure from an initial inwardly directed radialfield. Large-scale fields increase toward the Galactic Center, with a meanvalue of about 2~$\mu$G in the Solar neighbourhood and 4~$\mu$G at aGalactocentric radius of 3 kpc.Comment: ApJ accepted. 16 pages, 14 figures, 2 tables, 223 pulsar RM