Pulsar Rotation Measures and Large-scale Magnetic Field Reversals in the Galactic Disk

We present the measurements of Faraday rotation for 477 pulsars observed by the Parkes 64 m radio telescope and the Green Bank 100 m radio telescope. Using these results, along with previous measurements for pulsars and extragalactic sources, we analyze the structure of the large-scale magnetic field in the Galactic disk. Comparisons of rotation measures of pulsars in the disk at different distances, as well as with rotation measures of background radio sources beyond the disk, reveal large-scale reversals of the field directions between the spiral arms and interarm regions. We develop a model for the disk magnetic field, which can reproduce not only these reversals but also the distribution of the observed rotation measures of background sources.