The Local Direction and Curvature of the Galactic Magnetic Field Derived from Starlight Polarization

Abstract

We use the Mathewson & Ford compilation of starlight polarization data to derive the local direction and curvature of the Galactic magnetic field lines. We find the field lines are spirals, with a local radius of curvature 8.8±1.8 kpc and a local inclination angle i = 7°.2±4°.1. Despite these apparently large errors, we show that with high statistical confidence our results are inconsistent with those derived from pulsar data by Rand & Lyne, with a concentric-circle model, and with a spiral-arm model having inclination i = 12°.5 (the standard value for the Galaxy). In external galaxies, the field lines follow the spiral pattern closely. Unless the magnetic field pattern of the Galaxy is unique, we must invoke local distortions of the spiral pattern or the field lines; such distortions might conceivably be Galactic in scale and related to the field reversal that occurs just inside the solar circle.