Localized Measurements of the Galactic Magnetic Field with Photoionized Interstellar Plasmas (HII Regions)

The Milky Way galaxy has a large scale magnetic field that is in the plane of the Galactic disk, is mainly azimuthal with a smaller radial component, and has a magnitude of approximately 4 microGauss. Most astrophysical plasma measurements are path integrals along the entire line of sight through the plasma. In this paper, we discuss a method which provides information on the Galactic magnetic field on scales much smaller than the Galactic radius (tens of parsecs rather than thousands of parsecs). We measure Faraday rotation due to HII regions, roughly spherical regions of photoionized plasmas surrounding hot stars. We have made measurements of Faraday rotation on lines of sight passing through two HII regions, the Rosette Nebula and W4. In both cases, the Faraday rotation measure (RM) is dominated by the HII region, and in both cases the sign of the RM is consistent with the sign of the large scale Galactic field in that direction. We will discuss the implications of these measurements for interstellar turbulent magnetic fluctuations on spatial scales of 10 - 100 parsecs.