MAGNETIC FIELDS IN LARGE-DIAMETER HII REGIONS REVEALED BY THE FARADAY ROTATION OF COMPACT EXTRAGALACTIC RADIO SOURCES

We present a study of the line-of-sight magnetic fields in five large-diameter Galactic HII regions. Using the Faraday rotation of background polarized radio sources, as well as dust-corrected Ha surface brightness as a probe of electron density, we estimated the strength and orientation of the magnetic field along 93 individual sight lines through the HII regions. Each of the HII regions displayed a coherent magnetic field. The magnetic field strength (line-of-sight component) in the regions ranges from 2 to 6 mu G, which is similar to the typical magnetic field strength in the diffuse interstellar medium. We investigated the relationship between magnetic field strength and electron density in the five HII regions. The slope of magnetic field versus density in the low-density regime (0.8 cm(-3) < n(e) < 30 cm(-3)) is very slightly above zero. We also calculated the ratio of thermal to magnetic pressure, beta(th), for each data point, which fell in the range 1.01 < beta(th) < 25. Finally, we studied the orientation of the magnetic field in the solar neighborhood (d < 1.1 kpc) using our data from five HII regions along with existing measurements of the line-of-sight magnetic field strength from polarized pulsars whose distances have been determined from their annual parallax. We identify a net direction for the magnetic field in the solar neighborhood, but find no evidence for a preferred vertical direction of the magnetic field above or below the Galactic plane.