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 H-alpha 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
microgauss, 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 5 HII regions. The slope of magnetic
field vs. density in the low-density regime (0.8 < n_e < 30 per cubic cm) 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 5 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.
