A Method for a Pseudo-Local Measurement of the Galactic Magnetic Field
  Much of the information about the magnetic field in the Milky Way and other
galaxies comes from measurements which are path integrals, such as Faraday
rotation and the polarization of synchrotron radiation of cosmic ray electrons.
The measurement made at the radio telescope results from the contributions of
volume elements along a long line of sight. The inferred magnetic field is
therefore some sort of average over a long line segment. A magnetic field
measurement at a given spatial location is of much more physical significance.
In this paper, we point out that HII regions fortuitously offer such a
``point'' measurement, albeit of one component of the magnetic field, and
averaged over the sightline through the HII region. However, the line of sight
(LOS) through an HII region is much smaller (e.g. 30 - 50 pc) than one through
the entire Galactic disk, and thus constitutes a ``pseudo-local'' measurement.
We use published HII region Faraday rotation measurements to provide a new
constraint on the magnitude of magnetohydrodynamic (MHD) turbulence in the
Galaxy, as well as to raise intriguing speculations about the modification of
the Galactic field during the star formation process.
