A Method for a Pseudo-local Measurement of the Galactic Magnetic Field

Abstract

Abstract 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 contributions of volume elements along a long line of sight (LOS). A magnetic field measurement at a given spatial location is of much more physical significance. In this paper, I point out that H ii regions fortuitously offer such a “point” measurement, albeit of one component of the magnetic field, and averaged over the sightline through the H ii region. However, the LOS through an H ii region is much smaller (e.g., 30–50 pc) than one through the entire Galactic disk, and thus constitutes a “pseudo-local” measurement. I use published H ii region Faraday rotation measurements to provide a new constraint on the magnitude of magnetohydrodynamic turbulence in the Galaxy.