HII Region Driven Galactic Bubbles and Their Relationship to the
  Galactic Magnetic Field
  The relative alignments of mid-infrared traced Galactic bubbles are compared
to the orientation of the mean Galactic magnetic field in the disk. The
orientations of bubbles in the northern Galactic plane were measured and are
consistent with random orientations - no preferential alignment with respect to
the Galactic disk was found. A subsample of HII region driven Galactic bubbles
was identified, and as a single population they show random orientations. When
this subsample was further divided into subthermal and suprathermal HII
regions, based on hydrogren radio recombination linewidths, the subthermal HII
regions showed a marginal deviation from random orientations, but the
suprathermal HII regions showed significant alignment with the Galactic plane.
The mean orientation of the Galactic disk magnetic field was characterized
using new near-infrared starlight polarimetry and the suprathermal HII regions
were found to preferentially align with the disk magnetic field. If
suprathermal linewidths are associated with younger HII regions, then the
evolution of young HII regions is significantly affected by the Galactic
magnetic field. As HII regions age, they cease to be strongly linked to the
Galactic magnetic field, as surrounding density variations come to dominate
their morphological evolution. From the new observations, the ratios of
magnetic-to-ram pressures in the expanding ionization fronts were estimated for
younger HII regions.
