Modeling the Magnetic Field in the Galactic Disk using New Rotation
  Measure Observations from the Very Large Array
  We have determined 194 Faraday rotation measures (RMs) of polarized
extragalactic radio sources using new, multi-channel polarization observations
at frequencies around 1.4 GHz from the Very Large Array (VLA) in the Galactic
plane at $17^\circ \leq l \leq 63^\circ$ and $205^\circ \leq l \leq 253^\circ$.
This catalog fills in gaps in the RM coverage of the Galactic plane between the
Canadian Galactic Plane Survey and Southern Galactic Plane Survey. Using this
catalog we have tested the validity of recently-proposed axisymmetric and
bisymmetric models of the large-scale (or regular) Galactic magnetic field, and
found that of the existing models we tested, an axisymmetric spiral model with
reversals occurring in rings (as opposed to along spiral arms) best matched our
observations. Building on this, we have performed our own modeling, using RMs
from both extragalactic sources and pulsars. By developing independent models
for the magnetic field in the outer and inner Galaxy, we conclude that in the
inner Galaxy, the magnetic field closely follows the spiral arms, while in the
outer Galaxy, the field is consistent with being purely azimuthal.Furthermore,
the models contain no reversals in the outer Galaxy, and together seem to
suggest the existence of a single reversed region that spirals out from the
Galactic center.
