New Constraints on the Galactic Halo Magnetic Field using Rotation
  Measures of Extragalactic Sources Towards the Outer Galaxy
  We present a study of the Milky Way disk and halo magnetic field, determined
from observations of Faraday rotation measure (RM) towards 641 polarized
extragalactic radio sources in the Galactic longitude range 100-117 degs,
within 30 degs of the Galactic plane. For |b| < 15 degs, we observe a symmetric
RM distribution about the Galactic plane. This is consistent with a disk field
in the Perseus arm of even parity across the Galactic mid-plane. In the range
15<|b|<30 degs, we find median rotation measures of -15+/-4 rad/m^2 and -62+/-5
rad/m^2 in the northern and southern Galactic hemispheres, respectively. If the
RM distribution is a signature of the large-scale field parallel to the
Galactic plane, this suggests that the halo magnetic field toward the outer
Galaxy does not reverse direction across the mid-plane. The variation of RM as
a function of Galactic latitude in this longitude range is such that RMs become
more negative at larger |b|. This is consistent with an azimuthal magnetic
field of strength 2 microGauss (7 microGauss) at a height 0.8-2 kpc above
(below) the Galactic plane between the local and the Perseus spiral arm. We
propose that the Milky Way could possess spiral-like halo magnetic fields
similar to those observed in M51.
