A Relation Between the Warm Neutral and Ionized Media Observed in the
  Canadian Galactic Plane Survey
  We report on a comparison between 21 cm rotation measure (RM) and the
optically-thin atomic hydrogen column density (N_HI) measured towards
unresolved extragalactic sources in the Galactic plane of the northern sky. HI
column densities integrated to the Galactic edge are measured immediately
surrounding each of nearly 2000 sources in 1-arcminute 21 cm line data, and are
compared to RMs observed from polarized emission of each source. RM data are
binned in column-density bins 4x10^20 cm^-2 wide, and one observes a strong
relationship between the number of hydrogen atoms in a 1 cm^2 column through
the plane and the mean RM along the same line-of-sight and path length. The
relationship is linear over one order of magnitude (from 0.8-14x10^21 atoms
cm^-2) of column densities, with a constant RM/N_HI -23.2+/-2.3 rad m^-2/10^21
atoms cm^-2, and a positive RM of 45.0+/-13.8 rad m^-2 in the presence of no
atomic hydrogen. This slope is used to calculate a mean volume-averaged
magnetic field in the 2nd quadrant of <B_||>~1.0+/-0.1 micro-Gauss directed
away from the Sun, assuming an ionization fraction of 8% (consistent with the
WNM). The remarkable consistency between this field and <B>=1.2 micro-Gauss
found with the same RM sources and a Galactic model of dispersion measures
suggests that electrons in the partially ionized WNM are mainly responsible for
pulsar dispersion measures, and thus the partially-ionized WNM is the dominant
form of the magneto-ionic interstellar medium.
