2013ApJ...773L..11F

We report on a comparison between 21 cm rotation measure (RM) and the optically thin atomic hydrogen column density (N_ H I_(τ ⟶ 0)) measured toward unresolved extragalactic sources in the Galactic plane of the northern sky. H I column densities integrated to the Galactic edge are measured immediately surrounding each of nearly 2000 sources in 1 arcmin 21 cm line data, and are compared to RMs observed from polarized emission of each source. RM data are binned in column density bins 4x10²⁰/cm2 wide, and one observes a strong relationship between the number of hydrogen atoms in a 1 cm² 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 to 14x10²¹ atoms/cm2) of column densities, with a constant RM/N_ H I_∼ -23.2±2.3 rad m^–2/10²¹ atoms/cm2, 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 second quadrant of < B_|| > ∼1.0±0.1 µG directed away from the Sun, assuming an ionization fraction of 8% (consistent with the warm-neutral medium; WNM). The remarkable consistency between this field and < B > = 1.2 µG found with the same RM sources and a Galactic model of dispersion measures (DMs) suggests that electrons in the partially ionized WNM are mainly responsible for pulsar DMs, and thus the partially ionized WNM is the dominant form of the magneto-ionic interstellar medium.