NEW CONSTRAINTS ON THE GALACTIC HALO MAGNETIC FIELD USING ROTATION MEASURES OF EXTRAGALACTIC SOURCES TOWARD THE OUTER GALAXY

We present a study of the Milky Way disk and halo magnetic field, determined from observations of Faraday rotation measure (RM) toward 641 polarized extragalactic radio sources in the Galactic longitude range 100[degrees]-117[degrees], within 30[degrees] of the Galactic plane. For |b| < 15[degrees], 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[degrees] < |b| < 30[degrees], we find median RMs of -15 + or - 4 rad m super(-2) and -62 + or - 5 rad m super(-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, then 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 mu G (7 mu G) 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.