Rotation measures of extragalactic radio sources

For |b|<15{deg}, 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{deg}<|b|<30{deg}, we find median RMs of -15+/-4rad/m2 and -62+/-5rad/m2 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|.