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 ^ { \circ } -117 ^ { \circ } , within 30 ^ { \circ } of the Galactic plane .
For | b | < 15 ^ { \circ } , 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 ^ { \circ } < | b | < 30 ^ { \circ } , we find median rotation measures of - 15 \pm 4 rad m ^ { -2 } and - 62 \pm 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 \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 .