In this talk, I will review observational constrains we have on the
magnetic field structure of our own Milky Way. In particular, I will
present results from two surveys of Faraday rotation measure of polarized
extragalactic sources. Based on rotation measures along 1,000 sight lines
towards the Galactic poles, we find a lack of vertical magnetic field
symmetry across the Galactic mid-plane. The observed RMs could be the
superposition of a symmetric disk field and an anti-symmetric field
produced by a separate dynamo effect in the Galactic halo. Furthermore, I
demonstrate that existing Galactic halo magnetic field models cannot
successfully reproduce extragalactic rotation measures at mid-Galactic
latitudes in the second Galactic quadrant. We suggest that halo fields
consist of magnetic spirals could potentially account for the observed
rotation measure pattern.
