Rotation measures of low-latitude extragalactic sources and the magnetoionic structure of the Galaxy

We have obtained Faraday rotation measurements of 56 extragalactic sources predominantly in the range 45^deg^ < l < 93^deg^, |b| < 5^deg^. The data complement the few rotation measures (RMs) that have been published for low-latitude extragalactic sources, and are used to probe the Galactic magnetic field and electron density to great distances. Within the longitude range sampled here, we find RM(l) = 1600 sin (l_0_ - l) rad m^2^, with a null in RM occurring at l_0_ = 62^deg^. Under the assumption of a uniform circular geometry for the magnetic field lines, the magnetoionic medium must exist to a Galactocentric radius R_m_ ~ 25 kpc to produce the observed magnitudes of the RMs, where we have assumed that n_e_ and |B| equal their local values of 0.03 cm^-3^ and 2.1 microG out to R_m_. The medium must exist to an even greater radius if n_e_ and/or |B| decrease with R, as is likely. The null in RM at l_0_ confirms that a field reversal occurs interior to R_0_ (the radius of the solar orbit about the Galactic center) and requires that the product quantity n_e_|B| be greater inside the field-reversed region than in the local ISM. Comparison of extragalactic and pulsar RMs along nearly coincident lines of sight is consistent with at least one field reversal exterior to R_0_. An anomalous RM contribution of >~1000 rad m^2^ is found at l = 92^deg^, b = 0^deg^, with an angular size of ~1^deg^-8^deg^. The phenomenon is akin to enhanced RMs observed behind the Cygnus region. The majority of our sources are components of double- lobed objects with lobe separations of 30"-200". This allows us to study variations in Galactic Faraday rotation over small angular scales. The differences exceed those expected from a large-scale ordered field by more than two orders of magnitude. The implied linear scales for fluctuations in the magnetoionic medium are ~0.1-10 pc. We are able to explain the small-scale variations in RM solely in terms of previously observed electron density turbulence in the interstellar medium.