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-degrees < l < 93-degrees, \b\ < 5-degrees. 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 (l0 - l) rad m-2, with a null in RM occurring at l0 = 62-degrees. Under the assumption of a uniform circular geometry for the magnetic field lines, the magnetoionic medium must exist to a Galactocentric radius R(m) congruent-to 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-mu-G 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 l0 confirms that a field reversal occurs interior to R0 (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 R0. An anomalous RM contribution of greater-than-or-similar-to 1000 rad m-2 is found at l = 92-degrees, b = 0-degrees, with an angular size of approximately 1-degrees-8-degrees. 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 approximately 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.