  We present an analysis of the rotation measures (RMs) of polarized
extragalactic point sources in the Southern Galactic Plane Survey. This work
demonstrates that the statistics of fluctuations in RM differ for the spiral
arms and the interarm regions. Structure functions of RM are flat in the spiral
arms, while they increase in the interarms. This indicates that there are no
correlated RM fluctuations in the magneto-ionized interstellar medium in the
spiral arms on scales larger than ~ 0.5 deg, corresponding to ~ 17 pc in the
nearest spiral arm probed. The non-zero slopes in interarm regions imply a much
larger scale of RM fluctuations. We conclude that fluctuations in the
magneto-ionic medium in the Milky Way spiral arms are not dominated by the
mainly supernova-driven turbulent cascade in the global ISM but are probably
due to a different source, most likely H II regions.
