  An analysis of structure in rotation measure (RM) across the sky based on the
RM catalog of Taylor et al. (2009) is presented. Several resolved RM structures
are identified with structure in the local ISM, including radio loops I, II,
and III, the Gum nebula, and the Orion-Eridanus super bubble. Structure
functions (SFs) of RM are presented for selected areas, and maps of SF
amplitude and slope across the sky are compared with H-alpha intensity and
diffuse polarized intensity. RM variance on an angular scale of 1 degree is
correlated with length of the line of sight through the Galaxy, with a
contribution from local structures. The slope of the SFs is less concentrated
to the Galactic plane and less correlated with length of the line of sight
through the Galaxy, suggesting a more local origin for RM structure on angular
scales ~ 10 degrees. The RM variance is a factor ~2 higher towards the SGP than
towards the NGP, reflecting a more wide-spread asymmetry between the northern
and southern Galactic hemispheres. Depolarization of diffuse Galactic
synchrotron emission at latitudes < 30 degrees can be explained largely by
Faraday dispersion related to small-scale variance in RM, but the errors allow
a significant contribution from differential Faraday rotation along the line of
sight.
