  We present an analysis of rotation measure (RM) fluctuations from the Test
Region of the Southern Galactic Plane Survey (SGPS), along with emission
measure (EM) fluctuations in the same field taken from the Southern H-Alpha Sky
Survey Atlas. The structure function of RM fluctuations shows a relatively
steep slope at small scales (1-5 arcmin), a break in slope to a flatter
structure function at intermediate scales (5-60 arcmin), and a systematic
variation of the strength of fluctuations as a function of position angle on
the sky at the largest scales (60-200 arcmin). The structure function of EM
fluctuations shows similar behavior, although the lower resolution of the data
prevents detection of a possible break in the spectrum. We interpret the
anisotropy in RM/EM structure on large scales as resulting from a large-scale
gradient in electron density (and possibly magnetic field) across the region.
The break in the slope of the RM structure function at scales of about 5 arcmin
can be explained by contributions from two spatially distinct magneto-ionized
screens, most likely in the Local and Carina spiral arms. The observed
structure function then implies that the outer scale of RM fluctuations in
these screens is about 2 pc. We conclude that we have identified an additional
source of enhanced turbulence, injected on scales of a few pc, possibly seen
only in the Galactic plane. The most likely source of such turbulence is
individual HII regions from relatively low-mass stars, whose characteristic
scale size is similar to the outer scale of turbulence inferred here. These
sources may be the dominant source of density and velocity fluctuations in warm
ionized gas in the Galactic plane.
