Magnetic Fields and Ionized Gas in the Inner Galaxy: An Outer Scale for Turbulence and the Possible Role of HiiRegions

We present an analysis of rotation measure (RM) fluctuations from the TestRegion of the Southern Galactic Plane Survey (SGPS), along with emissionmeasure (EM) fluctuations in the same field taken from the Southern H-Alpha SkySurvey Atlas. The structure function of RM fluctuations shows a relativelysteep slope at small scales (1-5 arcmin), a break in slope to a flatterstructure function at intermediate scales (5-60 arcmin), and a systematicvariation of the strength of fluctuations as a function of position angle onthe sky at the largest scales (60-200 arcmin). The structure function of EMfluctuations shows similar behavior, although the lower resolution of the dataprevents detection of a possible break in the spectrum. We interpret theanisotropy in RM/EM structure on large scales as resulting from a large-scalegradient 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 arcmincan be explained by contributions from two spatially distinct magneto-ionizedscreens, most likely in the Local and Carina spiral arms. The observedstructure function then implies that the outer scale of RM fluctuations inthese screens is about 2 pc. We conclude that we have identified an additionalsource of enhanced turbulence, injected on scales of a few pc, possibly seenonly in the Galactic plane. The most likely source of such turbulence isindividual HII regions from relatively low-mass stars, whose characteristicscale size is similar to the outer scale of turbulence inferred here. Thesesources may be the dominant source of density and velocity fluctuations in warmionized gas in the Galactic plane.