Small-scale variations in the Galactic Faraday rotation

Abstract

The variations of rotation measure across individual extragalactic 3C radio sources are analysed for two samples, selected at high and low Galactic latitudes, in order to investigate the RM contributions due to the Galactic foreground on angular scales 5 - 200 arcsec. These correspond to linear scales in the interstellar medium (ISM) of 0.05 - 2 pc, and RM variations due to the ISM are found on scales at least as small as 0.2 pc, with an amplitude requiring orders-of-magnitude enhancement of the typical magnetic field and/or thermal electron density in some regions. These variations may be due to small, faint H II regions along the line-of-sight. No plausible model of the local Faraday medium can account for the rotation measure structures in sources at high Galactic latitude without predicting even more Faraday rotation at low latitudes than is actually observed.