Structure in the local Galactic ISM on scales down to 1 pc, from multi-band radio polarization observations

We discuss observations of the linearly polarized component of the diffuse galactic radio background. These observations, with an angular resolution o f 4', were made with the Westerbork Synthesis Radio Telescope (WSRT) in 5 f requency bands in the range 341 - 375 MHz. The linearly polarized intensity P (with polarized brightness temperature going up to 10 K) shows a 'cloudy ' structure, with characteristic scales of 15 - 30', which contains relativ ely long, but very narrow 'canals' (essentially unresolved) in which P is o nly a small fraction of that in the neighbouring beams. These 'canals' are generally seen in more than one frequency band, although their appearance changes between bands. They are probably due to depolariz ation within the synthesized beam, because the change in polarization angle Delta phi(pol) across the deepest 'canals' is in general close to 90 degre es (or 270 degrees etc.). These very abrupt changes in phi(pol), which are seen only across the 'canals', seem to be accompanied by abrupt changes in the Rotation Measure (RM), which may have the right magnitude to create the difference of close to 90 degrees in phi(pol), and thereby the 'canals'. The structure in the polarization maps is most likely due to Faraday rotati on modulation of the probably smooth polarized radiation emitted in the hal o of our Galaxy by the fairly local (less than or similar to 500 pc) ISM. T herefore, the abrupt changes of RM across the 'canals' provide evidence for very thin (less than or similar to 1 pc), and relatively long transition r egions in the ISM, across which the RM changes by as much as 100%. Such dra stic RM changes may well be due primarily to abrupt changes in the magnetic field.