Depolarization canals and interstellar turbulence

Abstract

Recent radio polarization observations have revealed a plethora ofunexpected features in the polarized Galactic radio background thatarise from propagation effects in the random (turbulent) interstellarmedium. The canals are especially striking among them, a random networkof very dark, narrow regions clearly visible in many directions againsta bright polarized Galactic synchrotron background. There are no obviousphysical structures in the ISM that may have caused the canals, and sothey have been called Faraday ghosts. They evidently carry informationabout interstellar turbulence but only now is it becoming clear how thisinformation can be extracted. Two theories for the origin of the canalshave been proposed; both attribute the canals to Faraday rotation, butone invokes strong gradients in Faraday rotation in the sky plane(specifically, in a foreground Faraday screen) and the other only relieson line-of-sight effects (differential Faraday rotation). In this reviewwe discuss the physical nature of the canals and how they can be used toexplore statistical properties of interstellar turbulence. This opensstudies of magnetized interstellar turbulence to new methods ofanalysis, such as contour statistics and related techniques ofcomputational geometry and topology. In particular, we can hope tomeasure such elusive quantities as the Taylor microscale and theeffective magnetic Reynolds number of interstellar MHD turbulence.