Faraday ghosts: depolarization canals in the Galactic radio emission

Narrow, elongated regions of very low polarized intensity - so-called 'canals'- have recently been observed by several authors at decimetre wavelengths in various directions in the Milky Way, but their origin remains enigmatic. We show that the canals arise from depolarization by differential Faraday rotation in the interstellar medium and that they represent level lines of Faraday rotation measure RM, a random function of position in the sky. Statistical properties of the separation of canals depend on the autocorrelation function of RM, and so provide a useful tool for studies of interstellar turbulence.