DEPOLARIZATION AND FARADAY EFFECTS IN GALAXIES

Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto-ionic medium and relativisti c electrons are non-uniformly distributed in a layer containing both r egular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity, We demonstrate that non-uniformity of the magneto-ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whethe r or not the source is symmetric along the line of sight. The RM may c hange sign in a certain wavelength range in an asymmetric slab even wh en the line-of-sight magnetic field has no reversals. Faraday depolari zation in a purely regular magnetic field can be much stronger than su ggested by the low observed rotation measures, A twisted regular magne tic field may result in p increasing with lambda - a behaviour detecte d in several galaxies. We derive expressions for statistical fluctuati ons in complex polarization and show that random fluctuations in the d egree of polarization caused by Faraday dispersion are expected to bec ome significantly larger than the mean value of p at lambda greater th an or similar to 20-30cm, We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the so urce and in an external Faraday screen. We briefly discuss application s of the above results to radio polarization observations. We discuss how the degree of polarization is affected by the scaling of synchrotr on emissivity epsilon with the total magnetic field strength B, We der ive formulae for the complex polarization at lambda --> 0 under the as sumption that epsilon alpha (BBperpendicular to2)-B-2, which may arise under energy equipartition or pressure balance between cosmic rays an d magnetic fields. The resulting degree of polarization is systematica lly larger than for the usually adopted scaling epsilon alpha B-perpen dicular to(2); the difference may reach a factor of 1.5.