  Depolarization of diffuse radio synchrotron emission is classified in terms
of wavelength-independent and wavelength-dependent depolarization in the
context of regular magnetic fields and of both isotropic and anisotropic
turbulent magnetic fields. Previous analytical formulas for depolarization due
to differential Faraday rotation are extended to include internal Faraday
dispersion concomitantly, for a multilayer synchrotron emitting and Faraday
rotating magneto-ionic medium. In particular, depolarization equations for a
two- and three-layer system (disk-halo, halo-disk-halo) are explicitly derived.
To both serve as a `user's guide' to the theoretical machinery and as an
approach for disentangling line-of-sight depolarization contributions in
face-on galaxies, the analytical framework is applied to data from a small
region in the face-on grand-design spiral galaxy M51. The effectiveness of the
multiwavelength observations in constraining the pool of physical
depolarization scenarios is illustrated for a two- and three-layer model along
with a Faraday screen system for an observationally motivated magnetic field
configuration.
