Magnetic field tomography, helical magnetic fields and Faraday
  depolarization
  Wide-band radio polarization observations offer the possibility to recover
information about the magnetic fields in synchrotron sources, such as details
of their three-dimensional configuration, that has previously been
inaccessible. The key physical process involved is the Faraday rotation of the
polarized emission in the source (and elsewhere along the wave's propagation
path to the observer). In order to proceed, reliable methods are required for
inverting the signals observed in wavelength space into useful data in Faraday
space, with robust estimates of their uncertainty. In this paper, we examine
how variations of the intrinsic angle of polarized emission $\psi_{0}$ with the
Faraday depth $\phi$ within a source affect the observable quantities. Using
simple models for the Faraday dispersion $F(\phi)$ and $\psi_{0}(\phi)$, along
with the current and planned properties of the main radio interferometers, we
demonstrate how degeneracies among the parameters describing the magneto-ionic
medium can be minimised by combining observations in different wavebands. We
also discuss how depolarization by Faraday dispersion due to a random component
of the magnetic field attenuates the variations in the spectral energy
distribution of the polarization and shifts its peak towards shorter
wavelengths. This additional effect reduces the prospect of recovering the
characteristics of the magnetic field helicity in magneto-ionic media dominated
by the turbulent component of the magnetic field.
