Depolarisation and the study of galaxy cluster magnetic fields with the SKA

Future radio surveys, most notably with the SKA aim at determining the comological evolution of magnetic fields using Faraday rotation of a huge number of cosmological radio sources. In contrast to currently used techniques, these surveys are planned for sub-GHz radio frequencies. However, radio emission is typically depolarised at low frequencies due to differential Faraday
rotation along different lines of sight within the telescope beam. Therefore, it is vital for the experiment to predict the exact frequency below which we expect depolarisation to occur. We compare a new turbulent depolarisation model to the classical two-field case, finding many interesting similarities. We calculate the depolarisation frequency, below which magnetic field measurements become much more difficult for the new model. Extrapolating from recent data,
we find a median depolarisation frequency of 300 MHz for typical SKA sources, hence enabling the determination of magnetic fields via the SKA mid frequency band.