  Radio observations of Fanaroff-Riley class II sources often show correlations
between the synchrotron emission and the linear-polarimetric distributions.
Magnetic position vectors seem to align with the projected emission of both the
radio jets and the sources' edges. Using statistics we study such relation as
well as its unknown time evolution via synthetic polarisation maps of model FR
II sources formed in 3D-MHD numerical simulations of bipolar, hypersonic and
weakly magnetised jets. The magnetic field is initially random with a
Kolmogorov power spectrum, everywhere. We investigate the structure and
evolution of magnetic fields in the sources as a function of the power of jets
and the observational viewing angle. Our synthetic polarisation maps agree with
observations, showing B-field vectors which are predominantly aligned with the
jet axis, and show that magnetic fields inside sources are shaped by the jets'
backflow. Polarimetry is found to correlate with time, the viewing angle and
the jet-to-ambient density contrast. The magnetic structure inside thin
elongated sources is more uniform than inside more spherical ones. We see jets
increase the magnetic energy in cocoons in proportion to the jet velocity and
the cocoon width. Filaments in the synthetic emission maps suggest turbulence
develops in evolved sources.
