3D magnetohydrodynamic simulations of the evolution of magnetic fields in Fanaroff-Riley class II radio sources

Radio observations of Fanaroff–Riley class II (FR 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 polarization
maps of model FR II sources formed in 3D magnetohydrodynamics numerical simulations of
bipolar, hypersonic and weakly magnetized 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 polarization 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.