2014MNRAS.437.3405L

We model the kiloparsec (kpc)-scale synchrotron emission from jets in 10 Fanaroff-Riley Class I radio galaxies for which we have sensitive, high-resolution imaging and polarimetry from the Very Large Array. We assume that the jets are intrinsically symmetrical, axisymmetric, decelerating, relativistic outflows and we infer their inclination angles and the spatial variations of their flow velocities, magnetic field structures and emissivities using a common set of fitting functions. The inferred inclinations agree well with independent indicators. The spreading rates increase rapidly, then decrease, in a flaring region. The jets then recollimate to form conical outer regions at distance r0 from the active galactic nucleus (AGN). The flaring regions are homologous when scaled by r0. At ~0.1r0, the jets brighten abruptly at the onset of a high-emissivity region and we find an outflow speed of ~0.8c, with a uniform transverse profile. Jet deceleration first becomes detectable at ~0.2r0 and the outflow often becomes slower at its edges than it is on-axis. Deceleration continues until ~0.6r0, after which the outflow speed is usually constant. The dominant magnetic-field component is longitudinal close to the AGN and toroidal after recollimation, but the field evolution is initially much slower than predicted by flux-freezing. In the flaring region, acceleration of ultrarelativistic particles is required to counterbalance the effects of adiabatic losses and account for observed X-ray synchrotron emission, but the brightness evolution of the outer jets is consistent with adiabatic losses alone. We interpret our results as effects of the interaction between the jets and their surroundings. The initial increase in brightness occurs in a rapidly falling external pressure gradient in a hot, dense, kpc-scale corona around the AGN. We interpret the high-emissivity region as the base of a transonic `spine' and suggest that a subsonic shear layer starts to penetrate the flow there. Most of the resulting entrainment must occur before the jets start to recollimate.