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We describe approximate axisymmetric computations of the dynamical evolutionof material inside radio lobes and X-ray cluster gas cavities in Fanaroff-RileyII sources such as Cygnus A. All energy is delivered by a jet to thelobe/cavity via a moving hotspot where jet energy dissipates in a reverseshock. Our calculations describe the evolution of hot plasma, cosmic rays (CRs)and toroidal magnetic fields flowing from the hotspot into the cavity. Manyobserved features are explained. Gas, CRs and field flow back along the cavitysurface in a "boundary backflow" consistent with detailed FRII observations.Computed ages of backflowing CRs are consistent with observed radio-synchrotronage variations only if shear instabilities in the boundary backflow are dampedand we assume this is done with viscosity of unknown origin. Magnetic fieldsestimated from synchrotron self-Compton (SSC) X-radiation observed near thehotspot evolve into radio lobe fields. Computed profiles of radio synchrotronlobe emission perpendicular to the jet are dramatically limb-brightened inexcellent agreement with FRII observations although computed lobe fields exceedthose observed. Strong winds flowing from hotspots naturally create kpc-sizedspatial offsets between hotspot inverse Compton (IC-CMB) X-ray emission andradio synchrotron emission that peaks 1-2 kpc ahead where the field increasesdue to wind compression. In our computed version of Cygnus A, nonthermal X-rayemission increases from the hotspot (some IC-CMB, mostly SSC) toward the offsetradio synchrotron peak (mostly SSC). A faint thermal jet along the symmetryaxis may be responsible for redirecting the Cygnus A non-thermal jet.

Dynamics Inside the Radio and X-ray Cluster Cavities of Cygnus A and Similar FRII Sources