  Cosmic rays, thermal gas and magnetic fields in FRII radio cavities are
assumed to come entirely from winds flowing from just behind the jet shocks.
Combining analytic and computational methods, it is shown that the computed
radio-electron energy distribution and synchrotron emissivity spectra
everywhere in the Cygnus A radio cavity agrees with radio observations of the
Cygnus A lobes. The magnetic field energy density is small everywhere and
evolves passively in the post-shock wind. Most synchrotron emission arises in
recent post-shock material as it flows back along the radio cavity wall.
Because it experienced less adiabatic expansion, the magnetic field in this
young backflow is larger than elsewhere in the radio lobe, explaining the
observed radio synchrotron limb-brightening. The boundary backflow decelerates
due to small cavity pressure gradients, causing large-scale fields
perpendicular to the backflow (and synchrotron emission) to grow exponentially
unlike observations. However, if the field is random on subgrid (sub-kpc)
scales, the computed field reproduces both the magnitude and slowly decreasing
radio synchrotron emissivity observed along the backflow. The radio synchrotron
spectrum and image computed with a small-scale random field agree with VLA
observations. The total relativistic energy density in the post-jet shock
region required in computations to inflate the radio cavity matches the energy
density of relativistic electrons observed in the post-shock region of Cygnus
A. This indicates that the component in the jet and cavity that dominates the
dynamical evolution is a relativistic pair plasma.
