The interaction between beams and their cocoons

It is demonstrated that large-scale density structure may exist within the cocoons of FRII radio sources, due to instability of the working surface of these sources. It is suggested that this structure is responsible for multiple deflections of the beam, and that it is convected into the beam path by a large-scale circulation in the cocoon, so that the pattern of deflections changes on a time-scale of 106 yr. This effect can explain the slight lack of collinearity of nucleus and hotspots, beam deflection such as is inferred for Cygnus A, and multiple hotspot structure. (author)