Time-dependent structure of perturbed relativistic jets

The internal structures found in a set of relativistic axisymmetric jet simulations are analyzed. The structures are induced by a conical pressure wave associated with the jet inlet on the computational grid and by cocoon vortices around the propagating jet. The observed structures and differences between structure in the different simulations are found to be fully understandable in terms of the structure and growth or damping of the normal axisymmetric Fourier modes of a cylindrical jet. Differences in detailed internal structure are largely the result of differences in the wavelength of perturbations applied by the conical pressure wave at the inlet and by cocoon turbulence. In some cases the perturbations can strongly couple to a normal mode of the jet. In other cases strong damping occurs when the perturbations cannot easily couple to a normal mode of the jet. In particular, the relative stability and lack of strong internal structure in the highly relativistic numerical simulations performed by Duncan & Hughes is a result of weak coupling to a normal mode of the jet.