The dynamics of relativistic plasmoids and rapidly varying radio sources

Variability and superluminal expansion of compact radio sources are investigated in terms of a theoretical model for the dynamics of radio sources involving relativistic motion of ram-pressure confined plasmoids (defined to be aggregates of magnetized plasma and relativistic particles). It is shown that relativistic spherical blast waves are subject to fragmentation leading to the formation of a number of discrete, relativistic plasmoids. It is shown that the initial temperature of the thermal component of the plasma fragments is likely to be on the order of 10/sup 12/ K. At this temperture the electrons in the plasma are relativistic, although the proton thermal velocity is nonrelativistic. As a result, the Faraday depolarization of such plasmoids is negligible, in agreement with observations of compact sources. Because of relativistic beaming, the plasmoids whose trajectories lie closest to the observer's line of sight are preferentially seen, causing the sources to exhibit variability and superluminal expansion effects which are also consistent with observations of rapidly varying radio sources.