FINE-SCALE STRUCTURE IN RELATIVISTIC JETS AND RAPID VARIABILITY IN BLAZARS

Several versions of the shock-in-jet model of the blazars' variability have been proposed in recent years. This family of models resorts to the interaction between a relativistic shock and a feature in the pars ec-scale jet associated to an AGN in order to explain the origin of th e rapid variability recently observed in many extragalactic radio sour ces. The nature of the involved features varies from one particular mo del to another. Here I discuss a possible origin for this variety of s ubparsec-sized features in the frame of the two-fluid model of extraga lactic jets. Macroscopic Kelvin-Helmholtz instabilities are shown to b e capable of producing the required plurality of morphologies in the i nnermost part of the beams, in such a way that the different models ha ve particular ranges of applicability according to the differencies in the parameters of the flows.