The variations observed in compact extragalactic radio sources will be investigated both observationally and theoretically. These variations manifest themselves as changes both in the structure as revealed by very-long baseline interferometry (VLBI)studies and in the total flux density. The VLBI technique will be used to monitor two quasars, 4C39.25 and NRAO 140, which are under going particularly interesting variations which may also lead to new understandings of phenomena previously observed. Investigations of theoretical explanations for the observed variability in quasars and similar sources will be carried out. Models involving shock waves propagating through turbulent regions in relativistic jets appear capable of explaining the general characteristics of the observed variations, especially the frequency dependence of the variability timescales. Numerical calculations of emission patterns and spectra of shocks in turbulent jets will be performed and compared with the observational data. The ultimate goal of the proposed project is to better understand the nature and origin of high-speed compact jets in quasars, and through this the nature of the "central engine" in these objects.
