Oblique Polarization Structures in AGN and QSO Radio Jets
  We interpret the linear polarization structures observed in extragalactic radio sources, even those oriented at oblique angles to the jet flows, to be due to oblique, relativistic shock fronts in the emitting regions. Many sources exhibit indications of such oblique structures, and the goal of this investigation is to test this hypothesis quantitatively. A selected group of ten highly-variable extragalactic sources were observed with the VLBA at 15 and 43 GHz, on nine epochs spanning a 30-month period; five of these objects were also observed at 8.0 and 22 GHz. The integrated total flux densities and linear polarizations of the selected objects were also observed several times a month at 4.8, 8.0 and 14.5 GHz with the University of Michigan 26-meter telescope. All objects exhibited variability with several exhibiting more than one independent outburst during the period. We show the evolution of the polarized components with time and discuss the relativistic shock parameters required to match the observed polarization structures. Even cases where the magnetic field is apparently oriented along the jet flow can be fit by oblique shock models when relativistic aberration effects are included. 
H. D. Aller, M.F. Aller, & P.A. Hughes