2003MNRAS.338..312G

Polarization-sensitive λ= 6 cm global VLBI images of BL Lac for data taken at three epochs are presented and analysed, together with an additional epoch for which only total intensity data are available. Previous λ= 6 cm and λ= 3.6 cm VLBI polarization observations of BL Lac have shown that the jet components in this source tend to have magnetic field transverse to the jet direction. On the whole, these images confirm this tendency, though there is evidence for possibly oblique or longitudinal magnetic fields in some features at some epochs. The flux densities of individual features moving from the core can initially drop rapidly, but the rate of flux decrease slows with time, so that many features are, in fact, remarkably long-lived. This has made it possible to trace the movements of some components over more than a decade. The trajectories of individual components show oscillations whose amplitude appears to grow with distance from the VLBI core.