ar X iv : a st ro - p h / 99 03 33 0 v 3 2 8 A pr 1 99 9 Radio Jet - Ambient Medium Interactions on Parsec Scales in the Blazar 1055 + 018

As part of our study of the magnetic fields of active galactic nuclei, we have recently observed a large sample of blazars with the Very Long Baseline Array. Here we report the discovery of a striking two-component jet in the source 1055+018 that consists of an inner spine with a transverse magnetic field and a fragmentary but distinct boundary layer with a longitudinal magnetic field. The polarization distribution in the spine strongly supports shocked-jet models, while that in the boundary layer suggests interaction with the surrounding medium. This behavior suggests a new way to understand the differing polarization properties of strongand weak-lined blazars. Subject headings: galaxies: jets, magnetic fields — polarization — quasars: individual (1055 +018)