Blazars with spine-sheath structures

Multi-frequency (5, 8, 15 GHz) total intensity and polarization VLBA images of three sources from a complete sample of BL Lac objects (0745+241, 1418+546, and 1652+398) are presented together with global VLBI images of the quasar 1055+018 at 1.6 and 5 GHz. These images show signs of a spiral magnetic field in the central channel of the jet, as well as a region of longitudinal magnetic field in “sheaths” at the jet edges, which may indicate an interaction between the jet and the surrounding medium on spatial scales of several parsecs.