2001MNRAS.320L..49G

VLBI total intensity and linear polarization images of the BL Lacertae object OJ 287 have been obtained at λ=6cm using a global ground array and the HALCA orbiting antenna, and at λ=1.3cm two weeks earlier using the VLBA. In the ground-based 6-cm images, the source is dominated by a core-jet double structure the components of which are essentially unresolved. The baselines to the orbiting antenna resolve both of these compact components. In the VSOP images, the ground-based `core' breaks up into several distinct components, demonstrating that this region is dominated by the contribution of bright, optically thin knots of jet emission. A very similar structure is observed in the 1.3-cm image. The magnetic field in the core is transverse, becomes longitudinal in the inner jet, then makes a sharp transition to a region of transverse field further from the core. This suggests that the field in the outer jet has become highly ordered in the transverse direction owing to the action of a shock; the physical nature of the extended region of longitudinal field closer to the core is not clear. The availability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a ≃90° rotation in polarization position angle for the core, owing to the transition from the optically thick (6cm) to the optically thin (1.3cm) regime.