VSOP polarization observations of the BL Lacertae object OJ 287

VLBI total intensity and linear polarization images of the BL Lacertae obje ct OJ 287 have been obtained at lambda = 6 cm using a global ground array a nd the HALCA orbiting antenna, and at lambda = 1.3 cm two weeks earlier usi ng 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 unresolve d. The baselines to the orbiting antenna resolve both of these compact comp onents. In the VSOP images, the ground-based 'core' breaks up into several distinct components, demonstrating that this region is dominated by the con tribution of bright, optically thin knots of jet emission. A very similar s tructure 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 tran sition 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 transvers e direction owing to the action of a shock; the physical nature of the exte nded region of longitudinal field closer to the core is not clear. The avai lability of nearly simultaneous observations with comparable resolution at widely spaced frequencies enabled detection of a similar or equal to 90 deg rees rotation in polarization position angle for the core, owing to the tra nsition from the optically thick (6 cm) to the optically thin (1.3 cm) regi me.