EVOLUTION OF THE PARSEC-SCALE LINEAR-POLARIZATION STRUCTURE OF THE SUPERLUMINAL QUASAR-3C-345

Six epochs of linear polarization sensitive VLBI observations spanning more then 6 yr are used to study the evolution of the parsec-scale st ructure of the superluminal quasar 3C 345 at 5 GHz (lambda = 6 cm). Th ese images form the first few frames of a ''magnetic movie'' showing t he structure and evolution of the magnetic held. Most of the polarized flux is associated with the moving components in the jet. New compone nts emerge with a small degree of polarization that increases as they separate from the core. The most highly polarized part of the jet has fractional polarization m(max) similar or equal to 20%, so there is si gnificant order in the magnetic field. The magnetic field orientation along the jet varies in a complicated fashion with both position and t ime. Near the core, where components have been observed to accelerate, the field orientation also changes rapidly. In the gently curving par t of the jet the magnetic field is very nearly parallel to the local j et axis. At the outermost knot visible at this frequency and resolutio n, the polarization is confined to the leading edge of the component, and the magnetic field is offset from parallel to the jet axis by simi lar to 40 degrees. The difference between the polarization on milliarc second and arcsecond scales shows the ''intermediate-scale'' jet to be modestly polarized (similar to 4%) with its magnetic field lying roug hly parallel to the jet direction. At all epochs a peak in the fractio nal polarization occurs between components, in the underlying jet. Thi s is strong evidence that the knots are regions of compression in a je t containing both longitudinal and tangled components of magnetic fiel d. Such a model is described in detail in a companion paper (Wardle et al. 1994).