86, 43, and 22 GHz VLBI observations of 3C 120

We present the first 86 GHz VLBI observations of the radio galaxy 3C 120, t ogether with contemporaneous 43 and 22 GHz polarimetric VLBA observations. The very high angular resolution obtained at 86 GHz provides an upper limit to the size of the core of 54 mu as (0.025 h(-1) pc). This represents a di rect determination of the base of the jet that is independent of variabilit y arguments (which depend on uncertain estimates of the Doppler factor) and places it below approximately 1 lt-month. Comparison with previous VLBA ob servations after a 1 yr interval shows pronounced changes in the structure and polarization of the jet. Most of the components are found to follow a c urved path while undergoing a steepening of their spectra accompanied by a decrease in total and polarized emission. However, at least one component i s observed to follow a quasi-ballistic motion, accompanied by a flattening of its spectrum as well as an increase in total and polarized flux. This ma y be explained by its interaction with the external medium, resulting in a shock that enhances the emission and aligns the magnetic field perpendicula r to the component motion, thereby producing an increase of the degree of p olarization from undetected values to as high as 15%. A second strong compo nent, with the highest degree of polarization (23%), is found to have exper ienced a displacement from the ridge line of the structural position angle of the jet as it moved downstream. We have found a mean swing to the south of the position angle of the innermost components of similar to 6 degrees b etween late 1996 and 1997, which may be responsible for the jet curvature o bserved at parsec and kiloparsec scales.