  Very long baseline interferometric observations of the radio galaxy 3C 120
show a systematic presence of gradients in Faraday rotation and degree of
polarization across and along the jet. These are revealed by the passage of
multiple superluminal components throughout the jet as they move out from the
core in a sequence of 12 monthly polarimetric observations taken with the VLBA
at 15, 22, and 43 GHz. The degree of polarization has an asymmetric profile in
which the northern side of the jet is more highly polarized. The Faraday
rotation measure is also stratified across the jet width, with larger values
for the southern side. Superposed on this structure we find a localized region
of high Faraday rotation measure (about 6000 rad/m^2) between approximately 3
and 4 mas from the core. This region of enhanced Faraday rotation may result
from the interaction of the jet with the ambient medium, which may also explain
the stratification in degree of polarization. The data are also consistent with
a helical magnetic field in a two-fluid jet model, consisting of an inner
emitting jet and a sheath of nonrelativistic electrons.
