Magnetic field and RM structure in the jet of 3C 120

We present a multi-frequency polarimetric VLBI observation of 3C 120. We investigated the Faraday rotation measure (RM) distribution of the central parsecs of the host of the jet. Our analysis revealed that the RM feature closely associated with the variation of magnetic field (B-field) orientation and spectral distribution of the jet. The variation of the B-field and spectral distribution are presumably brought about by some physical mechanism of the synchrotron emitting plasma of the jet. On the other hand, Faraday rotation is responsible for the low energy plasma, therefore the variation of RM is brought about by the electron density and/or B-field strength of the low energy plasma. This result leads us to expect that the low energy plasma surrounds the synchrotron emitting plasma, and that they are physically related.