Modeling the 3C 120 Radio Jet from 1 to 30 Milliarcseconds

In this paper we use the predicted spatial development of helical structures along an expanding jet to model observed structures and motions in the 3C 120 jet. New results of VLBI imaging of the parsec-scale radio jet in 3C 120 at 5 GHz are examined along with older long-term monitoring results at 5 GHz and older results obtained at 22 and 43 GHz. The high-frequency observations provide detailed information on motions and structure from 0.5 to 10 mas from the core and the lower frequency observations from 1 to 30 mas from the core. Proper motions of helical components associated with the pattern and of other components that move through the pattern provide estimates of flow and helical pattern speeds. Theoretical modeling of the motion and appearance of the helical pattern allows determination of sound speeds as a function of the jet viewing angle. The jet sound speed declines although probably not as fast as adiabatically. At a 12° viewing angle the most likely scenario involves a decline in jet sound speed from c/3 < aj < c/ at ~0.5 mas from the core to 0.1c < aj < 0.25c at ~25 mas from the core accompanied by some acceleration in the jet flow from Lorentz factor γ ≲ 5 to γ ≳ 7. The sound speed in the cocoon medium around the jet is less well determined but is less than the sound speed in the jet probably by a factor of 1.5-5. A largest possible viewing angle of 15° implies a jet sound speed at the upper limit of these estimates and somewhat higher flow Lorentz factors. However, jet morphology argues against viewing angles larger than 12°. At smaller viewing angles the jet sound speed is lower, and at a 6° viewing angle the jet sound speed is about a factor of 2 less, but the flow Lorentz factor is comparable. The decline in radio intensity is on the order of what would be associated with isothermal jet expansion. Knot-interknot intensity variations are greater than would be expected from adiabatic compressions associated with the helical twist, and we infer the presence of a shock along the leading edge of the helical twist in addition to shock or density structures flowing through the helical pattern. Our results imply that the macroscopic heating of the expanding jet fluid is less than the microscopic energization of the synchrotron radiating relativistic electrons.