VLBI Monitoring of the Sub-parsec-scale Jet in the Radio Galaxy 3C 66B
  at 22 GHz
  We present measurements of proper motion of the sub-parsec scale jet at 22
GHz in the nearby FR I galaxy 3C 66B. Observations were made using the VLBA at
six epochs over four years. A phase-referencing technique was used to improve
the image quality of the weak and diffuse jet components. We find that the
inner knots are almost stationary, though one of them was expected to be
detected with the apparent speed of 0.2 mas/yr according to 8 GHz monitoring at
the same observation epochs. Clear flux variations are not observed in the core
at 22 GHz, in contrast, clear flux enhancement is observed in the core at 8
GHz. We discussed that this can be explained, if the jet has helical structure,
that the viewing angle of the jet between 8 and 22 GHz differs by a few degree
in case the jet direction is almost along our line of sight. Although these
results may imply the existence of a two-zone jet, which has been suggested in
certain radio galaxies, it cannot explain the fact that the jet at the higher
frequency jet is slower than that at the lower frequency.
