MOJAVE. X. Parsec-Scale Jet Orientation Variations and Superluminal
  Motion in AGN
  We describe the parsec-scale kinematics of 200 AGN jets based on 15 GHz VLBA
data obtained between 1994 Aug 31 and 2011 May 1. We present new VLBA 15 GHz
images of these and 59 additional AGN from the MOJAVE and 2 cm Survey programs.
Nearly all of the 60 most heavily observed jets show significant changes in
their innermost position angle over a 12 to 16 year interval, ranging from 10
deg to 150 deg on the sky, corresponding to intrinsic variations of ~0.5 deg to
~2 deg. The BL Lac jets show smaller variations than quasars. Roughly half of
the heavily observed jets show systematic position angle trends with time, and
20 show indications of oscillatory behavior. The time spans of the data sets
are too short compared to the fitted periods (5 to 12 y), however, to reliably
establish periodicity. The rapid changes and large jumps in position angle seen
in many cases suggest that the superluminal AGN jet features occupy only a
portion of the entire jet cross section, and may be energized portions of thin
instability structures within the jet. We have derived vector proper motions
for 887 moving features in 200 jets having at least five VLBA epochs. For 557
well-sampled features, there are sufficient data to additionally study possible
accelerations. We find that the moving features are generally non-ballistic,
with 70% of the well-sampled features showing either significant accelerations
or non-radial motions. Inward motions are rare (2% of all features), are slow
(<0.1 mas per y), are more prevalent in BL Lac jets, and are typically found
within 1 mas of the unresolved core feature. There is a general trend of
increasing apparent speed with distance down the jet for both radio galaxies
and BL Lac objects. In most jets, the speeds of the features cluster around a
characteristic value, yet there is a considerable dispersion in the
distribution. (abridged)
