Many of the compact extragalactic radio sources that are used as
fiducial points to define the celestial reference frame are known to
have proper motions detectable with long-term geodetic/astrometric very
long baseline interferometry (VLBI) measurements. These changes can be
as high as several hundred microarcseconds per year for certain objects.
When imaged with VLBI at milliarcsecond (mas) angular resolution, these
sources (radio-loud active galactic nuclei) typically show structures
dominated by a compact, often unresolved "core" and a one-sided "jet."
The positional instability of compact radio sources is believed to be
connected with changes in their brightness distribution structure. For
the first time, we test this assumption in a statistical sense on a
large sample rather than on only individual objects. We investigate a
sample of 62 radio sources for which reliable long-term time series of
astrometric positions as well as detailed 8 GHz VLBI brightness
distribution models are available. We compare the characteristic
direction of their extended jet structure and the direction of their
apparent proper motion. We present our data and analysis method, and
conclude that there is indeed a correlation between the two
characteristic directions. However, there are cases where the ~1-10 mas
scale VLBI jet directions are significantly misaligned with respect to
the apparent proper motion direction.