On the connection of the apparent proper motion and the VLBI structure
  of compact radio sources
  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 micro-arcseconds per year for certain objects. When imaged with VLBI at
milli-arcsecond (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, not only for 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.
