Quasar Apparent Proper Motion Observed by Geodetic VLBI Networks

In our standard geodetic VLBI solutions, we estimate the positions of quasars assuming that their positions do not vary in time. However, in solutions estimating proper motion, a significant number of quasars show apparent proper motion greater than 50 uas/yr. For individual quasars, there are source structure effects that cause apparent proper motion. To examine how coherent the pattern of apparent proper motion is over the sky, we have estimated the vector spherical harmonic components of the observed proper motion using VLBI data from 1980 to 2002. We discuss the physical interpretation of the estimated harmonic components.