  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.
