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 μas/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. 1. History of Radio Source Observing by Geodetic Networks Since 1979, geodetic VLBI networks have observed in 3555 24-hour experiment sessions. Most of these observations were made by about 40 antennas. The number of radio sources observed in geodetic sessions has grown significantly during the last two decades. Until 1986, only 65 sources had been observed and these were nearly all in the Northern Hemisphere. From 1987 to 1989, 210 more sources were observed and the distribution of sources between hemispheres was greatly improved. In our current geodetic solutions, we estimate positions for 610 radio sources. Due to the fact that most of the geodetic antennas are in the Northern hemisphere, the distribution of radio sources is still better in the Northern than the Southern hemisphere. Since 1990, there have been 2-3 24-hour geodetic sessions each week, where typically 40-50 radio sources are observed per session. Typically each session used a network of 4-7 antennas, but we also have data from the bimonthly RDV series of sessions (1997-2003) that mostly used 10 of the standard geodetic antennas along with the 10-station VLBA. In each session, the choice of radio sources to be observed depended on the geometry of the network. The result is that the sampling history of the set of all sources that have been observed is generally uneven. 2. Observed Proper Motion Field A VLBI terrrestrial reference frame solution was performed using all VLBI observations from 1979-2003. In this type of solution, station positions, station velocities, and radio source positions and velocities are estimated from all of the 1