Interferometric phase scintillation in the IPM

Using a global VLBI array operating at 327 MHz, we find strongly correlated scintillations in the fringe phase obtained on various baselines. The magnitude of the fringe phase scintillations is approximately proportional to the baseline length. Cross correlatin of the fringe phase on different baselines reveals maximum correlation at some small lag, with the value dependent upon baseline pair. We find that the lags of maximum correlation (for different baseline pairs) are fit very well by a model in which the phase fluctuation pattern is moving across the array with fixed velocity. Since the fit velocity is 410 km/sm almost radially outwards from the sun, we confirm that the interplanetary medium is responsible. The implied phase power spectrum is in good agreement with other observations.These results have important implications for VLBI, particularly at low frequencies