Spaced receiver scintillation data for a 250-MHz signal transmitted from a geosynchronous satellite and received at an equatorial station are subjected to correlation analysis to determine the averge drift of equatorial F region irregularities transverse to the signal path, as well as the characteristic velocity which is a measure of the temporal variability of the irregularities. These parameters are computed for periods wtih significant amplitude scintillations during different seasons and magnetic activity levels. Three features of the characteristic velocity Vc have been observed in this study. Firstly, a steep decline in Vc following the onset of scintillation during the pre-midnight period is reconfirmed as has been observed previously. Secondly, the characteristic velocity shows a gradual increase in the post-midnight period which, when combined with the pre-midnight decline, gives a minimum around 0100 LT. Thirdly, the pattern followed by the characteristic velocity in the course of a magnetically disturbed night is on some occasions substantially different from that of a quiet period. Theoretical calculations of the space-time correlation function of moderate to strong intensity scintillations show that the steep decline of Vc, which is almost the same as the standard deviation of the velocity fluctuations, calls for a geophysical explanation while the minimum around 0100 LT may be attributed to the phenomenon of scintillation itself. The possibility that the velocity fluctuations are caused by electric field fluctuations associated with the generalized Rayleigh-Taylor instability is explored in this paper. This conjecture provides a plausible explanation for the steep decline of the characteristic velocity observed in the pre-midnight hours. ¿ American Geophysical Union 1989