San Marco D electric field measurements have been averaged in terms of the equivalent plasma drifts for days near the 1988 autumn equinox. The observed plasma drifts provide a satellite-based composite in altitude and local time of the evening enhancements in the zonal and vertical drifts and the shear in the zonal drift. The satellite data reveal details of the postsunset plasma drift vortex, which have been studied in radar and chemical release experiments. The averaged drift data show a proportional relationship between the prereversal enhancement of the vertical drift and the zonal derivative of the zonal drift. This relationship suggests that the vertical drift enhancement is caused by a curl-free response to the rapid increase of the zonal drift, which in turn is tied to the F region zonal winds and the E region sunset. The data also show a curl-free drop-off in altitude of the vertical plasma drift above and below the peak drift at 450 km altitude during the prereversal enhancement. Comparisons of the averaged data with results of coupled ionosphere-electrodynamics models are used to examine the underlying causes of the evening plasma drifts. The evening plasma drift vortex is shown to be a single drift structure with a single source. The source of the evening drift vortex resides on the bottomside of the integrated F region, just as originally supposed by Rishbeth . This dominant single source, which is produced by the divergence of the wind-driven current of the F region, causes the evening enhancement of the eastward plasma drift, the shear in the eastward drift, and the prereversal enhancement of the vertical drift.