Large-scale horizontal gradients in ion density and vertical drift observed by the Atmospheric Explorer E satellite in the South Atlantic region (latitudes 10¿S--20¿S, longitudes 50¿W--10¿E) during the June solstice at solar maximum are presented and analyzed. These features occur during the nighttime period. The observations near 450-km altitude show vertically downward ion drift velocities exceeding 120 m s-1 and depleted regions where the ion density is around 2 ¿ 104 cm-3. It is shown, using values modeled by the Sheffield University Plasmasphere Ionosphere Model (SUPIM) along the satellite trajectory, that the large ion density depletions appear as a result of large downward ion drifts driven by large southward winds along the magnetic meridian and by diffusion. During others seasons such behavior is not observed by the AE-E satellite, neither by SUPIM results. The roles played by the different physical processes responsible for the large downward drift velocities are investigated. The model results highlight the relationship between longitudinal variation of the ion densities and the location of the equatorial anomaly crest in the South Atlantic region.