The data of ionospheric electron density and S4 scintillation index from COSMIC/FORMOSAT-3 GPS occultation soundings are used to study the morphology of the equatorial F region irregularity (EFI) in the longitude sector (0 degrees similar to 75 degrees W) where the South Atlantic magnetic anomaly (SAA) is located. Launched into low Earth orbits on 14 April 2006, the six-satellite constellation COSMIC provides daily global measurements of the ionosphere at altitudes below similar to 800 km. The large data set enables us a statistically correlative study of electron density depletion and GPS L1 scintillation, which allows the global and altitudinal distributions of irregularity to be examined. Under solar minimum and geomagnetic quiet conditions, postsunset EFI/scintillation events are found to concentrate in the SAA longitude sector during northern winter months (D months), but nearly disappeared in the same longitude sector during the opposite season (J months). The D months' average pattern of EFI reveals that most of irregularities occurred in the bottom side F region (200-350 km altitude) and at low magnetic latitudes adjacent to the locations of the maximum postsunset N(max) and H(max). This is consistent with the early postsunset irregularities that are generated in the region where plasma upward drifts have been greatly enhanced. By examining the UT snapshots of the global distributions of electron density and scintillation events, strong spatial correlations are found to exist among the sunset terminator, the westward declined geo-magnetic field, strong scintillation event, and density depletion in the SAA longitude sector only during D months, where the E region sunset terminator is most possibly parallel to the magnetic field direction at equatorial latitudes. Density depletion structures within this longitude sector were bounded by two postsunset ionization enhancement bands (EIA) that extended over central SAA and its conjugated region, respectively. Such features provide further evidence that the enhanced conductivity gradients due to particle induced ionization in the vicinity of SAA contribute additional electric field enhancements in the equatorial F region. At the walls of the depletion region near the sunset lines, prominent horizontal density gradients exist in both westward and southward directions due to sun setting and SAA ionization enhancement, respectively. The combination of horizontal density gradients with antiparallel (north-eastward) neutral winds during D months can contribute additional growth rate to the EFI generation. These COSMIC observations not only confirm that the sunset equatorial electrodynamics plays a key role in controlling the seasonal and longitudinal occurrences of the quiet time EFI, but also reveal that seasonally dependent ionospheric responses to the energetic particle precipitation in SAA can affect considerably the morphology of EFI in the SAA longitude sector.
