AbstractThis study investigated the latitudinal variations of postâsunset enhancements in the ionospheric electron density during the geomagnetic quiet period in May 2021 with a combination of highâprecision ionospheric parameters obtained from four ionosondes, Beidou geostationary satellite (BDâGEO) receiver network and Sanya incoherent scatter radar (SYISR). We identified four categories of postâsunset enhancement phenomena (Types 1â4), each with unique spatial and temporal evolutions, yet uniformly accompanied by a decrease in hmF2. Measurements of plasma drift vector velocities from SYISR and hmF2 gradients across various latitudes provided pivotal insights, confirming that the ionospheric postâsunset enhancements can result from downward plasma motion due to westward electric field, downward fieldâaligned drift, or a combination of both. For Type 1, dominated by fieldâaligned drift, plasma density enhancements not only intensify at low latitudes but may also extend to midâlatitudes, exhibiting a distinct temporal delay with increasing latitude. In contrast, Type 4, primarily driven by the westward electric field, is characterized by modest increases in plasma density confined to localized lowâlatitude regions, with no observable latitudinal time delay in the peak of enhancements. Types 2 and 3, which are subject to the combined influence of the westward electric field and fieldâaligned drift, exhibit plasma density increases at certain lowâlatitude areas, with Type 2 presenting a delayed pattern and Type 3 showing none with rising latitude. Meanwhile, neutral winds can partially account for the observed postâsunset enhancement from low to middle latitudes. These findings offer new insights into the factors influencing ionospheric behavior after sunset.
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