Structure of Post-Midnight Enhancements in Electron Density at the Low Latitude F-Layer Ionosphere

It has previously found that at Sanya (109.6 degrees E, 18.3 degrees N) a post-midnight enhancement in the F2-layer peak electron density (NmF2) is often accompanied by the increase in the bottom side F layer electron density (Ne) and a reduction in the top side F layer Ne. We conduct a case study to explore the complete spatial coverage of the post-midnight enhancements of electron density in the low latitude ionosphere. The total electron content (TEC) data are retrieved from the Beidou Geostationary Earth Orbit (GEO) satellite signals recorded by a network of Global Navigation Satellite System (GNSS) receivers in Asia-Australia. The F2-layer peak parameters and the altitudinal profiles of Ne are measured by the ionosonde, and incoherent scatter radar at Sanya (109.6 degrees E, 18.3 degrees N). The GEO-TEC does not always completely follow the enhancement of NmF2. The enhancements may peak roughly simultaneously or with significant time shifts in the west-east direction, even in the GEO-TEC series recorded by a GNSS receiver in an event, which indicates possibly the enhancement of different longitudinal scales. Further, in the cases the enhancement is clustered in a limited area. The low latitude enhancements show a hemispheric asymmetry; they occur mainly in the northern hemisphere and usually have no correspondingly magnetic conjugate symmetrical features in the southern hemisphere. The hemispheric asymmetry of the enhancements highlights that both the field-aligned plasma transport and electric fields are possibly responsible for regulating the limited spatial presence of post-midnight enhancements in the low latitude ionosphere.