Statistical analysis was conducted to investigate the ionospheric diurnal double‐maxima (DDM) patterns using the total electron content (TEC) from the Beidou geostationary (GEO) satellites. The data were used over the Asian‐Australian sector during 2016–2018. It was found that the ionospheric DDM had the obvious seasonal and latitudinal variations as well as hemispheric differences. The occurrence rate of ionospheric DDM presented a semi‐annual variation with a main peak around the June solstice (local summer) and a secondary peak around the December solstice in the northern hemisphere and equator region, whereas an annual variation with the highest occurrence rate around the June solstice (local winter) was observed in the southern hemisphere. Furthermore, the occurrence rate of ionospheric DDM around the equatorial ionization anomaly crest region was lower than that at other latitudes. There was an anti‐correlation between the occurrence rate of ionospheric DDM and the background TEC, indicating that the drivers, which determine the variations of the background TEC, possibly play an important role in producing the latitudinal and seasonal variations of ionospheric DDM as well. Nevertheless, the causative mechanisms could be different for daytime and post‐sunset DDMs.

Ionospheric electron density enhancement at nighttime has been widely studied in the past decades. However, the mechanisms of this anomalous nighttime enhancement are not fully understood. In this study, we took advantage of the fidelity of geostationary total electron contents (GEO TECs) combined with the observations from three equatorial ionosondes to examine the morphology of equatorial ionospheric nighttime enhancements in the Asian-Australian sector. The equatorial ionospheric nighttime enhancements from GEO TECs during 2016-2018 can be characterized by three categories. Each of these categories showed similar seasonal variations, with the maximal occurrence in the equinoxes. Interestingly, there was a strong correlation between the pre-reversal enhancement (PRE) of the eastward equatorial electric field and these three types of equatorial ionospheric nighttime enhancements that were mostly accompanied by PRE inferred from foF2 and h'F2 from ionograms around dusk. Nevertheless, the formations of equatorial ionospheric nighttime enhancements could be associated with other factors, such as the equatorial electric fields and neutral winds, as well as associated plasma transports. This study provides a new perspective to explore the variations and the underlying physical processes of ionospheric nighttime enhancements.WANG ET AL.