Post-Sunset Equatorial Plasma Bubble Occurrence in the African Sector Observed by COSMIC-2 Radio Occultation

Post-sunset equatorial plasma bubble (EPB) occurrences in the African sector are investigated using Global Navigation Satellite System (GNSS) radio occultation (RO) observations acquired by the FormoSat-7/Constellation Observing System for Meteorology, Ionosphere, and Climate-2 (COSMIC-2) mission. A subset of the global COSMIC-2 GNSS RO database, containing EPB events identified during the period 2023, is analyzed. The analysis is restricted to RO observations located within the African longitude sector (20°W – 40°E) and within ±20° geomagnetic latitude of the magnetic equator. EPB events are identified using complete L1-band amplitude scintillation index (S4) profiles derived from RO signal-to-noise ratio measurements. An EPB event is defined when the maximum S4 value exceeds 0.3 at F-region altitudes. Only complete RO observations, for which the sampling spatial scale is smaller than the first Fresnel zone, are considered. To focus on post-sunset ionospheric irregularities, the analysis is further restricted to observations occurring within the local time interval between 18:00 and 24:00 local time (LT). The analyzed COSMIC-2 RO observations show that EPB occurrence rates in the African sector increase rapidly after local sunset and reach a maximum during the early post-sunset period. The highest occurrence rates are observed between approximately 19:00 and 22:00 LT, after which EPB occurrences decrease toward later nighttime hours. The latitudinal distribution of detected EPBs is mainly confined within ±20° geomagnetic latitude and exhibits a near-symmetric pattern with respect to the magnetic equator. Seasonal differences in EPB occurrence are also observed, with higher occurrence rates during equinoctial periods and relatively lower occurrence rates during solstitial seasons. COSMIC-2 RO profiles associated with EPB events also show pronounced reductions in F-region electron density, accompanied by enhanced L1-band S4 values during the post-sunset period.