Case Studies on the Day-to-Day Variability in the Occurrence of Post-Sunset Equatorial Spread F

The occurrence of post-sunset equatorial spread F (ESF) is well understood to be associated with the evening pre-reversal enhancement (PRE). Earlier studies have shown that when large datasets are examined, there is a significant correlation between the occurrence of post-sunset ESF and the evening PRE. However, this correlation is much lower when the events are examined on a day-to-day basis. This has led to the emergence of suggestions that the PRE may not be a necessary condition for the occurrence of post-sunset ESF. This study presents the results of the occurrence of post-sunset ESF using data obtained from the Lowell Digisonde at Ilorin, Nigeria (ILR; 4.68°E, 8.50°N; Dip latitude −1.25°), spanning May–September 2019. In this interval, data was available on 124 days and, of this number, post-sunset ESF occurred on 99 days (79.8%). Of the 99 days for which post-sunset ESF was observed, two events for which the evening PRE was absent were investigated in detail. Analysis of Digisonde data for these two selected events showed modulation of the bottomside F-layer plasma in the evening sector by wave structures. Spectral analysis of Global Positioning System Total Electron Content from multiple ground stations across Nigeria showed the appearance of wave structures, consistent with standing waves, in the ionosphere. The results presented show that standing waves are a possible mechanism for the generation of post-sunset ESF, even in the absence of the PRE.