The ionosphere is an indispensable medium for the propagation of communication and navigation signals. It has been extensively studied over the years to understand its properties and variability under different conditions. Spread F (SF) is a common feature of the ionosphere that is caused by irregularities in electron density distribution in the F-layer. Radio signals incident on these irregular plasma structures are scattered, thereby causing diffuse spreading in the F-layer trace of ionograms. Bottomside spread F was first reported by Booker and Wells (1938) and a number of mechanisms are understood to be responsible for its occurrence and development. Post-sunset SF is most commonly attributed to increased upward vertical plasma drift (also known as the pre-reversal enhancement [PRE]) in response to an enhanced eastward electric field. Due to the absence of a highly conducting E-layer at night, plasma instabilities can develop more easily at elevated F-layer heights and can subsequently grow via the Rayleigh-Taylor instability (RTI). This phenomenon is even more common at equatorial latitudes because the geometry of the geomagnetic field at these latitudes facilitates  E B plasma drifts (Sreehari et al., 2006). Apart from the PRE associated with post-sunset F-region electrodynamics, other mechanisms known to contribute to F-layer plasma instabilities include zonal winds, atmospheric gravity waves (AGWs) which manifest in the ionosphere as traveling ionospheric disturbances (TIDs), and trans-equatorial thermospheric winds (Abdu, 2012). The role of the RTI in the development of the equatorial spread F (ESF) was first suggested by Dungey (1956). The RTI develops when a heavy fluid rests on a lighter one. In such a state, gravity or other triggers can cause the heavier liquid to become unstable. Ossakow (1979) and references therein discussed the role of the F-layer peak and electron density scale length in the linear growth of the RTI. A higher F-layer peak and smaller electron density scale length are both favorable conditions for the linear growth of the RTI.In the mathematical formulation by Huang et al. (1993), it was shown that AGWs can directly seed the RTI. Huang et al. (1993) showed that AGWs with speed 5-20 ms −1 and wavelength in the order of 100 km Abstract Daytime equatorial spread F (ESF) is not as common as nighttime ESF due to the presence of a highly conducting E-layer during the daytime which counteracts the development of F-layer plasma irregularities. This study presents two rare daytime ESF-like events which occurred over an interval ∼2 h and were detected by the HF Doppler receiver located in Lagos (LAG: geographic: 3.27°E, 6.48°N; dip latitude −1.72°) and the Lowell Digisonde at Ilorin (ILR; 4.68°E, 8.50°N; dip latitude −1.25°), managed by Lowell GIRO Data Center (LGDC). Analysis of the first event revealed ∼30 min periodic oscillations in iso-heights of ionospheric electron density. Shorter period (∼15 min) oscillations appeared simultaneously in HF Doppler measurements and these oscillat...

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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.