## 3 Results and Discussion
### Characteristics at Low Latitudes  
Figure 7 exhibits the LT variations of the occurrence rate of plasma irregularities identified by RODI near the geomagnetic equator (\(\pm\)10\({}^{\circ}\)). For the Swarm A results (Figure 7a-c), the occurrence rate was higher during the December solstice and the two equinoxes and lowest during the June solstice. This could be related to the low-level background Ne during the June solstice, which leads to a small perturbation magnitude of EPIs; thus, the occurrence rate is lower based on the extract method with the absolute enhancement/attenuation threshold. The LT variations during the December solstice and the equinoxes showed similar characteristics. During the December solstice, the occurrence rate began to increase rapidly at about 19:00 LT, peaked at around 22:00 LT, and remained at high values before midnight; then it continued to decrease after midnight (Figure 7c). The prereversal enhancement (PRE) after sunset [36], corresponding to equatorial upward E\(\times\)B drift velocity, is conducive to the development of the R-T instability and the EPI formation. Therefore, the occurrence rate of EPIs usually peaks before midnight. Figure 6: Same as Figure 2 but for S4c after midnight. S4c denotes the approximate S4 index calculated from the carrier-to-noise ratio. The panels from top to bottom show results for Swarm A (F107 \(<\) 100, (**a–d**)), Swarm A (F107 \(>\) 100, (**e–h**)), Swarm B (F107 \(<\) 100, (**i–l**)), and Swarm B (F107 \(>\) 100, (**m–p**)), from left to right show results at March equinox, June solstice, September equinox, and December solstice. However, the situation was slightly different during the June solstice (Figure 7b,e), the whole occurrence rate was lower compared with other seasons, but the increase in occurrence rate stayed until midnight and peaked at around 1 LT (Figure 7b,e). [PERSON] et al. [29] showed that the occurrence of EPIs after midnight at an African longitude was comparable to or even higher than that before midnight during the equinoxes and June solstice. For the mechanism of EPI formation after midnight, some studies have suggested that the postmidnight EPIs are not a continuation of the premidnight EPIs during the June solstice, but they are newly generated due to an increase in the growth rate of the R-T instability caused by an upward drift of the plasma [37]. The duration of the vertical drift of the plasma at African longitudes is longer during the June solstice compared with other seasons. In addition, due to the equatorward meridional neutral winds associated with midnight temperature maximum (MTMs), the F-layer height rises significantly after midnight during the June solstice, leading to an increase in the growth rate of the R-T instability. Additionally, [PERSON] and [PERSON] [38] used numerical simulations to investigate the properties of large-scale equatorial F-region irregularities generated by gravity waves and showed that a zonally propagating gravity wave can seed the R-T instability, and the spatial resonance of gravity wave and plasma motion can promote the formation of a plasma bubble. [PERSON] [39] suggested that circularly phased gravity waves can generate electric field perturbations through the F-region dynamo mechanism, thereby initiating the R-T instability effectively. Furthermore, [PERSON] et al. [40] suggested that MSTID accompanied by a polarized electric field can also seed the R-T instability at low latitudes after midnight. Overall, after midnight, gravity waves, MSTID, and equatorial meridional winds might all contribute to EPIs. If interpreted as the postmidnight EPIs are mostly the continuation of EPIs that were generated before midnight, then it cannot explain that the peak of the June solstice occurred after midnight. Thus, the peak occurrence rate after midnight is \(\sim\) 1.5 \(\mu\)m, which is consistent with the observed peak of the June solstice.