## 3 Results and Discussion
### Post-Midnight EPBs from CSES-01 Observations
In addition, [PERSON] et al. [76] and [PERSON] et al. [77] noted that Field-Aligned Irregularities (FAI) \begin{table}
\begin{tabular}{c c c} \hline \hline
**Instrument** & **Number of Detected EPBs** & **Number of Semiorbits** \\ \hline LP & 3973 & 1937 \\ EFD & 2411 & 1598 \\ LP \(\cap\) EFD & 1658 & 800 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Number of EPBs automatically detected from 1 January 2019 to 31 May 2024, by LP and EFD onboard CSES-01. The table also reports the number of semiorbits in which EPBs are detected. Figure 6: Latitudinal occurrence (**top**) and latitudinal extension (**bottom**) of EPBs detected from 1 January 2019 to 31 May 2024 by EFD (**left**) and LP (**right**) onboard CSES-01. echoes from 2006 to 2010, associated with EPBs, appeared more frequently after midnight between May and August. Since, differently from previous studies, the period under investigation also covers the rising phase of Solar Cycle 25, we tested the goodness of our results by investigating the monthly distribution of the occurrence of quiet days evaluated accordingly [78]. As in Figure 8, the main occurrence of quiet days was recorded during June, December, and January, suggesting that EPBs recorded by CSES-01 occur primarily during quiet days. Following the results of [PERSON] et al. [79], reporting 4-year statistics (2009-2012) for post-midnight irregularities observed with the C/NOFS satellite, and indicating that the rate of occurrence of such irregularities peaks predominantly in the African sector during the June solstice, we investigated the regional dependence of EPBs during all June solstices covered by the present study. Our results, shown in Figure 9, agree well with those of [79]; in fact, the rate of occurrence of EPBs detected by the CSES-01 LP as a function of geographic longitude peaks predominantly in the African sector. According to the authors [79], this behavior could be attributed to the abundant appearance of the ionospheric sporadic E  
Figure 8: Monthly occurrence of quiet days from 1 January 2019 to 31 May 2024. Figure 7: Monthly occurrence of EPBs automatically detected from 1 January 2019 to 31 May 2024 by LP (**left**) and EFD (**right**) onboard CSES-01. layer (Es) in the June solstice, which in turn could be a source of eastward electric field that leads to bubble formation during the local time after midnight. In accordance with the results of [PERSON] et al. [80], who studied plasma density variations retrieved by the C/NOFS satellite, we investigated the longitudinal occurrence of post-midnight EPBs separated by seasons. Figure 10 shows the longitudinal occurrence of EPBs automatically detected from 1 January 2019 to 31 May 2024 by CSES-01 LP during northern summer (left), equinox months (center), and northern winter (right). As seen in the figure, during northern summer (left), EPBs peak predominantly in the Atlantic-African sector; differently, during northern winter (right), the major occurrence rate was found in correspondence with the mid-Pacific and the American sectors. At the equinoxes, EPBs seem to mostly affect both the American and the African sectors. The longitudinal structures in which these irregularities occur are very similar to those reported by [80], suggesting that the lower atmospheric tides may have actively contributed to EPB formations. Our results support findings of previous studies (see, e.g., [81] and references therein) and suggest that possible mechanisms for generating observed post-midnight irregularities could be both attributed to the seeding of the Rayleigh-Taylor instability by atmospheric gravity waves (propagating from below into the ionosphere) and, subsequently, to the uplift of the F layer induced by the meridional neutral winds in the thermosphere. Figure 10: Longitudinal occurrence of EPBs during the northern summer (**left**), equinox months (**center**), and northern winter (**right**), automatically detected by LP algorithm from 1 January 2019 to 31 May 2024 by CSES-01.