## 3 Results and Discussion
### Global Seasonal Distribution
Additionally, the occurrence rate at middle latitudes decreased as solar activity increased. Under low solar activity, for Swarm B observations, the overall occurrence rate was decreased at low and high latitudes but enhanced slightly at middle latitudes compared with Swarm A, which indicates that the midlatitude irregularities might be more concentrated at the altitude of Swarm B. Figure 5 shows the postmidnight occurrence rate of phase scintillation. Due to the weaker topside ionospheric scintillation, the range of the color bar is set to 0-0.1. The occurrence of scintillation at low and high latitudes exhibited similar seasonal and solar activity variations to that of the postmidnight irregularities. At middle latitudes, an enhanced occurrence rate of phase scintillation was also seen during the June solstice  
Figure 4: Same as Figure 2 but after midnight. 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. Figure 3: Same as Figure 2 but for ROTI. 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. by both satellites (Figure 5b,j); however, Swarm A at lower altitudes registered a higher occurrence rate than Swarm B at higher altitudes. Note that this altitudinal dependence of the occurrence of scintillation contradicts the occurrence rate of midlatitude irregularities, as the latter argument showed greater value at the altitudes of Swarm B (Figure 4b,j). At low latitudes, the occurrence rate of EPIs after midnight was positively correlated with solar activity, using absolute density perturbations. This result is in contrast to that of [PERSON] et al. [27], who detected EPIs by high pass filters and showed a negative correlation between postmidnight EPIs and F107 after midnight. In a study by [PERSON] et al. [25], the overall occurrence rate of identifying EPIs based on relative density perturbations was negatively correlated with solar activity, as can be seen in their Figures 3 and 4. [PERSON] et al. [29] identified EPIs by a combination of absolute and relative criteria, and in their Figure 5, it can be seen that the occurrence of postmidnight EPIs was higher overall under high solar activity than under low solar activity. Both absolute and relative density perturbations can be used as a criterion for identifying ionospheric irregularities. The lower ambient electron density after midnight may lead to a lower fluctuation in the electron density of EPIs as well, thus making the occurrence of identification based on absolute criteria positively correlated with solar activity. Additionally, for the opposite feature identified based on relative criteria after midnight, it was found that gravity wave activity is stronger at low solar flux levels [34], which may lead to an increase in the occurrence of EPIs under low solar activity. At middle latitudes, some studies have shown that the neutral-particle density of the upper atmosphere is highest at the equinoxes and lower during the solstices [35]. Meanwhile, the growth rate of the Perkins instability is inversely proportional to neutral-particle density, and the amplitude of gravity waves becomes greater at low neutral-particle density [10]. These facts may lead to the high occurrence of irregularities at middle latitudes during the solstices. Besides, due to the lower altitudes of Swarm A, the corresponding integration path of TEC is longer and may contain more irregularities, so both low and middle latitudes have a greater occurrence rate of scintillation on Swarm A.