## 3 Results and Discussion
### Characteristics at Middle Latitudes
During the December solstice, the occurrence was slightly stronger in NH than in SH and peaked around 60\({}^{\circ}\)-100\({}^{\circ}\)W (Figure 12b), which is in good agreement with previous results in Figures 4 and 5. Some studies have shown that the middle-latitudinal band structure appeared more frequently in the winter hemisphere during the June solstice and under low solar activity [12]. The middle-latitudinal band structure may promote the creation of irregularities. In addition, [PERSON] et al. [45] found that midlatitude plasma density fluc  
Figure 11: Variations of the plasma irregularity occurrence rate as a function of F107 after midnight by RODI for different satellites at middle latitudes (\(\pm\)30\({}^{\circ}\)=\(\pm\)40\({}^{\circ}\) geomagnetic latitudes). The panels from left to right show results during the equinoxes (**a**,**d**), June solstice (**b**,**e**), and December solstice (**c**,**f**), from top to bottom show results for Swarm A and Swarm B. The red lines are connected by the mean values of the F107 index every 20 fsu. tuations also showed a strong hemisphere asymmetry, with a preference for the summer hemisphere based on Challenging Mini-Satellite Payload (CHAMP) (altitude \(\sim\)400 km) and Korea Multi-Purpose Satellite-1 (KOMPSAT-1) (\(\sim\)685 km) observations. [PERSON] et al. [42] showed that the northern and southern hemisphere occurrence of MSTID after midnight even reverses as solar activity decreases. Unlike the MSTID signature, the midlatitude irregularities from Swarm (\(\sim\)450-500 km) occurred more frequently in the winter hemisphere after midnight under low solar activity, based on results from Figures 10-12. [PERSON] et al. [14] used Republic of China Satellite 1 (ROCSAT-1) to extract midlatitude irregularities and found that their predominant occurrence was in the winter hemisphere and after midnight. Further research is needed on the hemispheric asymmetry of midlatitude irregularities. In order to study the influence of middle-latitude irregularities, we inspected the same distribution of topside ionospheric phase fluctuations, as shown in Figure 12e-h. The overall occurrence rate in Swarm A was significantly higher than in Swarm B, especially  
Figure 12: Variations of the plasma irregularity occurrence rate by RODI and ROTI as a function of Doy for different satellites at middle latitudes during Doy 0–360, F107 \(<\) 100. The panels from top to bottom show results for RODI occurrence in northern hemisphere (**a,b**), RODI occurrence in southern hemisphere (**c,d**), ROTI occurrence in northern hemisphere (**e,f**), and ROTI occurrence in southern hemisphere (**g,h**), from left to right show results for Swarm A and Swarm B. Note that in order to distinguish peak values in different periods, color-bar ranges for irregularities and scintillation occurrence are set differently. in SH. Although the occurrence rate identified by ROTI was lower than that of RODI, their season-longitude variations were consistent, and similar features are not discussed here. However, unlike the former, the middle-latitude topside ionosphere did not produce strong hemispheric asymmetry in scintillation. During the June solstice, the scintillation difference in both hemispheres was not significant. However, during the December solstice, the occurrence was stronger in SH than in NH, mainly concentrated in the southwest Pacific to South America. Small-scale plasma perturbations may not be identified by RODI, but they could cause phase scintillations. The differences between the scintillation and the irregularities' distribution characteristics should be associated with the scale size of irregularities. However, at midlatitudes, the differences also suggest that scintillation in the topside ionosphere could be also influenced by other factors. Topside ionospheric scintillation may be also related to the distribution of Ne at higher altitudes. In a study by [PERSON] et al. [12], the Ne distribution at an altitude of 800 km can be seen in their Figures 4 and 5.