## 3 Results and Discussion
### Characteristics at Middle Latitudes  
We note that the occurrence rate of irregularities at middle latitudes was much lower than that at low or high latitudes (Figures 2 and 4). To further study the variations of irregularities at middle latitudes, Figure 10 displays the local time variations of midlatitude irregularities' (\(\pm 30^{\circ}\)\(\pm 40^{\circ}\)) occurrence rate identified by RODI. During the equinoxes, the occurrence rate was lowest, and the occurrence rate after midnight was higher than that before midnight. During the June solstice, the occurrence rate of midlatitude irregularities reached the highest level, compared with the other seasons. In Figure 10b,e, the irregularities started to appear at about 20:00 LT, and the first peak occurrence was witnessed at 21:00 LT. Afterward, the occurrence kept growing and reached its maximum at about 03:00 LT. Then the occurrence rate of the irregularities decreased rapidly during 03:00-06:00 LT. During the December solstice, the LT variations were similar to those during the June solstice, but at a smaller magnitude. To further investigate solar activity dependence, Figure 11 shows the postmidnight occurrence rate calculated by the RODI of midlatitude irregularities with F107. Note that the range of the occurrence rate is 0-0.15 for the equinoxes and 0-0.3 for the solstices. It can be seen that the solar activity dependence of the occurrence rate exhibits negative trends  
Figure 10: Variations of the plasma irregularity occurrence rate as a function of LT by RODI for different F107 and satellites at midlatitude (\(\pm 30^{\circ}\)\(\Rightarrow\)\(40^{\circ}\) 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 display the 0.5 h averaged variation of the occurrence. Each dot indicates the average daily occurrence and local time of the satellite’s ascending and descending orbits. after midnight, and the midlatitude irregularities only have a larger occurrence rate around the solar minimum. [PERSON] et al. [12] found that the middle-latitudinal band structure became more pronounced as solar activity decreased. The irregularities at middle latitudes exhibited similar solar activity dependence features to the middle-latitudinal band structure. A large number of observations suggested that the occurrence of MSTID at right is inversely correlated with solar activity [14; 42], which is consistent with the characteristics of observed midlatitude irregularities in this study. Although many mid-latitude irregularities have been explained by the Perkins instability, the problem with the Perkins instability theory is that the growth rate is too low to produce significant irregularities in the F region. Some studies have shown that the occurrence rate of sporadic E layer (Es) instability is higher under low solar activity at nighttime [43], and an electrodynamically coupled E and F region containing Es instability would lead to a faster growth rate of the Perkins instability at middle latitudes [44]. Thus, it can be explained that mid-latitude irregularities are higher under low solar activity. Figure 12 displays the season-longitude variations of the midlatitude irregularities and scintillation occurrence rate after midnight. During the equinoxes, the occurrence was the lowest in both hemispheres; this feature also can be seen in Figures 10 and 11. The midlatitude irregularities had a longitudinal preference over the Pacific, from Doy 120 to 230. During Doy 320-40, the large values concentrated over the west of America and the Pacific. In addition, we can see the obvious hemispheric asymmetry. During the June solstice, midlatitude irregularities were evident in the winter hemisphere.