## 3 Results and Discussion  
### Global Seasonal Distribution  
After the detection of the irregularities, the occurrence rate is calculated by the ratio of the irregularity data points to the total data points in a grid with 10\({}^{\circ}\) in longitude and 5\({}^{\circ}\) in latitude. To systematically present the spatial, seasonal, and solar activity variations of the nighttime ionospheric irregular structures, the occurrence rate of irregularities calculated from RODI at different seasons (March equinox, June solstice, September equinox, and December solstice), altitudes (460 km for Swarm A and 520 km for Swarm B), and solar activities (F107 \(<\) 100 and F107 \(>\) 100) is compared in geographic coordinates. Figure 2 shows the global distribution of the occurrence rate of irregularities before midnight (18:00-24:00 LT). During low solar activity for Swarm A observations (Figure 2a-d), the peak occurrence of irregularities was concentrated at low and high latitudes, with the weakest occurrence at middle latitudes. At low latitudes, the occurrence showed a clear seasonal and longitudinal dependence near the geomagnetic equator. Among the four seasons, the occurrence of EPIs (equatorial plasma irregularities) was highest during the December solstice and lowest during the June solstice. The occurrence was similar during March and September equinoxes. The high occurrence rate of EPIs during the March/September equinox showed a longitudinal preference from Africa to South America (Figure 2a,c). During the June solstice, EPIs were concentrated over North Africa, and during the December solstice, they were prevalent at Atlantic to South American longitudes (Figure 2b,d). At middle latitudes, the occurrence rate of irregularities was low in all seasons. Under low solar activity, the symmetrical band structure in the northern and southern hemispheres could be seen at middle latitudes with an occurrence of about 10%, concentrated in the Asia Pacific longitudes, and was more apparent in Swarm B (Figure 2i-l). Under high solar activity, the occurrence rate was larger in the summer hemisphere during the solstices (shown in Figure 2f,h,n,p). At high latitudes, the pattern of occurrence during the equinoxes showed a large value in both hemispheres, especially in the southern hemisphere. The occurrence rate at high latitudes was low during the June solstice, with peak occurrence concentrated in the northern hemisphere (NH), and the occurrence rate only showed great value in the southern hemisphere (SH) during the December solstice. Under high solar activity (Figure 2e-h), from the observations of Swarm A, the overall occurrence rate of irregularities was higher than that under low solar activity. At low latitudes, the high occurrence rate region of EPIs became significantly larger, compared with that under low solar activity. During the June solstice, in addition to the prominent peak occurrence rate over Africa, an enhanced occurrence can be also observed over the Pacific sector. At middle latitudes, the occurrence of irregularities remained low in all seasons, similar to the results under low solar activity, except for a slight enhancement in  
Figure 2: Global distribution of occurrence rate by RODI for different seasons, satellites, and F107 in geographic coordinates before midnight. The mean local time was marked in the subtitle. The pixels has a resolution of 10\({}^{\circ}\) in geographic longitude and 2.5\({}^{\circ}\) in geographic latitude. The contour lines of the geomagnetic latitudes at \(\pm\)20\({}^{\circ}\) (white line) are also plotted for reference. The color-bar ranges are set as the same at each column. 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. the northern hemisphere during the June solstice (Figure 2f).