## 3 Results and Discussion
### Global Seasonal Distribution
At high latitudes, the overall occurrence rate was enhanced with increasing solar activity, and the seasonal distribution characteristics remained similar during low solar activity periods. Many studies have shown that the occurrence of the premidnight EPIs peaks at solar maximum [13; 27; 28; 29]. In addition, at a given longitude, EPIs occur more frequently when the geomagnetic field lines are aligned with the sunset terminator [13; 30]. A small angle between the equatorial geomagnetic line and the sunset terminator could result in a decrease in the flux tube-integrated conductance and, thus, an increase in the growth rate of the R-T instability. For example, in the South American and Atlantic sectors, a small angle would occur from October to March, thus leading to a high occurrence rate in these regions during the December solstice and equinoxes. In addition, transequatorial winds may lead to an increase in local conductivity in the winter hemisphere and a decrease in the summer hemisphere [31; 32]. Since the decrease in conductivity would enhance the growth of the R-T instability, it results in a higher occurrence rate in the Asian and African sectors, where the geomagnetic equator is located in the summer hemisphere during the June solstice, and a higher occurrence rate in the American and Atlantic Ocean region during the December solstice. For Swarm B observations (Figure 2i-p), the overall occurrence of the irregularities decreased, compared with that of Swarm A. At higher altitudes, the region of the high occurrence rate of EPIs became smaller, during all seasons and solar activities. The remaining similar features of solar activity and seasonal distribution are not repeated here. In addition, the occurrence rate of RODI is much low between \(-60\) and \(60^{\circ}\) longitudes at midlatitudes in the southern hemisphere. The geomagnetic latitude is lower in this region, and the higher occurrence rate at other longitudes should be related to the Weddell Sea Anomaly (WSA) [33]. To compare the in situ observations of irregularities with the topside ionospheric scintillation before midnight, Figure 3 gives the occurrence of phase scintillation based on ROTI results. The occurrence of scintillation showed a good agreement with the characteristics of the irregularities based on RODI. At low and high latitudes, the premidnight irregularities and phase scintillation were positively correlated with solar activity, and they showed similar longitudinal/seasonal patterns. The occurrence rate of phase scintillation at middle latitudes was still at a very low level, which was also a little larger in the summer hemisphere during the solstices under high solar activity (Figure 3f,h,n,p), but the enhanced occurrence rate was almost not visible under low solar activity. Figure 4 illustrates the occurrence rate of irregularities after midnight, with a smaller color-bar range of 0-0.3, which is different from that of Figure 2. At low latitudes under low solar activity, the occurrence rate of irregularities was higher during the solstices than during the equinoxes (Figure 4a-d,i-l). Under high solar activity, the overall occurrence rate of EPIs was higher than that under low solar activity. The peak occurrence rate of EPIs during the equinoxes was concentrated in the region from the east coast of South America to Africa (Figure 4e-h), with an eastward movement compared with the distribution before midnight (Figure 2e-h). At middle latitudes under low solar activity, a striped structure of enhanced occurrence can be clearly observed at Asia Pacific longitudes in both hemispheres. The enhanced occurrence at the middle latitudes was most pronounced during the June solstice and least pronounced during the equinoxes. The irregularities at middle latitudes occurred more frequently in the winter hemisphere (Figure 4b,d,j,l). In addition, the enhanced midlatitude occurrence during the December solstice covered a wider longitude range from Asia Pacific to the America Atlantic.