## 3 Results and Discussion
### Characteristics at Low Latitudes
Figure 7: Variations of the plasma irregularity occurrence rate as a function of LT by RODI for different F107 and satellites near the geomagnetic equator (\(\pm 10^{\circ}\)). 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. midnight during the June solstice may be a combination of EPIs already generated before midnight and the newborn EPIs after midnight. Previous studies have shown that the EPIs with great perturbation magnitudes of plasma density could cause amplitude scintillation obviously [25]. However, how the EPIs affect phase scintillation is still not clear. In comparison with the occurrence of the irregularities, Figure 8 shows the same occurrence rate distribution calculated from ROTI, representing the topside ionospheric phase scintillation. There is a clear difference in the occurrence of the ROTI and RODI. As expected, the ROTI occurrence rate was much smaller than RODI. The enhancement of the scintillation occurrence rate before midnight during the December solstice and the equinoxes was more consistent with the results of EPIs as shown in Figure 7. During the equinoxes, the occurrence rate increased sharply from 18:00 to 20:00 LT, and after 20:00 LT, the occurrence rate began to fall steadily until 2:00 LT, remaining at an extremely low level throughout the postmidnight period. However, during the June solstice, the occurrence rate of ROTI was slightly higher before midnight than after midnight and presented a very low level at all LT. We can see that the total phase scintillation at the topside ionosphere is not always sensitive to the EPIs after midnight. Note that, unlike the in situ observed irregularities, the scintillation in this study is indicative of the signal being between the Swarm satellites and the GPS satellites. [PERSON] et al. [41] conducted continuous observations of the field-aligned irregularities (FAI) over Indonesia at night and found that the FAI was not accompanied by GPS scintillation after midnight. The background electron density in the top ionosphere is low after midnight, and there is little fluctuation in the total electron density of topside ionospheric irregularities, which may not generate significant scintillation. Note that small-scale plasma perturbations may not be identified by RODI, but they could cause phase scintillations. During the postmidnight hours, the small-scale plasma perturbations that cause phase scintillations could dissipate more quickly. As a consequence, the phase scintillations could be at a low level after midnight. In addition to the solar activity and LT variations, Figure 9 shows the season-longitude variations of the irregularities and scintillation occurrence within \(\pm 10^{\circ}\) geomagnetic lat  
Figure 8: Same as Figure 7 but for ROTI. 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.  
itudes during the whole night. During the period of low solar activity (Figure 9a,b), the occurrence pattern displayed larger values from the September equinox to the March equinox (Doy 270-80; Doy means the day of the year), appearing in the Atlantic American sector. The region with a high occurrence rate stretched eastward from 60\({}^{\circ}\)W to 20\({}^{\circ}\)E longitude during Doy 1-90. Later, around Doy 100, the region of high occurrence rate began to gradually separate into two parts, toward the east and the west. Then, during Doy 120-250, two regions of high occurrence rate were observed over the African and Pacific regions, but the overall occurrence decreased. Around the September equinox, the two separated regions with high occurrence rate began to merge into one again.