## 3 Results
### Seasonal Variations
The differences in the semi-annual variations in the GPS-TEC, GIM and IRI-2016 models were more pronounced during the daytime between 08:00 and 18:00 UT (06:30-16:30 LT) than during the nighttime between 20:00 and 06:00 UT (18:30-04:30 LT). This shows that day-to-day variations in TEC were higher during the daytime than the nighttime. The ranges of the semi-annual variations in GPS-TEC were  
Figure 11: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at PPPC. higher during the equinoxes (28-48 TECU) than the solstices (20-32 TECU). The semi-annual variations of the GIM and IRI-2016 models also followed a similar trend to that of GPS-TEC. The semi-annual variations of the GIM during the equinoxes and solstices were between 26 and 55 TECU and between 22 and 33 TECU, respectively. The semi-annual variations of IRI-2016 during the equinoxes were found to be higher (28-47 TECU) than during the solstices (20-39 TECU). It is equally important to mention that the highest values of the seasonal variations in GPS-TEC, GIM and IRI-2016 mostly occurred during the March equinox, while their lowest values mostly occurred during the June solstice. At ADIS (Figure 9), seasonal variations showed decreases in the GPS-TEC, GIM and IRI-2016 models during all seasons from the descending to the minimum phase of solar cycle 24. We observed semi-annual variations showing higher values of the GPS-TEC, GIM and IRI-2016 models during the equinoxes than the solstices, as expected. In the semi-annual variations in GPS-TEC and GIM, which were in about the same range, the maximum values (\(\sim\)52 TECU) occurred during the March equinox for the descending the phase year of 2016. The lowest values of GPS-TEC and GIM (\(\sim\)20 TECU) were observed in the June solstice during the minimum phase year of 2019. Semi-annual variations in ionospheric parameters, which comprise a peculiar feature in the equatorial and low-latitude ionospheres, have been widely reported in the past [51]. The shapes of GPS-TEC, GIM and IRI-2016 are further very important features. While the shapes of GPS-TEC and GIM showed a single peak during the daytime, especially during the equinoxes, the peaks of GPS-TEC and GIM were slightly wider (plateau-like) during the June solstice than the other three seasons. The difference in peak size during the June solstice, showing wider peaks, may be attributed to the dominant oxygen/nitrogen (O/N\({}_{2}\)) ratio during the June season [7]. On the other hand, the shape of IRI-2016 showed a typical noon bite-out profile, with a higher post-noon than pre-noon peak during the four seasons with respect to the UT scale. Another observation is that at CPVG, the GPS-TEC, GIM and IRI-2016 models showed a single peak that was shifted to LT post-noon periods (14:00-17:00 LT) during both phases of the solar cycle. These results are in contrast with the shape of the peak of IRI-2016 at ADIS, which showed a noon bite-out curve. Lastly, the seasonal variations at both stations in the African longitude showed the absence of the nighttime enhancement of TEC, except at the station along the GE (ADIS). These results were in contrast to the results of [PERSON] et al. [43], who reported nighttime enhancement at stations away from the dip equator. In their investigation of TEC variations over the magnetic equator, [PERSON] et al. [41] also reported the nighttime enhancement of TEC during the equinoxes at ADIS. In this study, nighttime enhancement could only be observed during the post-sunset hour (18:00-22:00 UT), corresponding to 21:00-01:00 LT of the March equinox and the December solstice for the descending phase year of 2016 (Figure 9) at ADIS. This feature has been widely reported at equatorial and low latitudes [51]. In the Asian longitude at HUTB (Figure 10), GPS-TEC values were only available during the descending phase year of 2016, making it difficult to follow the trend of GPS-TEC in comparison with GIM and IRI-2016.