## 3 Results
### Seasonal Variations  
The plots for the seasonal variations in the GPS-TEC, GIM and IRI-2016 models for the descending phase years (2016-2017) and the minimum phase years (2018-2019) at SALU and POVE (American), CPVG and ADIS (African), and HUTB and PPPC (Asian) longitudes are shown in Figures 6-11. Figures 6 and 7 show the seasonal variations in the GPS-TEC, GIM and IRI-2016 models at SALU and POVE, respectively, during the descending phase (2016-2017) and the minimum phase (2018-2019) in the American longitude. At the American longitude, Figures 6 and 7 show seasonal variations with higher values during the daytime than the nighttime with respect to the UT, as expected. At SALU, the seasonal variations showed semi-annual variations with higher magnitudes during the equinoxes than the solstices for both the descending and minimum phases. The seasonal profiles of GPS-TEC and GIM for all seasons were quite similar for both phases, showing a single peak that was shifted to 14:00-16:00 UT (11:00-13:00 LT). The IRI-2016 model showed a nonontime bite-out profile with two unequal peaks during most seasons for both solar cycle phases, except for the September equinox and the December solstice, in which the noon bite-out profile appeared to be insignificant. Figure 6: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at SALU. Figure 8: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at CPVG. Figure 7: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at POVE. Figure 10: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at HUTB. Figure 9: Seasonal variations in GPS-TEC, IRI-2016 and GIM models for descending (2016–2017) and minimum (2018–2019) phases of solar cycle 24 at ADIS. There were differences in the magnitudes of GPS-TEC and GIM that were more pronounced in the equinoxes during the descending phase (2016-2017) than in the minimum phase (2018-2019), except for the minimum phase year of 2019, in which GPS-TEC data were not available. Generally, for all seasons at SALU, GIM values were higher than those of both GPS-TEC and IRI-2016, except for the June solstice of the minimum phase year (2018), in which GPS-TEC and GIM values were in about the same range. Additionally, for the December solstice of the same year, the values of the GIM and IRI-2016 were almost equal. The maximum range of the GIM during the equinoxes of the descending phase (32-52 TECU) was higher than the maximum range of the GIM during the equinoxes of the minimum phase (21-32 TECU). The maximum value of GPS-TEC for both the descending and minimum phases during the equinoxes was between 29 and 43 TECU. The maximum value of the IRI-2016 model for both the minimum and maximum phases during the equinoxes was between 23 and 32 TECU. Although both the GIM and IRI-2016 assessments of GPS-TEC were poor, the GIM assessment looked better than that of IRI-2016. At POVE, the seasonal variations in GPS-TEC, GIM and IRI-2016 were very similar to the seasonal variations at SALU, showing similar seasonal profile shapes and magnitudes. In addition, the GIM assessment of GPS-TEC looked better than that of IRI-2016. In the African longitude, during the descending phase (2016-2017) at CPVG (Figure 8), seasonal variations in GPS-TEC, GIM and IRI-2016 showed semi-annual variations, with higher values during the equinoxes than the solstices. Meanwhile, during the minimum phase (2018-2019) depicted in the Figure 8, the seasonal variations in GPS-TEC, GIM and IRI-2016 showed slightly higher values during the March equinox than the September equinox and the solstices.