## 3 Results  
### Diurnal Variations  
Figures 2-4 and 5a-f show the plots of diurnal variations in the GPS-TEC, GIM and IRI-2016 models in the American (top panel), African (middle panel), and Asian (bottom panel) longitudes for the descending (2016-2017) and minimum (2018-2019) phases of solar cycle 24. These figures show decreases in the values of the GPS-TEC, GIM and IRI-2016 models from the descending to the minimum phase of solar cycle 24, as expected. During the descending phase year of 2016 in the American longitude, GPS-TEC was observed to start increasing from 09:00 UT (06:00 LT) at SALU (Figure 2a) and 10:00 UT (06:00 LT) at POVE (Figure 2b) from the lowest value of \(\sim\)2 TECU to the maximum value between 14:00 and 22:00 UT (10:00-18:00 LT; \(\sim\)45-52 TECU), and then it was reduced to the minimum value afterwards. The GIM showed a similar diurnal trend to GPS-TEC, though with slightly higher values. The IRI-2106 model values increased close to midday with respect to UT (07:00-08:00 LT; \(\sim\)2 TECU) to the maximum between 16:00 and 22:00 UT (12:00-18:00 LT; \(\sim\)20-26 TECU) before reducing to the minimum value afterward. The diurnal variations in the GIM values were higher in the American longitude, while the IRI-2016 showed the lowest diurnal values compared with the GPS-TEC values. Hence, GIM revealed a better assessment of GPS-TEC compared with the IRI-2016 model. In the African longitudinal sector, GPS-TEC showed an increase from 03:00 UT (06:00 LT) at ADIS (Figure 2d) and 08:00 UT (06:30 LT) at CPVG (Figure 2c) from the lowest value of \(\sim\)2 TECU to the maximum value between 10:00 and 14:00 UT (13:00-16:00 LT) at ADIS and between 14:00 and 18:00 UT (12:30-16:30 LT) at CPVG of \(\sim\)52-60 TECU at both stations before reducing to the minimum value afterward. The GIM also showed a similar diurnal trend to GPS-TEC, though with slightly higher values. The GIM early increase was from 2-3 TECU between 02:00 and 03:00 UT (05:00-06:00 LT) at ADIS and between 07:00 and 08:00 UT (05:30-06:30 LT) at CPVG to 56-65 TECU during the daytime with respect to UT at both stations, and then it reduced to the minimum value afterward. The IRI-2016 model increased from 04:00 UT (07:00 LT) at ADIS and 08:00 UT (06:30 LT) at CPVG (\(\sim\)2-3 TECU) to the maximum value of  
-20-40 TECU between 14:00 and 16:00 UT (17:00-19:00 LT) at ADIS and between 14:00 and 20:00 UT (12:30-18:30 LT) at CPVG before reducing to the minimum value afterward. Again, the GIM showed a closer assessment of GPS-TEC than the IRI-2016 model at both stations. Lastly, for the descending phase year of 2016 in the Asian longitude, GPS-TEC values increased from -2-3 TECU between 22:00 and 01:00 UT, corresponding to 03:00-06:30 LT at HUTB and 06:00-09:00 LT at PPPC, to maximum values of \(\sim\)35-43 TECU between 06:00 and 12:00 UT, corresponding to 12:30-17:30 LT at HUTB (30-34 TECU) and 14:00-20:00 LT at PPPC, before reducing to the minimum value towards the post-sunet hour with respect to UT at both stations. The GIM showed a similar diurnal trend to GPS-TEC but with higher values (48-60 TECU) at both stations. The IRI-2016 model showed similar diurnal variations to GPS-TEC but with lower values (21-26 TECU) at both stations. The GIM also showed a closer magnitude to GPS-TEC than the IRI-2016 model. The GPS-TEC, GIM and IRI-2016 models showed slightly higher values at PPPC (Figure 2_I_) than at HUTB (Figure 2e). It is also very important to mention that most of the empty spaces for the GPS-TEC for the three longitudes showed that there were no available GPS data for the period. In some cases, data were available but could not be processed, so they were considered bad data that could not be used in the present research. Figure 2: Diurnal variations in GPS-TEC and assessment of GIM and IRI-2016 models at the American (GPS stations: **(a)** SALU and **(b)** POVE), African (GPS stations: **(c)** CPVG and **(d)** ADIS), and Asian (GPS stations: **(e)** HUTB and **(f)** PPPC) longitudes for the year 2016.