## 3 Results
### Diurnal Variations
The details of the GPS stations are mentioned in Table 1.  
Figure 4: 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 2018. The details of the GPS stations are mentioned in Table 1.  
Figure 3: 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 2017. The details of the GPS stations are mentioned in Table 1.  
During the descending phase year of 2017 in the American longitude, GPS-TEC was observed to start increasing from 2-4 TECU between 09:00 and 10:00 UT, corresponding to 06:00-07:00 LT at SALU and 05:00-06:00 LT at POVE (Figure 3a,b), to the maximum value of \(\sim\)24-31 TECU between 14:00 and 20:00 UT, corresponding to 11:00-17:00 LT at SALU and 10:00-16:00 LT at POVE, before reducing to the minimum value toward the pre-sunrise hour with respect to UT at both stations. The GIM diurnal variation pattern was similar to that of GPS-TEC, though with slightly higher values with a difference of up to \(\sim\)5-12 TECU from post-noon to nighttime with respect to UT at both stations. The diurnal variations of the IRI-2016 model also showed a similar trend to that of GPS-TEC, though with lower values up to differences of \(\sim\)14-15 TECU from post-noon to nighttime with respect to UT at both stations. In the African longitude, CPVG (Figure 3c) did not have GPS-TEC data for many days, but according the available data, GPS-TEC began its early rise from \(\sim\)0-2 TECU between 02:00 and 07:00 UT, corresponding to 00:30-05:30 LT at CPVG (Figure 3c) and 05:00-10:00 LT at ADIS (Figure 3d), and reached maximum values of \(\sim\)28-32 TECU at both stations between 10:00 and 18:00 UT, corresponding to 08:300-16:30 LT at CPVG and 13:00-21:00 LT at ADIS, before reducing to minimum values afterwards at both stations. The diurnal variations in the GIM and IRI-2016 models showed a similar pattern with GPS-TEC, except that the maximum value of the IRI-2016 model, especially at ADIS (\(\sim\)16 TECU), was significantly less than those of GPS-TEC and GIM, whereas the GPS-TEC and GIM values were in about the same range at both stations. During the descending phase year of 2017 in the Asian longitude, HUTB (Figure 3e) did not present GPS-TEC values. Additionally, there were a lot of missing data at PPPC (Figure 3f), hence leading to a difficulty in understanding the diurnal variations in GPS-TEC. However, from the available data at PPPC, there were observed increases in the GPS-TEC, GIM and IRI-2016 models from minimum values of 1-3 TECU between 20:00 and 23:00 UT (04:00-07:00 UT) to enhanced values between 04:00 and 12:00 UT (12:00-20:00 LT) of 18-22 TECU for GPS-TEC, 26-32 TECU for GIM, and 19 TECU for IRI-2016, before reducing to minimum values  
Figure 5: 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 2019. The details of the GPS stations are mentioned in Table 1.  
afterwards. In terms of the assessment of GPS-TEC values, the GIM assessment reproduced the observed values better than the IRI-2016 model. For the minimum phase year of 2018 in the American longitude, GPS-TEC presented data for more than 3 months at SALU (Figure 4a). Irrespective of the large sum of missing data at SALU, we were able to observe the diurnal trend of GPS-TEC in the American sector (Figure 4a,b).