## 3 Results
### Longitudinal Variations  
Figures 2-4 and 5a-f also demonstrate the universal time (UT) effect and the influence of the geomagnetic field lines on the longitudinal variations in the GPS-TEC, GIM and IRI-2016 models during the descending and minimum phases in the American, African and Asian longitudes. In this study, longitudinal variations were observed to mostly show LT with respect to universal time, seasonal, and solar cycle effects. The proximity of a station to the magnetic equator was also found to play a major role in TEC variations. During the descending phase (2016-2017), GPS-TEC showed higher magnitudes in the African longitude, followed by the American longitude and, finally, the Asian longitude, for stations lying along the geomagnetic equator (GE) and slightly away from the GE. For stations located along the GE, GIM values were observed to be highest at the American longitude, followed by the Asian longitude and least in the African longitude, but for stations slightly away from the GE, GIM values were observed to be higher in the African \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station ID** & **Years** & **March Equinox** & **June Solstice** & **September Equinox** & **December Solstice** \\ \hline \multirow{4}{*}{ADIS} & 2016 & 2.90 & 1.56 & 1.85 & 2.15 \\  & 2017 & 1.77 & 2.00 & 1.63 & 1.81 \\  & 2018 & 2.96 & 1.39 & 2.14 & 2.70 \\  & 2019 & 2.82 & 1.22 & 1.36 & 2.71 \\  & 2016 & 8.05 & 3.96 & 7.51 & 2.78 \\  & 2017 & - & - & - & - \\ \multirow{4}{*}{HUTB} & 2018 & - & - & - & - \\  & 2019 & - & - & - & - \\  & 2016 & 7.70 & 5.21 & 13.42 & 2.50 \\ \multirow{4}{*}{PPPC} & 2017 & 6.02 & 3.84 & 3.24 & 2.21 \\  & 2018 & 3.15 & 2.81 & 4.40 & 3.38 \\ \cline{1-1}  & 2019 & 2.82 & 2.01 & 4.50 & 3.48 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._ \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Station ID** & **Year** & **March Equinox** & **June Solstice** & **September Equinox** & **December Solstice** \\ \hline \multirow{4}{*}{SALU} & 2016 & 4.94 & 3.14 & 4.19 & 2.83 \\  & 2017 & 3.45 & 2.90 & 3.05 & 3.48 \\  & 2018 & 3.29 & 2.76 & - & 3.86 \\  & 2019 & - & - & - & - \\  & 2016 & 6.44 & 3.26 & 4.54 & 3.50 \\ \multirow{4}{*}{POVE} & 2017 & 4.76 & 2.82 & 4.17 & 3.85 \\  & 2018 & 3.89 & 2.37 & 3.58 & 4.36 \\  & 2019 & 4.02 & 1.92 & 4.02 & 4.92 \\  & 2016 & 5.01 & 3.31 & 3.16 & 5.23 \\ \multirow{4}{*}{CPVG} & 2017 & 4.16 & - & 2.01 & 4.88 \\  & 2018 & 6.10 & 3.86 & 5.43 & 5.91 \\  & 2019 & 3.94 & 3.88 & - & 7.34 \\  & 2016 & 9.87 & 4.17 & 5.81 & 6.33 \\ \multirow{4}{*}{ADIS} & 2017 & 7.34 & 3.50 & 4.74 & 4.79 \\  & 2018 & 5.95 & 3.32 & 4.21 & 4.67 \\ \multirow{4}{*}{HUTB} & 2019 & 6.27 & 2.95 & 3.46 & 4.63 \\  & 2016 & 5.31 & 2.76 & 3.37 & 3.52 \\ \multirow{4}{*}{HUTB} & 2017 & - & - & - & - \\  & 2018 & - & - & - & - \\  & 2019 & - & - & - & - \\ \multirow{4}{*}{PPPC} & 2016 & 5.57 & 2.01 & 6.64 & 3.59 \\  & 2017 & 2.96 & 3.44 & 3.86 & 1.69 \\ \cline{1-1}  & 2018 & 2.56 & 2.07 & 3.44 & 3.18 \\ \cline{1-1}  & 2019 & 2.78 & 2.34 & 4.49 & 3.79 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: RMSE between GPS-TEC and IRI-2016. longitude than both the American and Asian longitudes. Though IRI-2016 showed higher values at the African and Asian longitudes than the American longitude for stations located along the GE, for stations away from the GE, IRI-2016 model values were higher in the African longitude than both the American and Asian longitudes in nearly the same range. During the minimum phase years (2018-2019), GPS-TEC values were higher in the African longitude than the American and Asian longitudes for stations along the GE and stations away from the GE, although SALU in the American longitude and HUTB in the Asian longitude (stations away from the GE) did not present GPS-TEC data.