## 2 Materials and Methods
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Longitude Sector** & **Station Code** & **Location** & **Geographic Lat.** & **Geographic Long.** & **Magnetic Lat.** \\ \hline \multirow{2}{*}{American} & SALU & Sao Luis, Brazil & \(-2.59^{\circ}\) & \(-44.21^{\circ}\) & \(-0.25^{\circ}\) \\  & POVE & Porto Velho, Brazil & \(-8.7^{\circ}\) & \(-63.9^{\circ}\) & \(2.87^{\circ}\) \\ \hline \multirow{4}{*}{African} & CPVG & Cape Verde & \(16.73^{\circ}\) & \(-22.94^{\circ}\) & \(4.94^{\circ}\) \\  & ADIS & Addis Ababa & & & \\  & ADIS & University, & \(9.02^{\circ}\) & \(38.81^{\circ}\) & \(0.16^{\circ}\) \\  & Ethiopia & & & & \\ \hline \multirow{4}{*}{Asian} & HUTB & Hutbay, Nicobar Islands & \multirow{2}{*}{\(10.62^{\circ}\)} & \(92.51^{\circ}\) & \(2.28^{\circ}\) \\  & PPPC & Puerto Princesa, Phillipines & & \(9.77^{\circ}\) & \(118.74^{\circ}\) & \(1.93^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The geographic coordinates and magnetic latitude of the GPS stations in American, African, and Asian longitude sectors. angles, an elevation mask angle cut-off of 30\({}^{\circ}\) was used for an ionospheric shell height of approximately 350 km [28; 45; 46; 47]. This allowed only GPS signals at a 350 km height to fall near the GPS receiver on the ground. The GPS-TEC analysis application program uses the 30-second sampling observables from each GPS satellite as input to provide 1-minute averages of \(VTEC\) calculated over all the visible GPS satellites during the epochs, which were further downsampled to hourly averaged \(VTEC\) values for analysis in this study. \(STEC\), which is the \(TEC\) along a satellite pass, is expressed in Equation (1) as:  
\[STEC=\int\limits_{receiver}^{satellite}N(s)ds \tag{1}\]  
where \(N(s)\) is the electron density and s is the satellite-receiver distance. \(VTEC\), which is the perpendicular projection of \(STEC\), can be obtained using Equation (2). \[VTEC=STEC\times cos\bigg{[}sin^{-1}\bigg{(}\frac{R_{E}cos\theta}{R_{E}+h} \bigg{)}\bigg{]} \tag{2}\]  
where \(R_{E}\) is the radius of the Earth, \(\theta\) is the elevation angle at the ground station, and \(h\) is the ionospheric layer height, which is mostly assumed to be 350 km at equatorial and low latitudes. The values of the \(STEC\) and \(VTEC\) are expressed in \(TEC\) units (TECU), given that 1 TECU = \(10^{16}\) electrons/m\({}^{2}\). There are possibilities for monitoring global ionosphere maps (GIMs) with the data from a huge number of permanent GPS receivers established under the IGS Associate Analysis Centers (IAACs). GIMs are semi-empirical models that are products of the International GNSS Service (IGS). The GIMs used in the present study were those developed by one of the IAACs, i.e., the Center for Orbit Determination in Europe (CODE) at the University of Bern, as the accuracy of the CODE-GIMs (1h is believed to be equivalent to that of the combined IGS-GIMs (2h) but with a relatively better temporal resolution. The IRI  
Figure 1: Location of GPS stations at different longitude sectors considered in this study, a red contour line in the center of the map indicates the magnetic equator, whereas the black counters in either hemisphere represent the approximate position of the EIA crest during the period. The red colour stars indicate the locations of GPS stations as listed in Table 1.  
is an empirical ionospheric model that also plays a supplementary role in understanding TEC variability in the global ionosphere. A brief explanation of the IRI, which can be found in the introduction section of this research, was well-explained in the work of [PERSON] et al. (2020, 2020). Diurnal variations in the GPS-TEC, GIM and IRI-2016 models (NeQuick topside option) were plotted using daily values, while the seasonal variations in the GPS-TEC, GIM and IRI-2016 models were plotted using monthly values. We adopted the universal time (UT), with an understanding of the local time (LT) difference at the stations spreading over three longitude sectors.