## 2 Materials and Methods  
In the present study, we considered the GPS observation data in Receiver Independence Exchange (RINEX) format from three different longitudinal regions, namely, American (Sao Luis, Brazil (SALU) and Porto Velho, Brazil (POVE)), African (Cape Verde (CPVG) and Addis Ababa University, Ethiopia (ADIS)), and Asian (Hutbay, Nicobar Islands (HUTB) and Puerto Princesa, Phillipines (PPPC)) longitudes, when extracting the ionospheric TEC at the respective locations. The geographic coordinates, as well as geomagnetic latitudes, of individual stations are listed in Table 1. The 30-second sampling GPS observation data from the three longitudinal sectors were downloaded from the UNAVCO archives ([[https://data.unavco.org/archive/gnss/rinex/obs/](https://data.unavco.org/archive/gnss/rinex/obs/)]([https://data.unavco.org/archive/gnss/rinex/obs/](https://data.unavco.org/archive/gnss/rinex/obs/)); accessed on 25 October 2021). Figure 1 shows the exact location of the stations used in this research, while Table 1 shows their geographic coordinates and magnetic latitudes, as well as the full meaning of all the GPS observation station codes. Further, the TEC values at corresponding locations predicted by the IRI were obtained by executing the instant run version of the IRI-2016 model ([[https://ccmc.gsfc.nasa.gov/modelweb/models/iri2016_vitmo.php](https://ccmc.gsfc.nasa.gov/modelweb/models/iri2016_vitmo.php)]([https://ccmc.gsfc.nasa.gov/modelweb/models/iri2016_vitmo.php](https://ccmc.gsfc.nasa.gov/modelweb/models/iri2016_vitmo.php)); accessed on 19 November 2021). The TEC values were also extracted from the hourly GIMs developed by the Center for Orbit Determination in Europe (CODE) at the University of Bern by adopting a four-point inverse distance weighted interpolation technique [1]. The CODE GIMs are available in the IONosphere map EXchange (IONEX) format, with the global map represented in the form of grids having a spatial resolution of 2.5\({}^{\circ}\) in latitude, a spatial resolution of 5\({}^{\circ}\) in longitude, and a temporal resolution of 1 h (25 maps per day). The corresponding GIM TEC at the GPS station locations in the grids can be extracted by interpolating the values from the four nearest grid points' vertexes. The CODE GIMs were accessed from the CDDIS-NASA server ([[https://cddis.nasa.gov/archive/gnss/products/ionex/](https://cddis.nasa.gov/archive/gnss/products/ionex/)]([https://cddis.nasa.gov/archive/gnss/products/ionex/](https://cddis.nasa.gov/archive/gnss/products/ionex/)); accessed on 16 December 2021). The obtained GPS-TEC data were processed using the GPS-TEC analysis application program developed by [PERSON] [44], which was obtained from his webpage ([[https://seemala.blogspot.com](https://seemala.blogspot.com)]([https://seemala.blogspot.com](https://seemala.blogspot.com)); accessed on 12 October 2021). To compute the relative values of vertical TEC (_VTEC_), the TEC analysis program uses the phase and code values for both L1 (1575.42 MHz) and L2 (1227.60 MHz) to minimize the impact of clock errors and tropospheric water vapor. The absolute values of TEC were determined by considering the differential code biases of the satellites and receivers, as given by the University of Bern, which were calculated by minimizing the TEC deviation between 02:00 and 06:00 LT [45]. In order to minimize the effects of multipath associated with low elevation