## 4 Discussion  
The F2 layer of the ionosphere is very useful for radio wave propagation, communication, and navigation, as well as other relevant purposes, and it also poses some degrees of danger to highly dynamic technological systems, as mentioned in Section 1. The region near the F2 peak height of the ionosphere is usually a subject of major concern because of the large number of electron densities it contains. However, above the F2 layer height, where the topside ionosphere-plasmosphere is positioned, the various physical processes dominating this region (which are quite complicated and difficult to understand) are sometimes different from the processes controlling the bottom-side ionospheric region [42; 54]. Hence, there is an urgent need for additional studies on the topside ionosphere-plasmosphere for more understanding of the physical processes associated with this region. The results from this research reveal a clear solar cycle effect on TEC, showing higher TEC values during the descending phase (2016-2017) than the minimum phase (2018-2019) at the three longitudes. Solar activity indicates the intensity of X-rays and extreme ultraviolet (EUV) radiations from the Sun. The variations of these solar electromagnetic radiations cause enormous variations in temperature, neutral wind, electric field, and ion and electron densities in the ionosphere [7; 25; 55; 56; 57]. The diurnal variations in GPS-TEC, GIM and IRI-2016 were observed to show similar features at each longitude, except in the African longitude, where GPS-TEC, GIM and IRI-2016 were observed to show unique features at the station away from the GE (CPVG). At CPVG, the peaks of the GPS-TEC, GIM and IRI-2016 models were observed to be delayed from post-noon in the December solstice to post-sunset in the equinoxes and June solstice hours with respect to UT for both phases of the solar cycle. The delayed peaks were more obvious in GPS-TEC values than in GIM and IRI-2016 values, which was in contrast to the behavior of the TEC in the African longitude, where maximum TEC values were observed around the noon to post-noon LT periods [7; 27]. This observation at CPVG could have been due to its location with respect to the geographic and geomagnetic equator. The local time difference (4 h) between both stations in the African longitude may also have been a contributing factor to the unique behavior of TEC at CPVG. On a general note, GIM predictions looked more suitable as a proxy for GPS-TEC measurements in the sense that they were able to reveal the actual shape of GPS-TEC at the three longitudes, although there were some cases of underestimation and overestimation. However, the IRI-2016 model was not able to reveal the true shape of GPS-TEC at the three longitudes, except at CPVG in the African longitude, although the IRI-2016 model values were often slightly higher than the GPS-TEC values at CPVG. At the other stations in the three longitudes, we observed double peak-like structures (noon bite-out) in the IRI-2016 model. This is a major shortcoming for the IRI-2016 model; even though the noon bite-out is an equatorial ionospheric feature, the TEC usually does not conform to a noon bite-out profile because it is extended beyond the bottom-side ionosphere into the plasmsphere, where plasmaspheric electron density contributions are dominant. Furthermore, IRI-2016 is restricted to the whole bottom-side and partially topside ionosphere [20; 27]. The disparity between the ionospheric models (GIM and IRI-2016 models) and GPS-TEC, showing either the underestimation or overestimation of GPS-TEC values, are evident in the RMSE values (Tables 2 and 3). GIM assessment generally looked better than IRI assessment, especially in the African longitude compared with the other two longitudes. IRI-2016 predictions were sometimes better than GIM predictions in some seasons, especially during the March equinox of the descending phase year of 2016 at CPVG in the African longitude and the June solstice for the descending phase year of 2016 at PPPC in the Asian longitude.