## 4 Discussion
Most of the time, there were discrepancies between the ionospheric models and GPS-TEC. These discrepancies were likely due to the fact that most ionospheric models are designed based on limitations in the upper limit prediction [1; 58]. On a general note, the seasonal values of GPS-TEC were observed to be higher in the African longitude than the American and Asian longitudes. This could be attributed to the fact that the Sun rays arrive at the African longitude at a more oblique angle, giving rise to higher electron densities at the African longitude than at the other two longitudes. Irrespective of the fact that the orientation ofthe geomagnetic field lines in the African and Asian longitudes are nearly horizontal, one would expect that GPS-TEC values in the Asian longitude would be higher than GPS-TEC values in the American sector, but this is not the case. GPS-TEC values in the Asian longitude were generally the smallest of the three longitude sectors. There could be possibilities of dominating physical processes such as diffusion, thereby reducing the height of the ionosphere over the Asian longitude that therefore reduced the electron densities and gave rise to the smallest GPS-TEC values in the Asian longitude compared with the American and African longitudes. Interestingly, GPS-TEC values were mostly higher at stations on the GE than stations away from the GE at the three longitudes, especially in the African longitude during both phases of the solar cycle as a result of the ionospheric morphology near the dip equator. Semi-annual variations in TEC were evident during the descending and minimum phases of solar cycle 24, with some exceptions (semi-annual anomaly) showing equal peaks during the September equinox and the December solstice or with the December solstice peak being slightly higher than the peak of the September equinox. These results partly agree with those of [PERSON] et al. [43], who reported semi-annual variations in their research. These observations were more evident during the minimum phase at stations along the GE in the American (POVE) and African (ADIS) longitudes. An important point to note is that the peak of TEC from the various sources was slightly wider during the June solstice than the other three seasons during the descending and minimum phases at the three longitudes. Furthermore, the peak of TEC was generally wider during the minimum phase than the descending phase for stations along the GE than stations away from the GE at the three longitudes. The phenomenon of the semi-annual variations of ionospheric parameters such as TEC has been widely reported [7; 59; 57; 59; 60]. Therefore, we intend not to dwell on the mechanism giving rise to the semi-annual variations of ionospheric parameters. However, it is worth mentioning that when there is a seasonal variation in the solar zenith, there could be possible deviations in the semi-annual pattern of ionospheric parameters such as TEC [7]. The equinoctial asymmetry reported during both phases at the three longitudes is not new; however, they did not show any significant longitudinal variations in this study. This is in contrast to the results reported by [PERSON] et al. [61] at low latitudes. They reported the presence of longitudinal variations in the equinoctial asymmetry of TEC and NmF2 at low latitudes. However, the results on equinoctial asymmetry in this research agree with those of [PERSON] et al. [53], who reported that the behavior of equinoctial asymmetry has no significant longitudinal variations. Equinoctial asymmetry is an ionospheric phenomenon associated with differences in the electron density distribution as a result of differences in electric fields, temperatures, thermospheric neutral winds, and sometimes the extreme ultraviolet radiation from the Sun during both equinoxes [61; 39; 62]. However, we are quite surprised by the insignificant longitudinal variations of the equinoctial asymmetry because the electron density distribution and ionospheric morphology at the three longitudes were quite different [48; 63; 64; 65].