## 3 Results
### Longitudinal Variations
The GIM values were at about the same range at the three longitudes for the stations along the GE and stations away from the GE, while IRI-2016 model values were highest at the station away from the GE in the African longitude and lowest at the station along the GE in the American longitude. During the descending and minimum phases, we observed LT with respect to universal time effects of longitudinal variations in GPS-TEC, GIM and IRI-2016 within the equatorial ionosphere. It was necessary to harmonize the time frame of observation to the universal time (UT) scale in order to show TEC variations at the three longitudes. The result of the UT with respect to the LT effect was peak shifting in the African, American and Asian longitudes to LT hours between 10:00 and 18:00 LT. Longitudinal variations in plasmaspheric TEC were investigated by [PERSON] et al. [42], who reported significant longitudinal variations that may be similar to the longitudinal variations observed in GPS-TEC but different from the longitudinal variations in electron densities at the F2 peak. The universal time with respect to the LT effects of the longitudinal variations of ionospheric parameters such as TEC are strongly related to the solar zenith angle, which is linked to the rotation of the Earth around its axis. The seasonal effects of the longitudinal variations during the descending and minimum phases are better observed in Figures 6-11. On a general note, GPS-TEC, GIM and IRI-2016 during the descending and minimum phases showed semi-annual variations, i.e., higher magnitudes during the equinoxes than the solstices at the three longitudes, although, in some cases, the September equinox and December solstice magnitudes were at about the same range at the three longitudes. Hence, the seasonal effects at the three longitudes for both stations along the GE and stations away from the GE were quite similar. However, the GPS-TEC, GIM and IRI-2016 values were generally highest during the March equinox and lowest during the June solstice at the three longitudes. In the American longitude (Figures 6 and 7), the GPS-TEC (22-24 TECU) and GIM (25-27 TECU) values were at about the same range during the September equinox and the December solstice, apart from the fact that their peaks appeared to be slightly broader during the December solstice than the September equinox. Contrastingly, the IRI-2016 model values were higher during the December solstice (2-31 TECU) than the September equinox. We also noticed unequal magnitudes of GPS-TEC, GIM and IRI-2016 during the March and September equinoxes at the three longitudes, which is known as equinoctial asymmetry. This asymmetry is an unusual ionospheric phenomenon because the solar zenith angles in the March equinox and the September equinox are expected to be equal [52; 53]. There were not many differences in the seasonal effects at the other two longitudinal regions (African and Asian), apart from the arrival time of their peak values and the nighttime enhancement in the GPS-TEC, GIM and IRI-2016 models observed in the station along the GE in the African longitude (ADIS) during the March equinox and the December solstice for the descending phase year of 2016. The nighttime enhancement of TEC is mainly controlled by the pre-reversal enhancement (PRE) of the zonal electric field above the magnetic equator near the local dusk terminator, which develops vertical ion drift and therefore impacts the generalized Rayleigh-Taylor instabilities (RTIs). An RTI is considered to be mechanism of the occurrence and development of equatorial plasma bubbles (EPBs) at equatorial anomaly regions [7; 27; 41; 51].