## 4 Discussion  
Ionospheric perturbations following geomagnetic storms are caused by solar wind shockwaves that interact with Earth's magnetosphere, causing magnetic field disturbances and subsequent TEC variations that last from several hours to several days [64]. TEC variability is linked to several physical and chemical phenomena, including chemistry, ambipolar diffusion, transport due to E \(\times\) B drift, and transport due to neutral wind (e.g., [65; 66]). During the second half of February 2014, four M-class CMEs caused three clear SSCs on 15, 20, and 23 February, and one HSSW event on 19 February with the absence of classical SSC features. The magnetospheric reaction to the HSSW on 19 February resulted in an SYM-H peak of \(-\)112 nT. For the HSSW storm, no classical SSC is seen, most probably due to the prevalent magnetospheric convection caused by the preceding southward IMF on 15 February. Negative IMF B\({}_{2}\) and increased solar wind pressure resulted in an increased merging electric field E\({}_{m}\), and thus intensified the magnetospheric convection (reflected by the Am index). Low-latitude TEC enhancements resulting from the HSSW on 19 February reached over 20 TECU in the IGS profiles, and these were longer-lasting at the dip equator than in the EIA crests along the successive SSCs events on 20 and 23 February. From this point, the observations were of a gradual increase in GEC up to 3.5 GECU at the end of the month, showing 1 GECU differential intensification. This increase was not reflected by the low-latitude IGS TEC profiles (Figure 9g), but clear rTEC enhancements over 60% were observed during the same period on the night side at the RABT ground station. This increased gradient may be localized at higher latitudes following the EIA expansion generated by the day-time eastward electric fields causing E \(\times\) B vertical plasma uplift. All the GNSS stations showed irregular TEC variability due to the EIA expansion towards mid-latitudes. Day- and night-side TEC enhancements were observed as plasma flowed along the magnetic field lines to the mid- and high latitudes [67]. It is clear that the low-latitude TEC variability is strongly influenced by the storm-time EIA expansion, as shown by, e.g., Ref. [68]. The ionospheric TEC disturbances in the African sector differed significantly at stations located at different magnetic latitudes. The TEC depletion at the dip equator could be due to the enhancement of the fountain effect. The observed TEC variations respond to energy injection at storm onset [69], which also depends on thermospheric chemistry processes. We consider the E \(\times\) B drift disturbance to be the main contribution of the observed TEC variability. DDEF phenomena and PPEF processes change the E \(\times\) B drift and its effect; the change depends on magnetic latitude nonlinearly [70]. The PPEF plays an important role in the ionosphere processes at the equatorial regions during geomagnetic storms, whereas [71; 72] provided similar conclusions. We suspect that EIA expansion may be produced by eastward PPEF in the Asian sector, where the main phase of the geomagnetic storms occurred at the day side. The post-sunset PPEF of the eastward (daytime) and westward (nighttime) drifts combine to drag plasma from the dip equator by E \(\times\) B convection, sustaining the plasma pile at the EIA crests. The polarity of the PPEF was eastward and westward during day- and night-time, respectively ([40], and references therein), while, during day-side and post-sunset, it has eastward polarity intensified through plasma induction at the EIA crest. The EEJ data show that the storm-time PPEFs are an important contribution of the observed ionospheric responses to this multiphase geomagnetic storm. Contributions of neutral winds should also be considered and investigated from, for example, LEO satellites (e.g., thermospheric winds estimated from accelerometers [73; 74] onboard Swarm, Gravity Recovery and Climate Experiment, etc.) and other sources in the future.