## 4 Discussion
The third uplift phase can be identified in all three types of ionosonde parameters portrayed on Figure 4, however, it is important to highlight that the virtual height (h\({}^{7}\)F, hF2) data is the most suitable for the identification of the uplifts and F1-layer height decreases. [PERSON] and [PERSON] [59] analyzed the equatorial ionosphere at Asian longitudes (54-76\({}^{\circ}\) W) with ionosonde and Swarm data during the main phase of this storm and they identified three IMF phases. They also analyzed the height variation of the F2-layer, but they came up with different conclusions as we were during our study. This is most probably connected to the fact that, during this superstorm interval, the equatorial and the low- and midlatitude ionosphere reacted differently to the incident interplanetary effects. Between 21.4 to 21.8 UT, IMF Bz was mostly constant at \(--\)40 nT and IMF By fluctuated around zero, the study of [PERSON] and [PERSON] [59] named this interval as Condition I. They observed almost steady height in ionosonde data taken from near the magnetic equator in contrast to our observations at European longitudes. Between 22.2 and 22.5 UT the IMF Bz turned northward without much variation in IMF By (strongly negative with \(--\)60 nT, [PERSON] and [PERSON] [59] named this as Condition II). In their ionosonde data, they observed a variable response. During northward IMF Bz turnings, around the equator due to overshielding, electron density decrease should be observed. Between 22.5 and 23 UT, IMF Bz turned northward to zero, and IMF By turned sharply from negative to positive (Condition III after [PERSON] and [PERSON] [59]). They observed in ionospheric layer height data a very large increase at all three analyzed stations [59]. [PERSON] and [PERSON] [59] concluded that this enhancement probably occurred due to the post-sunet pre-reversal enhancement (PRE) in that local time sector. Our observations reveal similar ionospheric height increasement during the period of Condition III (Figure 4), at SO station the height increased from 597.7 km to 642.2 km during this half hour interval, in the meantime VT height data show enhancement from 505 to 549.9 km indicating that the European midlatitude ionosphere behave similarly as the Asian longitude one. Ionosonde data of BVJ04 (Geographic: 2.8\({}^{\circ}\)S, 60.6\({}^{\circ}\) W; Geomagnetic: 11.98\({}^{\circ}\)N) (see Figure 2 of [PERSON] and [PERSON] [59]) show the most similar variation to our investigated height parameters. It is important to note that our data show height fluctuations around the interval of Condition III, which can be attributed to TID activity as it discussed above. Therefore, this increasement cannot be clearly connected to the effect of positive IMF By turning in our case. In the European sector the effect of the strong eastward PPEF (caused by the strong positive IMF By turning, reported by [PERSON] and [PERSON] [59]) between 22.5 and 23 UT manifested as electron density decrease at the latitude of VT (4 to 3.12 MHz), on the contrary no plasma density variation was observed at the latitude of Sopron. In the early recovery phase, the G-condition was followed by an exceptionally prolonged disappearance of the layers from the ionograms (\"fade-out\"), lasting for 6-7 h at SO (07:10-14:10 UT) and VT (08:07-14:07 UT), which uniquely occurred during daytime. Meantime, the Swarm C and B observed a density decrease of a factor of 3 to 4 around 6:00 and 9:30 UT (7 and 11.5 MLT) compared to the previous day's quiet values (Figure 7i,l). Swarm's vTEC and electron density measurements also showed extreme (30-50% decrease in TEC) and latitudinally extended plasma depletion in the Northern Hemisphere (Figure 7h,k) during this interval. The \"fade-out\" from the ionograms likely resulted also from the fact that electron density dropped below the daytime detectability level of the Digisondes, approximately 2.3 MHz (6.58-10\({}^{4}\) 1/cm\({}^{3}\)) [20; 77].