## 4 Discussion  
Multi-instrumental observations captured unique ionospheric conditions at mid- and low-midlatitudes during the storm's main phase. Initially, F2 density rapidly decreased, followed by an unusual G-condition at both SO and VT, where F2 density fell below F1 in the early recovery phase. This was followed by an exceptionally long fade-out of the layers during daytime in the early recovery phase. By 22 UT, the F2-layer raised to the altitude of Swarm, and Swarm confirmed the severe nighttime depletion of this layer (Figure 7f). Meanwhile, a horizontal redistribution also took place. By this time, MIT reached SO's latitude. Afterwards, the equatorward shift of the MIT went to the extremely low 35\({}^{\circ}\) malt (Figure 6), which was indicative of the simultaneous dramatic shrinkage of the plasmasphere [24]. The observed density depletion was limited to the extent of the MIT and to the F2-layer as vTEC remained nearly unchanged. The low F2 density observed by ionosondes in the main phase can also be primarily attributed to the arrival of MIT. Our findings are in good agreement with the result of [58]. [PERSON] and [PERSON] [59] recorded IMF By turning at 22:30 UT on 10 May which also coincided with northward turning of IMF Bz, see also on Figure 1a,b. Storm-enhanced plasma density (SED) developed at nighttime low latitudes and expanded toward mid-latitudes (Figure 6), approaching the counter-shifting MIT, while a deepening equatorial plasma depletion (EPD) evolved at the equator [2]. [PERSON] et al. [2] found that the daytime EIA extended up to \(+/-40^{\circ}\) malt due to a \"Strong Super fountain Effect\" which were linked to eastward PPEFs and DDEFs during the main and the early recovery phase of the present storm. EPDs generally develop due to the generalized Rayleigh-Taylor instability (e.g., [60; 61; 62]) and prefer the early morning [63; 64; 65; 66] and post sunrise sectors [67] and can extend to \(+/-36^{\circ}\) malt [68]. By the end of the main phase, [PERSON] observed that the poleward-expanding SED and the equatorward-shifting ssFAC-dominated (precipitation) zone nearly merged, with the MIT sandwiched between them (Figures 6 and 7). At this point, the nightside midlatitude ionosphere was confined to the MIT. The movement of the MIT was closely followed by the AO. The poleward wall of MIT is believed to be shaped by auroral precipitation in the evening MLT sector [69; 70]. The observations confirm the rapid equatorward shift of the AO (optical images, auroral Es- and F-layers and intense Swarm ssFAC). The unique observations included the first-ever Digisonde recording of auroral Es at SO, and auroral F observations at unprecedented low latitudes (down to 35\({}^{\circ}\) malt), confirmed by optical all-sky observations (see Figure 3c,f,i). The source of these recorded auroral Es was most probably due to particle impact to the region of auroral oval [15]. Using multi-instrumental observations, [PERSON] et al. [3] examined the American-Atlantic longitude sector during this event and found that the EIA expanded and merged with the AO. [PERSON] et al. [14] reported repeated overlaps between the poleward-expanding EIA and the AO during the storm's main phase, based on NASA's Global-scale Observations of the Limb and Disk (GOLD) mission data. The northern and southern EIA crets reached approximately 38\({}^{\circ}\)N and 35\({}^{\circ}\)S mlat around UT midnight (20 MLT), while the AO extended to 40\({}^{\circ}\)S and 38\({}^{\circ}\)N (the latter estimated from a GOLD image taken at 22:10 UT, presented in Figure 5 of [PERSON] et al. [14]). These observations confirm a remarkable absence of a mid-latitude ionosphere also on the dayside. Previous studies during extremely intense storms have also registered similar auroral expansions up to \(\sim\)40-50\({}^{\circ}\) malt [71; 72; 73].