## 3 Observations
### Swarm Measurements  
The electron density during the pre-storm and storm period (9-15 May 2024), observed by Swarm B at MLT = 23.5, is presented in Figure 6. Dotted red and blue lines mark the SSC and Dst minimum times, respectively. The quasi dipole (QD) mlat for SO and VT is indicated by grey horizontal lines. Black dots represent trough minima from the Swarm MIT product, with additional manual selections where the automated algorithm failed. Swarm observations reveal that the trough moved rapidly equatorward during the storm's main phase, nearing VT's latitude by phase end. This is supported by all-sky records (Figure 3i), which show the Stable Auroral Red (SAR) arc approaching the zenith at Manciano (36.14\({}^{\circ}\)N). In the Southern Hemisphere, the MIT also reached low latitudes below 35\({}^{\circ}\). During the recovery phase, the MIT gradually retreated poleward. Enhanced density was observed equatorward of the MIT at night in both hemispheres, particularly early in recovery, while a density hole formed around the equator. This latter phenomenon was partially discussed by [PERSON] et al. [2] and is not the focus here. Poleward of the MIT, the ionosphere was severely depleted at the satellite's altitude. Swarm passed over VT and SO four times within a day following the SSC. The first pass occurred shortly after the SSC at 17:47 UT (SO) and 17:49 UT (VT) at MLT 19.1. Compared to the previous day (shown in grey in Figure 7a-c), EIA crests on 10 May (shown in black) were more pronounced in both Swarm C's plasma density and LEO vTEC observations (Figure 7a-c). Additionally, ssFACs intensified poleward of mlat \(-60^{\circ}\) (Figure 7a). Figure 5: The ionospheric absorption investigation with the amplitude method for Sopron (**a,b,d,e**) and the portray of the GOES satellite X-ray flux and integrated proton flux data for the quiet and the disturbed period, respectively. Panel (**c**) shows the calculated ionospheric absorption data at 3 MHz for the quiet period and panel (**f**) the calculated ionospheric absorption for the disturbed period at 3 MHz. The green bars mark the intervals when the layer frequency went under 3 MHz; the grey bars mark the intervals of the “fade-out periods”. More dramatic changes were observed a few hours later at 22:04 UT when Swam B passed over VT and SO (Figure 7d-f). Poleward of 40\({}^{\circ}\) malt in both hemispheres, ssFAC intensity increased by orders of magnitude. This zone of intense ssFACs (and particle precipitation) corresponds to the auroral zone. Notably, its equatorward boundary shifted to as low as 40\({}^{\circ}\) malt (Figure 7d), south of SO. Local density peaks in both vTEC and plasma density (see Figure 7e,f), occurring between 40-60\({}^{\circ}\) malt, were likely generated by storm-enhanced particle precipitation. During this flyby, MIT was located at around 40\({}^{\circ}\) malt, marking the equatorward boundary of the auroral zone. On 11 May, Swarm C, followed by Swarm B, passed over the ground stations again. In both cases, the in-situ plasma density observed near the stations was three to four times lower (log density decreased by 0.44 at MLT 7.3 and by 0.63-.64 at MLT 10.8) than the previous day (Figure 7i,l). This decrease was also noted in the LEO TEC in the Northern Hemisphere, poleward of 30\({}^{\circ}\) at MLT 7.3 (5:58 UT) (Figure 7h) and 10\({}^{\circ}\) at MLT 10.8 (9:30 UT)  
Figure 6: Ionospheric plasma density observed by Swarm B at MLT 23.5 between 9–15 May 2024. Figure 7: Swarm passes over Sopron and San Vito between 9–11 May 2024. Black curves show observations following the SSC, while grey curves represent observations a day earlier for comparison. For each of the four passes (**a**–**c**), (**d**–**f**), (**g**–**i**), (**j**–**l**), ssFAC density (log scale), vertical TEC, and plasma density (log scale) are shown. MLT, UT (at Sopron), and orbit longitude are provided above each block. Vertical dashed and dotted lines mark the positions of San Vito and Sopron, respectively. (Figure 7k).