## 2 Data and Methods  
Manually scaled ionograms from Sopron, Hungary (SO148, quasi-dipole geomagnetic coordinates: 43.95\({}^{\circ}\)N, 90.69\({}^{\circ}\)E) and San Vito, Italy (VT139, geomagnetic coordinates: 35.94\({}^{\circ}\)N, 90.72\({}^{\circ}\)E) Digisonde stations [32; 33; 34] were analyzed. Key ionospheric parameters, including foF1, foF2, h'F1, and h'F2, which represent peak electron density and maximum layer altitude, were examined. Monthly median values of foF1, foF2, h'F2 and h'F2 were calculated to use them as references for each station. The time resolution is 5 min for Sopron(SO) and 7.5 min for San Vito (VT). In addition, Digisonde-derived B0 and B1 parameters were used, where B0 shows the height difference between hmF2 and the height where the electron density has dropped down to 0.24-NmF2 (maximum of electron density in the F2 layer) [29]. B1 characterizes the shape of the electron density profile: the larger B1 is the larger are the electron densities right below the F2 peak from hmF2 (F2 layer peak height) down to (hmF2 \(-\) B0) [29; 35]. See more about the calculation and usage of B0, B1 parameters in the article of [36; 37]. Figure A10 shows all the stations used in this study. In addition, the manually scaled data of the ionosondes, the received amplitudes at SO and VT stations, were also taken into account to determine the ionospheric absorption changes. At every sounding, the amplitude values were taken in the following frequency bins: 2, 2.5, 3, 3.5, 4, 4.5, 5, and 5.5 MHz +/\(-\)200 kHz, respectively. From these amplitudes, the relative loss of the electromagnetic signal was estimated according to the following equation based on the Friis Transmission Formula (for a more detailed explanation see [38]):  
\[10\lg(L)=10\lg(P_{t}G_{t}G_{r})+20\lg(\frac{\lambda}{4\pi 2h})-10\lg(P_{r}) \tag{1}\]  
In Equation (1), the known terms are \(\lambda\) (the wavelength of the received signal), \(h\) (height of the reflection), and \(P_{r}\) which is the power of the receiver. If we would like to calculate L, which is the loss in dB units, we have to determine the \(P_{t}G_{t}G_{r}\) (power of the transmitter, gain of the transmitter, and gain of the receiver, respectively) product first. To do so, one can estimate that during nighttime, under quiet solar conditions, the loss is relatively \"zero\" (that is the reason that this method can only be used for the 2-5.5 MHz frequency range, where there is reflected signal from the F-layer during nighttime, too, the method is described in more detail in [PERSON] et al. [38]). With this assumption, we calculated the \(P_{t}G_{t}G_{r}\) product based on Equation (1) for SO and VT stations using nighttime data from 173 quiet days in 2019 and 2020 (the same days as described in the Supplementary Material of [PERSON] et al. [38], just for the years of 2019 and 2020). With the \(P_{t}G_{t}G_{r}\) product now established for each station, we can calculate the loss during the investigated period, which is proportional to the relative ionospheric absorption [38]. Subsequently, the absorption values are compared to the X-ray and integrated proton fluxes as measured by the GOES-16 satellite. The X-ray and integrated proton fluxes and the absorption values are determined as a median diurnal variation of three quiet days around the storm (24-26 May 2024) for the sake of better comparison. For the analysis of the auroral region's storm-time condition, we used the IE, IU and IL indices (source: [[https://space.fmi.fi/image/www/il_index_panel.php](https://space.fmi.fi/image/www/il_index_panel.php)]([https://space.fmi.fi/image/www/il_index_panel.php](https://space.fmi.fi/image/www/il_index_panel.php)) (accessed on 8 July 2025). These auroral electrojet indicators are simple estimates of the total eastward and westward currents crossing the magnetometer network [39; 40], managed by the International Monitor for Auroral Geomagnetic Effects (IMAGE), described by [41]. The definition for IE, IU and IL is like the standard AL, AU, and AE indices [42].