## 2 Observation Methods
### Ionosonde Observations  
Ionosondes employ high frequency (HF) radio transmissions of various frequencies to determine the electron density profile above the instrument. Frequencies below the plasma frequency \(\omega=\sqrt{\frac{Ne^{2}}{\varepsilon_{0}m}}\) (where \(e\) is the electron charge, \(m\) is its mass, and \(\varepsilon_{0}\) is the permittivity of free space) are reflected by the plasma in the ionosphere. Therefore, the electron density profile \(N_{e}(h)\) can be reconstructed from the reflection of a range of different frequencies. The main limitation of this technique is that it allows observations only up to the height of the greatest electron density. This altitude varies significantly both during quiet conditions and as a result of particular disturbances such as geomagnetic storms. In the present study, we use data from three ionosondes in Europe (JR055, DB049, and EB040), at different latitudes, as well as two ionosondes in other longitude sectors: IC437 in South Korea and MHJ45 in the USA. Ionograms were obtained from the DIDBase of the GIRO repository [23]. This allows us to compare the storm effects at different latitudes as well as the effects of the local time at the onset of the event. The coordinates of the observatories are listed in Table 1. Each of these observatories uses a similar Digisonde DPS-4D ionosonde and produces regular soundings of the ionosphere at intervals of 5 (for the European observatories) or 7.5 min (for the American and South Korean observatories). All ionograms were manually inspected in order to derive the characteristics of the various layers (here, we use the critical frequencies _foF\({}_{1}\)_ and _foF\({}_{2}\)_ of the two F region density peaks), only retaining the data points which could be obtained reliably according to the standard URSI (International Union of Radio Science) rules. The exceptional ionospheric conditions during the events analyzed here cause some particular difficulties in the interpretation of the ionograms. There are three reasons for occasional gaps in the time series of the critical frequencies. First, during the nighttime, the spreading of the F\({}_{2}\) trace is observed (see the left panel of Figure 1 for an example). This can render it impossible to accurately determine the critical frequency _foF\({}_{2}\)_. During the first night of the 11 May storm, there were also sporadic E layers observed, produced by particle impacts (which are associated with the visible auroras seen at those same times even at low latitudes). An example of this is shown in Figure 1 on the right. In this example, the Es layer extends to above 4 MHz, making it impossible to detect the depleted F layer above. Finally, there are some gaps in the data related to the absorption of the ionosonde signals in the D region of the ionosphere due to an increased X-ray flux or polar and auroral absorption \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Name (Country)** & **URSI Code** & **Latitude** & **Longitude** \\ \hline Juliusruh (Germany) & JR055 & 54.60\({}^{\circ}\) N & 13.40\({}^{\circ}\) E \\ Dourbes (Belgium) & DB049 & 50.10\({}^{\circ}\) N & 4.60\({}^{\circ}\) E \\ Roquetes (Spain) & EB040 & 40.80\({}^{\circ}\) N & 0.50\({}^{\circ}\) E \\ Millstone Hill (USA) & MHJ45 & 42.60\({}^{\circ}\) N & 288.50\({}^{\circ}\) E \\ I-Cheon (South Korea) & IC437 & 37.14\({}^{\circ}\) N & 127.54\({}^{\circ}\) E \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Ionosondes from which data are used. Figure 1: Examples of spread-F condition ((**left**), recorded by IC437 on 11 May 2024 at 20:15:00 UTC) and particle-induced Es layer ((**right**), recorded by DB049 on 11 May at 00:50:02 UTC). Note that the height (vertical axis) ranges from 0 to 800 km in both cases, but the frequencies (horizontal axis) range from 1 to 12 MHz in the left panel and from 1 to 9 MHz in the right panel. (the latter mostly affects data from the Millstone Hill observatory).