## 3 Mother's Day Storm: 10-11 May 2024
### Ionosonde Observations
This sudden refilling thus occurs at the same time in the European and American sectors, despite the local time difference. The pattern of the F\({}_{1}\) layer at MHJ45 is of course delayed in time compared to the European observatories as a result of this local time difference. Figure 7: Peak electron densities for the F\({}_{2}\) (red) and F\({}_{1}\) (blue) obtained from the ionosonde measurements at Millstone Hill ((**top**), MHJ45) and I-Cheon ((**bottom**), IC437) from 10 May 2024 0:00 to 13 May 2024 0:00 UTC. The observations at IC437 (bottom panel of Figure 7) show a different pattern. A sudden depletion at the storm onset is again evident, but at this location, 18:00 UTC corresponds to nighttime. During the local daytime on the two succeeding days, there are multiple gaps in the data due to the various difficulties explained above. Nevertheless, from those ionograms that could reliably be scaled, it is clear that severe depletion persisted throughout these days, and even during the local nighttime on 12 May--the end of the time series shown in the Figure--the electron density has clearly not recovered yet to the quiet-time level seen before the storm onset. The peaks in _foF\({}_{2}\)_ seen during the local nighttime on 11 May correspond to periods of strong spread-F associated with auroral oval structures expanding to the latitude of this observatory. During the main phase of the storm, the electron density and the height of the F region decreased and increased, respectively, indicating an upward displacement of the ionosphere (depression and uplift) and irregularities (plasma bubbles) due to the stable aurora red arc convergence of low- and high-latitude effects. The electron density in the F\({}_{2}\) layer decreased due to the expansion of the atmosphere that modified the recombination time of the ionized particles. During the strong post-storm depletion, F\({}_{2}\) disappeared behind the F\({}_{1}\) layer. The strong spread-F initiated during the main phase was still present during the next nights and caused the scintillation of the satellite signals. After the arrival of the CME, the electron density decreased and remained significantly lower than during the quiet days for more than a full day, including during the recovery phase of the geomagnetic storm. This meant that for high-frequency (HF) communication, the portion of higher HF frequencies was not available for communication, and so several advisories were sent to civil aviation via PECASUS ([[https://pecasus.eu/](https://pecasus.eu/)]([https://pecasus.eu/](https://pecasus.eu/)), accessed on 12 May 2024) to warn for this so-called post-storm depression [40]. Unusually high spatially distributed values of the rate of TEC change index (ROTI) were detected on the nights of 10 and 11 May [2]. The ROTI enhancements on 10 May might be linked to stable auroral red arcs and an equatorward displacement of the main ionospheric trough. Instead, the ROTI enhancements on 11 May might be triggered by a joint action of low-latitude plasma pushed poleward by the pre-reversal enhancement in the post-sunset hours and wave-like perturbations propagating from the north.