## 3 Mother's Day Storm: 10-11 May 2024
### Ionosonde Observations  
Figure 6 shows the peak electron densities for the F\({}_{1}\) (in blue) and F\({}_{2}\) (in red) layers for the three European ionosondes for a three-day period covering 10 to 12 May 2024. The  
Figure 5: VTEC maps, differences with expected behaviour and variability over Europe on 11 May 2024 at 20:20 UTC. (**Top**): VTEC maps estimated in real-time. The dots represent the VTEC data used for the interpolation. (**Bottom left**): differences between VTEC maps and the expected VTEC (median over the past 15 days). (**Bottom right**): the VTEC variability reflecting the ionospheric state variations during the 5 min time span of the interpolation. E layer was found to be not affected by the geomagnetic storm. This is to be expected, because the E layer is known to be driven entirely by solar irradiance. At the onset of the storm, a very rapid depletion of the F\({}_{2}\) layer density can be observed. The depletion appears to be more rapid at the higher latitudes, although of course the peak density before the storm is lower at higher latitudes as well. For the most part of 11 May, the F\({}_{2}\) layer was not detected, because the electron density in the F\({}_{2}\) region was lower than in the F\({}_{1}\) layer and thus not visible to the ionosondes. This is referred to as the \"G-condition\" in the ionosonde community and is often seen during major storms (see for instance [38] for some recent examples). The density of the F\({}_{1}\) layer is also seen to be lower than during the quiet day before the storm, but the depletion is much less severe than in the F\({}_{2}\) layer because transport processes and chemical changes are less important at the F\({}_{1}\) layer altitude compared to the influence of solar irradiance. The strong depletion of the F region lasts throughout 11 May and the morning of 12 May. The reappearance of the F\({}_{2}\) layer on 12 May and its return to a quiet time peak density is almost as rapid as the depletion at the onset of the event. This indicates that the  
Figure 6: Peak electron densities for F\({}_{2}\) (red) and F\({}_{1}\) (blue) obtained from the ionosonde measurements at Juliiusruh ((**top**), JR055), Dourbes ((**middle**), DB049), and Ebre ((**bottom**), EB040), from 10 May 2024 0:00 to 13 May 2024 0:00 UTC. refilling of the F\({}_{2}\) layer was very quick. It is also evident that this refilling occurred earlier at the EB040 observatory, which is at the lower latitude. Here, the F\({}_{2}\) layer was detected again around 06 UTC. At the DB049 and JR055 ionosondes at higher latitudes, the F\({}_{2}\) layer was not detected until around 09 UTC, and the peak density reached its climatological values only around noon. Although the reappearance of the F\({}_{2}\) layer happens earlier at lower latitudes, it can be seen that the peaks of the highest electron density reached on 12 May occur earlier at the highest latitude. The highest density is seen on 12 May at 12:20 UTC by JR055, at 12:50 UTC by DB049, and at 13:45 UTC by EB040. This seems to indicate large-scale travelling ionospheric disturbances (LSTIDs) moving from the auroral region to lower latitudes. Such LSTIDs are usually seen during the beginning of a storm (e.g., [39]) but seem to also occur here at the time of the refilling of the F\({}_{2}\) layer. Figure 7 shows the critical frequencies of the F\({}_{1}\) and F\({}_{2}\) layers for the Millstone Hill (MHJ45) and I-Cheon (IC437) ionosondes. The behaviour of _foF\({}_{2}\)_ at MHJ45 (top panel) can be seen to be very similar to what was observed by the European ionosondes (see Figure 6). There is a sudden depletion starting at 18:00 UTC on 10 May and lasting until 12 May. G-condition was observed for the entirety of 11 May. Around 09:00 UTC on 12 May, there is a sudden increase again in the F\({}_{2}\) layer density. At MHJ45, this corresponds to the normal morning-time density increase (as is evident by comparing with the same time on 10 May, which is still before the onset of the storm).