## 3 Mother's Day Storm: 10-11 May 2024
### Ionospheric Vertical Total Electron Content  
Figure 4 shows the time series of the VTEC (in red), the expected VTEC, i.e., the 15 previous days median (in grey, with standard variation), and the 5 min VTEC variability (in blue) in the north of Europe, middle of Europe (Brussels) and more southward (north of Africa). The three near-real-time VTEC time series showed abnormal variations during the night of 10 to 11 May 2024, followed by a long depletion in VTEC until 12 May 2024. This is due to the strong CME impact detected during the afternoon of 10 May. By comparing with Figure 3, these VTEC fluctuations (high variations during the storm and long depletion after) cannot be explained by variations in F10.7 during these days.  
Figure 3: Geomagnetic activity indices of Bartels Kp (**top panel**), Ap in nT (**2 nd panel**), Disturbed Storm Time Dst in nT (**3 rd panel**), daily solar activity indices F10.7, solar _radio flux_ at 10.7 cm (**4 th panel**), and intensity of the Lyman alpha line in the solar spectrum (**5 th panel**) from 1 to 24 May 2024. The (**bottom panel**) illustrates Dst from 1 January 2000 to 31 January 2025. The two analyzed events are indicated by the red vertical lines.  
Consequently, the geomagnetic storm had a strong impact on the ionospheric electron content, with sudden increases and decreases of 20 TECu from 18:00 UTC 10 May to 12:00 UTC 11 May in the north of Europe. At mid-latitude, the VTEC tends to decrease starting at 18:50 UTC, with a minimum of \(-\)10 TECu, with respect to the expected quiet ionosphere, and with a sudden peak of an increase of 8 TECu (with respect to the quiet ionosphere) at 22:20 UTC. At the same time, at a low latitude, the VTEC starts to decrease with respect to the quiet ionosphere. For both mid- and low latitudes, the VTEC variability also remains high, until midnight on 11 th May. These VTEC variations and rapid variability at these three latitudes are due to the injection of the particles associated with the storm and leading to the occurrence of auroras at unusually low latitudes.  
The maximum variability in the VTEC is observed at 20:20 UTC (see Figure 5).  
In Figure 5, it can be observed that an increase in TEC with respect to the quiet ionosphere occurs at high and low latitudes, while this is negative at mid-latitude. Over Scandinavia, the VTEC variability reached a maximum of 5 TECu for a 5 min interval at 20:20 UTC. This could be linked with the height values in the rate of TEC change index (ROTI) observed in [2], supporting the fluctuations, implying disturbances in GNSS applications.  
The other effect of the storm is the depletion of the VTEC down to 30 TECu at low latitudes until 12 May around 05:00 UTC. As mentioned in [36; 37], the persisting depletion in TEC a few days after the onset is due to the contraction of the plasmasphere, implying disturbances in the thermospheric circulation and a chemical loss of ionization in the ionosphere. This contraction of the plasmasphere is coherent with the observation in Figure 9 Section 3.4 of the present paper.  
Figure 4: VTEC time series at 3 locations in Europe from 9 to 14 May 2024. The **(left panel)** shows the time evolution of the vertical total electron content (VTEC) (in red) at three locations illustrated in the **(right panel)** on a map of Europe: (a) in the northern part of Europe (61\({}^{\circ}\) N, 5\({}^{\circ}\) E), (b) above Brussels (50.5\({}^{\circ}\) N, 4.5\({}^{\circ}\) E), and (c) in North Africa (36\({}^{\circ}\) N, 5\({}^{\circ}\) E).