## 3 Mother's Day Storm: 10-11 May 2024
### Plasmasphere  
While ionosonde measurements are not affected by the density of electrons in the plasmasphere, this is not the case for VTEC. The sudden but short increase in density observed at the onset of the storm, in VTEC but not by the ionosondes, indicates that this additional ionization appears above the F\({}_{2}\) peak. The plasmaspheric density is thus very important to determine. The sharp density decrease measured by both techniques shows that it already takes place at low altitudes in the F layer. Considering also the plasmasphere erosion appearing during storms (e.g., [41]), VTEC observations allow us to determine the plasmaspheric part in comparison to the ionospheric reduction.  
The plasmasphere is the extension at the low and middle latitude of the ionosphere to a higher altitude [42], and its erosion can also explain the reduction in VTEC during and after the storm. Indeed, the extend of the plasmasphere is strongly reduced after storms, with a sharp plasmapause appearing at lower radial distances, as is illustrated in Figure 8 using the BSPM plasmasphere-ionosphere model [20]. The top panel shows the extended plasmasphere (orange region with high electron density) before the storm, with an average plasmapause position above 4 Re at all Magnetic Local Times (MLTs). The plasmasphere is seen in the geomagnetic equatorial plane (left panel) and in the meridian plane (right panel), with the Sun (thus noon 12:00 MLT) on the left.  
The bottom panels show the eroded plasmasphere after the storm, which is very strong due to the Kp reaching the maximum value of 9. The new plasmapause is located around 3 Re on average but is as low as 2.3 Re in the post-midnight sector and as high as 5 Re at the endpoint of the plume in the afternoon MLT sector. The plume co-rotates with the Earth  
Figure 8: Electron density obtained with the BSPM coupled with the ionosphere [25] on 10 May 2024 at 17:00 UTC (before the storm, (**top**) panels) and 11 May 2024 at 5:00 UTC (after the storm, (**bottom**) panels). For the two cases, the plasmasphere (orange region) is illustrated in the equatorial plane (**left**) and in the meridian plane (**right**). The Bartels geomagnetic index Kp from 1 day before to 1 day after the simulated day is shown in the top panels, with a red dashed line to indicate the exact illustrated time.  
after its formation. It takes typically 2 or 3 days after the storm to progressively refill the outer shells of the plasmasphere [20].  
The plasmasphere erosion after the storm can thus at least partially contribute to the observed VTEC. When the plasmapause comes closer to the Earth in the equatorial plane (see Figure 8 left bottom panel), the ionospheric trough also appears at lower latitudes (see right bottom panel) due to the motion of the particles along the magnetic field lines [25].  
The midnight plasmapause proxy derived from the magnetic and plasma observation of the low-Earth orbiting Swarm satellites also confirms a very low plasmapause after 11 May 2024 reaching less than 2 Re, as illustrated in Figure 9 (top panel) with the Kp index (bottom panel). Such plasmapauses lower than 2 Re are very rare. The green line corresponds to the midnight plasmapause position, and the black lines give the position \(\pm\) the standard variation. The squares correspond to the Swarm plasmapause observations. The plasmapause is directly related to the mid-latitude ionospheric trough (MIT) observed by Swarm [31]. The dots show the plasmapause index, based on the L value distance between the small-scale field-aligned current boundary and the MIT minimum.  
The position of the plasmapause is crucial because different waves are generated inside and outside the plasmasphere, and they affect the particles trapped in the radiation belts [43; 44]. Several unusual electron belts were observed after the 11 May 2024 geomagnetic storm, as well as a strong increase in the protons from 9.5 to 13 MeV trapped in the south part of the South Atlantic Anomaly [1].