## 5 Discussion and Conclusions  
In the present work, we show that a study of the ionospheric observations as a function of the latitude and the Magnetic Local Time using VTEC, ionosondes, and plasmaspheric observations is crucial for understanding the causes of the density variations associated with the magnetic storms. We have analyzed the electron density measured by ionosondes in the F\({}_{1}\) and F\({}_{2}\) layers, the VTEC measured from the ground to GNSS satellites, and the position of the midnight plasmapause measured by Swarm during the two biggest geomagnetic storms of the present solar cycle 25, in May and October 2024. The observations at different latitudes and longitudes allow us to determine the spatial and temporal effects and to study the fraction of electron density due to the ionospheric peak and to the plasmaspheric storm response. The observations during the two analyzed storms show the following:  
1. While the ionization increases during the main phase of the storms, the density of electrons decreases for at least one day after the storms. 2. The VTEC depletion is not only due to a decrease in the ionization in the F\({}_{2}\) layer but also to a closer plasmapause, as shown using Swarm plasmapause observations. This confirms that sharp electron density depletion is associated with plasmasphere  
Figure 16: Midnight plasmapause proxy derived from the magnetic and plasma observation of the low-Earth orbiting Swarm satellites (like in Figure 9) from 9 to 12 October 2024.  
erosion [24]. This was also observed in the studies of previous geomagnetic storms. For instance, it was found that the plasmasphere can lose 40% or more of its total mass during massive erosions [47]. The relative contribution of the plasmasphere to the nighttime (i.e., locally) total electron content (TEC) can easily go beyond 80% during severely disturbed periods [48]. The plasmasphere is often overlooked despite the direct interaction between the ionosphere-plasmasphere system. 3. The F\({}_{2}\) layer refills very suddenly after the Mother's Day event, which is very unusual. Measurements using different instruments (ionosondes, GNSS) at different latitudes and longitudes on different continents indicate that the sudden refilling occurs at the same time in the European and American sectors, despite the local time difference. The refilling occurred earlier at the lower latitudes. 4. The comparison of two superstorms with different intensities allowed us to determine how different mechanisms can take place depending on the events. Indeed, clear differences could be observed in the response of the ionospheric layers to both storms. The storms were similar in strengths and in the local time of the onset. However, the background conditions of the ionosphere in May and October are very different, at least in the lower ionosphere. The F\({}_{1}\) peak is more pronounced in May than in October, but the F\({}_{2}\) peak is more compact in October, with a peak density somewhat higher than in May. These differences in the structure of the ionospheric layers lead to the effects of the storm being visible for a longer time for the May event, with the F\({}_{2}\) layer only becoming visible again during the second day after the storm. 5. G-condition (i.e., when the F\({}_{2}\) layer is not detected because the density is higher in the F\({}_{1}\) layer than in F\({}_{2}\)) is observed for the entirety of 11 May, while it is almost absent in October 2024. G-condition is found to always be more severe and longer lasting at higher latitudes. This is largely due to the different climatological background conditions, because the storms happened in different seasons. This is consistent with the observations made in [38], where storms from March and April 2023 were discussed. Such seasonal differences are smaller at higher altitudes. 6.