## 1 Introduction: Strongest Storms of Solar Cycle 25  
The geomagnetic storm of 10-11 May 2024, also called the Mother's Day event, garners a lot of scientific interest because this event is the strongest storm of the last 20 years [1], and it caused auroras at unusually low latitudes [2, 3, 4]. Many studies have been published or are in progress about this event. Published works concerning the ionospheric effects of this superstorm in May 2024 focus, in general, on measurements at specific local places like Peru [5], Asia [6], Asian-Australian and American sectors [7, 8], Latin America [9], or Europe [10]. Near the equator, strong eastward ionospheric electric fields have been observed at dusk during the northward Interplanetary Magnetic Field [11]. Plasma fountainsduring the Mother's Day storm were unusually strong across different local time sectors. These fountains were shown to be sustained by the combined effects of a strong penetration electric field and meridional wind [12]. At high latitudes, plasma lifting during the storm caused mid-latitude displacements of ionospheric peak height by as much as 300 km over the course of 1 h [13]. TEC maps show the intensification and spread of the 11 May 2024 extreme auroral event across the continental US over a 20 min interval [14]. A typical sporadic E layer type was detected for the first time during nighttime in the South American Magnetic Anomaly [15]. At a lower altitude in the mesosphere and thermosphere, a thermospheric NO radiative cooling flux was observed [16], while strong temperature increases were also detected [17], as well as modifications of atmospheric composition [18]. The storm was produced due to a highly compressed magnetosphere, with the magnetopause pushed below geostationary orbit (6.6 R\({}_{\rm E}\)) continuously for 6 h [19]. The highly compressed magnetosphere led an intense ring current at a much closer distance to the Earth. These previous studies open some interesting questions:  
* Is it possible to know exactly how deep the ionospheric depletion will be, and how long it will last? * How does the atmospheric response to geomagnetic storms change depending on the latitude and longitude? * How is the atmospheric ionization modified as a function of the altitude, including above the ionospheric layers in the plasmasphere? In the present study, we use an innovative approach that significantly advances the understanding of geomagnetic superstorms beyond prior works by providing the following:  
1. Including the plasmasphere that is directly coupled to the ionosphere and provides magnetospheric effects in three dimensions;
2. Providing a comparison between measurements of different instruments (VTEC, ionosondes, and plasmopause), helping to differentiate the effects at different altitudes;
3. Analyzing these measurements at different places all around the world (Europe, North Africa, America, and Asia) to differentiate the effects at different latitudes and longitudes;
4. Comparing the Mother's Day event with the second strongest superstorm of this solar cycle, the event of October 2024, for which the present study is a pioneer to our knowledge. This allows us to determine how the intensity of the geomagnetic storms modifies the atmospheric ionization. The two strongest geomagnetic events of solar cycle 25 in 2024 studied here are as follows:  
1. The Mother's Day storm on 10-11 May 2024. 2. The successive storms on 8-11 October 2024. For these events, we show the observations of solar wind parameters that generate the geomagnetic activity, ionosonde measurements (up to the maximum of electron density in the F layer of the ionosphere), vertical total electron content (VTEC) up to the GNSS (Global Navigation Satellite System) orbit altitude, plasmapause measurements by Swarm satellites, and the three-dimensional BSPM plasmasphere model [20] in order to determine the evolution in time and space of the plasma density variations.