Effects of the Geomagnetic Superstorms of 10–11 May 2024 and 7–11 October 2024 on the Ionosphere and Plasmasphere

On 10 May 2024 at 17h:07 UTC, the arrival of solar a coronal mass ejection (CME) generated the strongest geomagnetic storm of the last twenty years with a minimum Dst = -412 nT, usually referred to as the Mother’s Day event. On 10 October 2024, the second strongest event of the solar cycle 25 appeared with Dst=-335 nT, preceded on 8 October by an event with Dst=-153nT. In the present work, we show with measurements of vertical total electron contents and with ionosonde observations from Europe, USA and South Korea that the ionization of the upper atmosphere shortly increased at the arrival of the CME for these different events, followed by a fast decrease at all latitudes. The ionization remained very low for more than a full day. While the recovery appears at the beginning of the second day after the onset for both events in October, the sudden recovery in the middle of the second day on 12 May is much more unusual. The analysis of the observations at different latitudes and longitudes shows that the causes of the ionization variations during the superstorms were mainly due strong perturbations in the ionospheric F layer, and amplified by the plasmasphere’s influence on the vertical total electron content (VTEC). The erosion of the plasmasphere during these two strong events led to a plasmapause located to exceptionally low radial distances smaller than 2 Re (Earth’s radii) in the post-midnight sector, and a rotating plume in the afternoon-dusk sector clearly visible in the BSPM plasmasphere model. Several days after the storms were necessary to recover normal ionization rates.