Ionospheric variations observed at Syowa Station, Antarctica, during large space weather events in May and October 2024

The largest geomagnetic storm to date of Solar Cycle 25 occurred in May 2024, followed by another significant event in October 2024. Analysis of ionosonde and GNSS scintillation data at Syowa Station, Antarctica, revealed three common features during both events: disappearance of ionosonde echoes, occurrence of ionospheric negative storms, and enhancement of phase scintillation. Echo blackouts were primarily caused by enhanced ionospheric absorption due to energetic particle precipitation. In October, the absorption was dominated by solar protons, whereas in May, it was caused by both solar protons and auroral electrons (10 s–100 s keV) associated with shifts in the auroral location as confirmed by comparison with particle measurements from the GOES and POES satellites. Ionospheric negative storms were identified through reductions in foF2 and TEC, with high-altitude echoes occasionally shifting during the recovery phase. Phase scintillation enhancements were closely associated with irregular E-region structures, such as auroral-type and slanted sporadic E layers. A new finding of this study is the persistent phase scintillation observed during ionospheric negative storm phases at Syowa Station in the auroral zone, in contrast to the suppression reported in the polar cap region.