Study of higher-frequency EMIC wave–particle interactions in the inner magnetosphere during the May 2024 storm using data from the PWING ground network and the Arase and POES satellites

The May 2024 storm exhibited a minimum Dst of -406 nT and was recorded as one of the largest geomagnetic storms in recent decades, causing various geomagnetic phenomena, such as the penetration of enhanced ring current particles into the deep inner magnetosphere, the acceleration and loss of radiation belt electrons, the generation of various plasma waves, etc. Electromagnetic ion cyclotron (EMIC) waves, also known as Pc1 geomagnetic pulsations on the ground, play an important role in the loss processes of energetic ring current protons and relativistic electrons in the radiation belts through pitch-angle scattering. In this study, we present observations of EMIC wave activity during the great geomagnetic storm of May 2024 obtained by the PWING ground-based observation network and the POES and Arase satellites. Using data obtained from the Arase satellite, we found that unusual high-frequency EMIC waves with frequencies above 5 Hz were predominantly observed at L * < 3 during the main and early recovery phases. POES observations also revealed significant 30–80 keV proton precipitation events by EMIC wave–particle interaction at L * ~ 2 during the main phase. PWING ground-based observations exhibit special types of Pc1 pulsations at subauroral and low-latitude stations with frequencies higher than those of typical Pc1 waves. We also found that the variation in Pc1 wave frequency shows a relationship with the plasmapause locations and the inner boundary of 33–78 keV proton fluxes. We investigate the evolution of ring current protons and their contribution to the generation of EMIC waves during the storm. From these observational facts, we suggest that the energetic ring current protons penetrate the deep inner magnetosphere (L * < 3) during the main and early recovery phase of the severe storms and drive EMIC waves in the near-Earth regions. These waves can contribute to the loss of enhanced ring current protons and relativistic electrons in the deep inner magnetosphere. These observations provide new insights into the generation processes and evolution of EMIC waves and the inner magnetospheric dynamics during intense geomagnetic storms.