Arase In Situ Observations of High-Frequency Electromagnetic Ion Cyclotron (EMIC) Waves in Regions Close to the Earth During the May 2024 Storm

Abstract

During the May 2024 storm, the minimum Dst index was approximately −412 nT, marking the largest geomagnetic storm of the past decade. This event caused the inner edge of the ring current to penetrate deeply into the inner magnetosphere during the main phase of the storm. We present observations of high-frequency electromagnetic ion cyclotron (HF EMIC) wave activity during this intense geomagnetic storm using data from the Arase satellite. Arase observations showed that HF EMIC waves with frequencies of 5–36 Hz at L ∼ 2, occurred mainly during the main and early-recovery phases. The minimum resonance energy of energetic protons and relativistic electrons associated with HF EMIC waves suggests their potential to cause the loss of relativistic electrons in the low L-shell region. Our observations provide new insights into the generation of EMIC waves and the dynamics of energetic particles at low L-shells in the inner magnetosphere.