Journal Article

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

Hori, T., Kim, K.‐H., Lee, J.‐H., Yamamoto, K., Jun, C.‐W., Shinbori, A., Kitamura, N., Lyons, L., Shinohara, I., Miyoshi, Y., Teramoto, M., Kasaba, Y., Bortnik, J., Kasahara, Y., Matsuoka, A., Matsuda, S.

2025

Overview

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. Plain Language Summary Intense geomagnetic storms can transport energetic particles into the deep inner magnetosphere, generating various magnetospheric plasma waves in regions close to the Earth. Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. In particular, EMIC waves can cause the loss of relativistic electrons and ring current ions. In this study, we present observations of high‐frequency (HF) EMIC waves during the May 2024 storm using data from the Arase satellite. Remarkably, these HF EMIC waves were detected in the deep inner magnetosphere during the main and early‐recovery phases of the storm and exhibited unusually high frequencies. The minimum resonance energy of HF EMIC waves suggests their capability to cause the loss of energetic protons and relativistic electrons in the low L‐shell region. The present results provide new insights into the generation processes and wave‐particle interactions of HF EMIC waves in the deep inner magnetosphere. Key Points High‐frequency (HF) EMIC waves were observed at regions close to Earth during a severe geomagnetic storm Most of HF EMIC waves occurred during the main and early‐recovery phase of the storm with the highest wave frequency HF EMIC waves at low L‐shells have the potential to cause loss of relativistic electrons in the slot regions of the radiation belt

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Arase in situ observations / Charged particles / Cyclotrons / DST Index / Earth magnetosphere / Electrons / EMIC waves in the deep inner magnetosphere / Energetic particles / Geomagnetic storms / Geomagnetism / high‐frequency electromagnetic ion cyclotron waves / Jupiter / Magnetic storms / Magnetospheric plasma / Magnetospheric plasma waves / Particle interactions / Particle physics / Plasma waves / Protons / Recovery / Relativistic effects / Resonance / Ring current ions / Ring currents / Satellite observation / Satellites / Storms / the May 2024 storm