Wave-particle interactions play a fundamental role in the transfer of energy between electromagnetic waves and plasma ions in collisionless space plasmas. This study investigates how electromagnetic ion cyclotron (EMIC) wave-particle interactions modify ion distribution functions in Earth’s magnetosphere using observations from the Magnetospheric Multiscale (MMS) mission. A recent study by Kitamura et al. [41] reported bidirectional energy transfer between EMIC waves and plasma ions. Building upon this work, we examine the response of hydrogen and helium ion velocity distributions during the EMIC wave activity using HPCA and FGM measurements from MMS. Our results show that energetic hydrogen ions exhibit a localized reduction in phase space density within the velocity range of approximately 800-1600 km/s during enhanced EMIC wave activity, consistent with cyclotron resonant interactions. In contrast, the helium ion distributions show a broader enhancement and increased perpendicular anisotropy without a distinct velocity-space depletion, suggesting energization over a wider region of velocity space during the EMIC wave activity. These results provide observational insight into species-dependent EMIC wave-particle interaction processes and energy exchange mechanisms in magnetospheric plasmas, with implications for radiation belt dynamics and geomagnetic activity.
