Bidirectional Energy Transfer via Simultaneous Wave‐Particle Resonances in Inhomogeneous Space Plasmas

Wave‐particle resonance is a key mechanism for energy transfer in collisionless plasmas with important implications for wave excitation and particle acceleration. Cyclotron resonance occurs when a particle's gyromotion synchronizes with the wave fields, with their interactions usually describable by quasilinear diffusion or nonlinear trapping. The nonlinear wave‐particle energy transfer is facilitated by background inhomogeneities such as magnetic field gradients and/or frequency sweeping. When wave amplitudes become large, the particle's gyromotion can be strongly perturbed, altering the classical cyclotron resonance condition and giving rise to a nonlinear process known as anomalous resonance or anomalous trapping. This mechanism has been explored in idealized, uniform settings; however, its behavior in realistic inhomogeneous environments remains poorly understood. Here, we present in situ evidence of simultaneous cyclotron and anomalous resonances between electromagnetic ion cyclotron (EMIC) waves and charged particles in Earth's dayside magnetosphere. The local magnetic‐field gradient enables bidirectional wave‐particle energy transfer: high‐energy ions drive the waves through cyclotron resonance, while lower‐energy ions absorb energy from the same waves via anomalous resonance. We also identify signatures consistent with anomalous resonance of sub‐keV electrons, suggesting that efficient wave‐particle coupling may extend far below the classical cyclotron‐resonant energy. This interplay of resonances reveals an inhomogeneity‐driven pathway for energy redistribution across a broader energy range than previously recognized. These findings highlight the need to incorporate anomalous resonance into models of magnetospheric dynamics, especially in regions with strong inhomogeneities, and open new avenues for understanding energy transfer in complex plasma environments.