Observations of Two-Layers Spectrum Formed by Simultaneous H+ and He+ anomalous resonance with EMIC Waves

In collisionless plasma environments,wave-particle interactions facilitate efficient energy exchange between particles and phase-coherent waves.Using Magnetospheric Multiscale mission observations,we report a novel two-layers bridge-like arcs structure in ion energy spectrum,formed by simultaneous nonlinear interactions between large-amplitude electromagnetic ion cyclotron waves and H+ and He+. Even after incorporating finite wave-field effects into the resonance condition,only a few instances exist where the minimum correction energy is lower than the observed value,thus confirming the presence of an anomalous heating mechanism. The wave-particle interaction provides direct evidence of continuous energy exchange between EMIC waves and ions. The lower-layer bridge-like arcs are formed by the nonlinear interaction between H+ and EMIC waves,while the upper-layer bridge-like arcs result from the nonlinear interaction between He+ and EMIC waves. These observations provide the first direct evidence of continuous energy exchange between ions and EMIC waves,providing a new perspective for studying the simultaneous nonlinear interaction between ions and EMIC waves.