Identifying the Ion Cyclotron Wave Excitation via Cyclotron Resonance in the Solar Wind

The wave-particle resonance is fundamental for mediating energy transfer, thereby facilitating particle heating and acceleration in the plasma universe. Cyclotron resonance between ion cyclotron waves and solar wind ions offers a compelling explanation for the long-standing solar wind heating problem.
Additionally, this resonance can drive wave excitation, although direct observational evidence remains limited. The Solar Orbiter spacecraft provides high-resolution three-dimensional ion velocity distributions, enabling detailed investigations of wave-particle interactions. Here, we present two events featuring counterpropagating ion cyclotron waves, in which the ion gyro-phase spectra exhibit phase-bunched signatures, providing solid evidence of cyclotron resonance. The anisotropic core and beam populations resonate with outward- and inward-propagating waves, respectively. The ion distributions denote pronounced agyrotropy, highlighting the pivotal role of nonlinear wave-particle resonances in driving wave excitation and particle energization in the solar wind.