Direct Observations of Solar Wind Proton Energization via Nonlinear Cyclotron Resonance

The heating of corona and solar wind remains a fundamental but unresolved problem in space and astrophysical plasma physics. Ion cyclotron waves (ICWs) have long been proposed as a potential mechanism, energizing solar wind ions through cyclotron resonance. The wave-particle energy transfer is typically evaluated using quasilinear diffusion theory, which assumes gyrotropic ion distributions and may underestimate the actual efficiency. Therefore, high-resolution measurements of three-dimensional ion velocity distribution functions are essential to capture agyrotropic signatures arising from kinetic or nonlinear effects. Here, we report Solar Orbiter observations showing that falling-tone ICWs can efficiently energize agyrotropic protons via nonlinear cyclotron resonance. These phase-bunched ions generate resonant currents that mediate substantial energy transfer, with efficiencies up to two orders of magnitude higher than previous quasilinear estimates. These findings highlight the critical role of nonlinear wave–particle interactions in solar wind heating and acceleration, which may operate more broadly across diverse plasma environments.