Resonant interactions between suprathermal protons and ion-scale waves near an interplanetary shock

Context. The interaction between waves and particles is crucial for particle acceleration near interplanetary shocks. Previously, research on these processes was constrained by limited data and the coarse time resolution of interplanetary missions. However, recent high-resolution observations from the Solar Orbiter mission are providing new insights into this area. Aims. We analyzed data measured by the Energetic Particle Detector, the Magnetometer, and the Solar Wind Analyzer on board Solar Orbiter, to investigate wave-proton interactions upstream an interplanetary shock observed on April 8, 2022. Methods. We performed a mean-field-transformed wavelet analysis on the magnetic field data to derive the wave properties. We reconstructed pitch angle distributions and gyrophase distributions in the solar wind frame of reference to analyze the proton behavior. Results. We find that the observed waves are quasi-parallel propagating, ion-scale transverse waves that exhibit alternating left-handed and right-handed polarization. Fluxes of suprathermal protons oscillate quasi-periodically with these waves and show signs of wave modulation. In addition, signatures hinting at resonance, such as phase shifts across energy, are revealed in proton fluxes. The proton phase space density near the calculated resonant energy increases during the interaction, which indicates the acceleration or scattering of protons. Conclusions. We present direct observations of particles resonating with waves close to an interplanetary shock, which captures these dynamics within single wave periods. Our results highlight the role of wave-particle interactions in dynamic processes occurring in the inner heliosphere.