Energetic proton bursts downstream of an interplanetary shock

Context. The Energetic Particle Detector (EPD) suite on board Solar Orbiter provides unprecedented high-resolution measurements of suprathermal and energetic particles in interplanetary space. These data can resolve particle dynamics near interplanetary shocks, offering new insights into particle acceleration and transport processes. Aims. We present observations of energetic proton bursts downstream of an interplanetary shock and discuss possible acceleration and formation processes. Methods. We combined data from two sensors of EPD, the SupraThermal Electron Proton (STEP) sensor and the Electron-Proton Telescope (EPT), to investigate the proton bursts across the full energy range. We examined the dynamic energy spectra, temporal flux profiles, pitch-angle distributions, and spectral features of these proton bursts. Results. We find that these proton bursts travel anti-parallel to the interplanetary magnetic field (IMF) in a region where the IMF is pointing southward, substantially out of the ecliptic plane. These bursts typically last for ∼10−20 s and span a wide energy range from ∼20 to ∼1000 keV. Their energy spectra typically show an evident bump in the ∼20−100 keV range, characterized by a valley at ∼20−30 keV, a peak at ∼40−50 keV, a full width at half maximum of ∼30 keV, and a positive spectral slope of ∼1 between the valley and peak. These proton bursts exhibit no velocity dispersion feature and their occurrences do not coincide with significant changes in the IMF direction or with enhancements in the 0.1−4 Hz magnetic field fluctuations. Conclusions. These results suggest that the proton bursts could originate from a source below the ecliptic plane, probably the part of the shock situated there. These protons could be accelerated through shock-drift acceleration or shock-surfing acceleration, with spatially varying efficiencies in the source region. The observed spectral bumps likely arise from the relatively low intensities of the low-energy ∼10−50 keV protons.