Acceleration of suprathermal protons near an interplanetary shock

Context. Interplanetary collisionless shocks are known to be sources of energetic charged particles up to hundreds of MeV. However, the underlying acceleration mechanisms are still under debate.
Aims: We determine the properties of suprathermal protons accelerated by the interplanetary shock on 2021 November 3 with the unprecedented high-resolution measurements by the SupraThermal Electron Proton sensor of the Energetic Particle Detector on board the Solar Orbiter spacecraft, in order to constrain the potential shock acceleration mechanisms.
Methods: We first reconstructed the pitch-angle distributions (PADs) of suprathermal protons in the solar wind frame. Then, we studied the evolution of the PADs, the temporal flux profile, and the velocity distribution function of this proton population close to the shock and compared the observations to theoretical predictions.
Results: We find that the suprathermal proton fluxes peak ∼12 to ∼24 s before the shock in the upstream region. The proton fluxes rapidly decrease by ∼50% in a thin layer (∼8000 km) adjacent to the shock in the downstream region and become constant farther downstream. Furthermore, the proton velocity distribution functions in the upstream (downstream) region fit a double power law, f(v)∼v^−γ, at ∼1000 − 3600 km s⁻¹, with a γ of ∼3.4 ± 0.2 (∼4.3 ± 0.7) at velocities (v) below a break at ∼1800 ± 100 km s⁻¹ (∼1600 ± 200 km s⁻¹) and a γ of ∼5.8 ± 0.3 (∼5.8 ± 0.2) at velocities higher than this. These indices are all smaller than predicted by first-order Fermi acceleration. In addition, the proton PADs in the covered pitch-angle range show anisotropies in the direction away from the shock in the region close to the upstream region and become nearly isotropic farther upstream, while downstream of the shock, they show a tendency of anisotropies towards 90° PA.
Conclusions: These results suggest that the acceleration of suprathermal protons at interplanetary shocks are dynamic on a timescale of ∼10 s, that is, few proton gyroperiods. Furthermore, shock-drift acceleration likely plays an important role in accelerating these suprathermal protons.