EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Acceleration of Suprathermal protons near an Interplanetary Shock

Liu Yang1, Verena Heidrich-Meisner1, Lars Berger1, Robert Wimmer-Schweingruber1, Linghua Wang2, Jiansen He2, Xingyu Zhu2, Die Duan2, Alexander Kollhoff1, Daniel Pacheco1, Patrick Kühl1, Zigong Xu1, Duncan Keilbach1, Javier Rodríguez-Pacheco3, and George Ho4
Liu Yang et al.
  • 1Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel, 24118 Kiel, Germany (
  • 2Peking University, Institute of Space Physics and Applied Technologies, Beijing, China
  • 3Universidad de Alcalá, Space Research Group, 28805 Alcalá de Henares, Spain
  • 4Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA

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 onboard the Solar Orbiter spacecraft, in order to constrain the potential shock acceleration mechanisms.
Methods. We first reconstruct the pitch-angle distributions (PADs) of suprathermal protons in the solar wind frame. Then, we study the evolution of the PADs, flux temporal profile and velocity distribution function of this proton population close to the shock and compare the observations to theoretical predictions.
Results. We find that the suprathermal proton fluxes peak 
12 to 24 seconds 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 further downstream. Furthermore, the proton velocity distribution functions in the upstream (downstream) region fit to 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 above. These indices are all smaller than predicted by first-order Fermi acceleration. In addition, the proton PADs show anisotropies in the direction away from the shock in the close upstream region and become nearly isotropic further upstream, while downstream of the shock, they show a clear tendency of anisotropies towards 90 PA.
Conclusions. These results suggest that the acceleration of suprathermal protons at interplanetary shocks are dynamic on a time scale of
10 seconds, i.e., few proton gyro-periods. Furthermore, shock drift acceleration likely plays an important role in accelerating these suprathermal protons.

How to cite: Yang, L., Heidrich-Meisner, V., Berger, L., Wimmer-Schweingruber, R., Wang, L., He, J., Zhu, X., Duan, D., Kollhoff, A., Pacheco, D., Kühl, P., Xu, Z., Keilbach, D., Rodríguez-Pacheco, J., and Ho, G.: Acceleration of Suprathermal protons near an Interplanetary Shock, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2236,, 2023.