Proton acceleration during the interaction of a coronal-mass-ejection-driven shock and a current sheet

¹ Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville Huntsville AL 35805, USA
² Institut für Experimentelle und Angewandte Physik, Christian-Albrechts-Universität zu Kiel 24118 Kiel, Germany
³ Department of Space Science, The University of Alabama in Huntsville Huntsville AL 35805, USA
⁴ Institut für Physik und Astronomie, Universität Potsdam Potsdam, Germany

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 13 August 2025
Accepted: 19 December 2025

Abstract

Conext. Shock geometry plays a critical role in the acceleration of energetic particles. To isolate its effect, it is essential to study particle behavior under different shock geometries while maintaining comparable shock properties such as strength, speed, and turbulence conditions within a continuous period of time.

Aims. We aim to study the physical process of the interaction between an interplanetary shock and a current sheet, and compare the dynamic behavior of energetic protons under different shock geometries separated by the current sheet using the Electron-Proton Telescope on board the Solar Orbiter (SolO) on 14 March 2023.

Methods. We applied a partial-variance-increment (PVI) method to detect the magnetic structure in the upstream region of the shock. We reconstructed the proton pitch-angle distribution (PAD) in the solar wind frame to analyze the particle dynamics. We calculated the first-order flux anisotropy in the solar-wind frame of reference.

Results. We find that the differential flux of 60 keV–2 MeV protons is enhanced by 2.5 times from the current sheet to the shock. During the quasi-parallel shock interval, the first-order flux anisotropy in the solar-wind frame is consistent with diffusive shock acceleration. Nevertheless, the flux anisotropy exhibits a bipolar shape during the quasi-perpendicular shock interval. The coexistence of positive and negative flux anisotropy suggests that the magnetic field is wandering around the shock and connected to the shock surface with different acceleration efficiencies. That the flux anisotropy peaks at a transition energy may indicate that it is influenced by the current sheet. The transition energy is greater during the quasi-perpendicular interval, implying more efficient proton acceleration.

Conclusions. We suggest that the current sheet is an important ingredient that affects the local shock geometry and, thus, the particle acceleration efficiency and flux anisotropy. This effect can accumulate as the shock propagates outward from the Sun and should be taken into account when interpreting particle spectral profiles at greater distances.