A tale of two shocks

¹ Institute of Experimental and Applied Physics, Kiel University Leibnizstraße 11 DE-24118 Kiel, Germany
² European Space Agency (ESA), European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villanueva de la Cañada Madrid, Spain
³ Institut de Recherche en Astrophysique et Planétologie (IRAP) Toulouse, France
⁴ University of Alcalá Alcalá de Henares, Spain
⁵ School of Earth and Space Sciences, Peking University Beijing 100871, People’s Republic of China
⁶ Southwest Research Institute San Antonio TX, USA
⁷ Applied Physics Laboratory, Johns Hopkins University Laurel MD, USA
⁸ Istituto per la Scienza e la Tecnologia dei Plasmi, Consiglio Nazionale delle Ricerche I-70126 Bari, Italy
⁹ Heliophysics Science Division, NASA Goddard Space Flight Center Greenbelt MD 20771, USA

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Received: 4 June 2025
Accepted: 4 November 2025

Abstract

Context. Energetic particles in interplanetary space are normally measured on timescales that are long compared to the ion gyroperiod. Such observations by necessity average out the microphysics associated with the acceleration and transport of 10 s–100 s keV particles.

Aims. We aim to investigate previously unseen nonequilibrium features that only become observable at very high time resolution and discuss possible explanations for these features.

Methods. We used unprecedentedly high time-resolution data that were acquired by the in situ instruments on Solar Orbiter in the vicinity of two interplanetary shocks observed on 29 Nov 2023 07:51:17 UTC and 30 Nov 2023 10:47:26 UTC at ∼0.83 astronomical units from the Sun.

Results. The solar-wind proton-beam population follows the magnetic field instantaneously and on timescales that are significantly shorter than a gyro-period. Energetic particles, despite sampling large volumes of space, vary on remarkably short timescales, typically on the order of the convection time of their gyro-radius. Nonequilibrium features such as bump-on-tail distributions of energetic particles are formed by small-scale magnetic structures in the interplanetary magnetic field.

Conclusions. High time-resolution observations show previously unobserved microphysics in the vicinity of two traveling interplanetary shocks, including ion reflection at a current sheet, which may explain where ions are reflected in shock acceleration.