Galactic and solar energetic particle observations during the increasing part of solar cycle 25 with EPD/HET and Metis on board Solar Orbiter

¹ DiSPeA, University of Urbino Carlo Bo Urbino (PU), Italy
² INFN Florence, Italy
³ LIGM, Univ Gustave Eiffel, CNRS F-77454 Marne-la-Vallée, France
⁴ University of Florence, Department of Physics and Astronomy, Via Sansone 1 50019 Sesto Fiorentino (FI), Italy
⁵ INAF – Astrophysical Observatory of Torino, Italy
⁶ Institut für Experimentelle und Angewandte Physik, Christan-Albrechts-Universität zu Kiel Kiel, Germany
⁷ Universidad de Alcalá, Space Research Group 28805 Alcalá de Henares, Spain
⁸ INAF – Astronomical Observatory of Capodimonte Naples, Italy
⁹ CNR – IFN Via Trasea 7 35131 Padova, Italy
¹⁰ CISAS, Centro di Ateneo di Studi e Attività Spaziali “Giuseppe Colombo” Via Venezia 15 35131 Padova, Italy
¹¹ INAF – Astrophysical Observatory of Catania Catania, Italy
¹² Institute of Physics, University of Graz Universitätsplatz 5 8010 Graz, Austria
¹³ Solar-Terrestrial Centre of Excellence – SIDC, Royal Observatory of Belgium Ringlaan -3- Av. Circulaire 1180 Brussels, Belgium
¹⁴ INAF – Astronomical Observatory of Trieste Loc. Basovizza n. 302 34149 Trieste, Italy
¹⁵ Dept. of Physics, University of Trieste Via A. Valerio 2 34127 Trieste, Italy
¹⁶ National Institute for Nuclear Physics, Division in Trieste Via A. Valerio 2 34127 Trieste, Italy
¹⁷ Dip. di Fisica e Astronomia “Galileo Galilei”, Università di Padova, Via G. Marzolo 8 35131 Padova, Italy
¹⁸ Max Planck Institute for Solar System Research Justus-von-Liebig-Weg 3 37077 Göttingen, Germany
¹⁹ ASI – Italian Space Agency Via del Politecnico snc 00133 Rome, Italy
²⁰ INAF – Arcetri Astrophysical Observatory Largo Enrico Fermi 5 I-50125 Florence, Italy
²¹ MPS Göttingen, Germany
²² INAF – Institute for Space Astrophysics and Cosmic Physics Milan, Italy

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Received: 28 July 2025
Accepted: 3 November 2025

Abstract

Context. Galactic cosmic rays (GCRs) and solar energetic particles (SEPs) with energies greater than tens of megaelectron volts are at the origin of spacecraft deep charging. The High Energy Telescope of the Energetic Particle Detector instrument measures the particle flux incident on the Solar Orbiter spacecraft. An algorithm implemented in the processing electronics of the visible light (VL) instrument of the Metis coronagraph generates cosmic-ray matrices containing the pixels fired by high-energy particles. These independent observations allow us to monitor the secondary particle production into the spacecraft.

Aims. We studied the GCR flux long-term variations during the ascending phase of solar cycle 25 and the evolution of two SEP events observed on July 24–26, 2023, and on February 9–14, 2024, above 80 MeV with the aim of evaluating the impact of galactic and solar high-energy particles on Metis.

Methods. A Python tool named REBECCA has been developed for the automated analysis of the Metis cosmic-ray matrices. The number of observed particle tracks is compared to Monte Carlo simulations of the Metis VL bidimensional CMOS sensor used as a particle detector.

Results. We present the modulation of the GCR energy spectrum from 2020 through 2024 above 100 MeV. The dynamics of two intense SEP events is also reported. Monte Carlo simulations indicate that the composition of particles in the cosmic-ray matrices is dominated by protons. Going from solar minimum to maximum, an increase in particles produced by cosmic rays in the spacecraft material surrounding Metis was observed. Conversely, during the whole evolution of SEP events, protons made up more than 90% of the particles. These observations were gathered near the ecliptic during a positive polarity epoch of the global solar magnetic field. Analogous studies will be conducted during the negative polarity epoch, within 1 au, both above and below the ecliptic plane, throughout the remaining duration of the mission.