Bidirectional anisotropic solar energetic particle events observed by Solar Orbiter

¹ Institute of Experimental and Applied Physics, Kiel University, Leibnizstrasse 11, D-24118 Kiel, Germany
² Center for Space Plasma and Aeronomic Research (CSPAR), The University of Alabama in Huntsville, Huntsville, AL 35805, USA
³ Universidad de Alcalá, Alcalá de Henares 28805, Spain
⁴ Southwest Research Institute, San Antonio, TX 78228, USA
⁵ Johns Hopkins Applied Physics Lab, Laurel, MD 20723, USA
⁶ State Key Laboratory of Lunar and Planetary Sciences and CNSA Macau Center for Space Exploration and Science, Macau University of Science and Technology, Macau, China
⁷ Czech Academy of Sciences, Institute of Atmospheric Physics, Department of Space Physics, Boní II 1401, Prague 4, Czech Republic
⁸ LIRA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université Paris-Cité, Meudon, France
⁹ Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK

^⋆ Corresponding author: ding@physik.uni-kiel.de

Received: 25 June 2025
Accepted: 18 July 2025

Abstract

Context. Solar energetic particle (SEP) events are critical for understanding particle acceleration and transport in the heliosphere. While most SEP events involve outward streaming particles along open magnetic field lines, bidirectional events characterized by simultaneous sunward and anti-sunward particle flows offer unique insights into magnetic field topology and the interplay of multiple acceleration sources.

Aims. We investigate the origin and transport of energetic particles in two rare bidirectional anisotropic SEP events observed by Solar Orbiter, with a particular emphasis on their association with magnetic flux ropes.

Methods. Energetic particles, solar wind plasma, magnetic field, and solar radio measurements were analysed. Via the velocity dispersion analysis, we determined release times and path lengths for distinct particle populations. Automated flux rope identification and magnetic helicity diagnostics were used to characterize magnetic flux ropes.

Results. Both events showed two clear velocity dispersion signatures with opposite particle anisotropies during their onset phase. The sunward streaming protons, characterized by a delayed release time, a harder spectral index, and higher intensities, may be due to coronal mass ejection-driven shock acceleration, while the promptly released anti-sunward streaming protons are likely linked to flare acceleration. Notably, in both cases, small-scale flux ropes were identified in situ during the time intervals corresponding to the bidirectional particle streaming. Path lengths derived for sunward and anti-sunward injections were substantially greater than nominal values of the Parker field lines, further evidence of the role of the flux rope in shaping particle trajectories.

Conclusions. These observations demonstrate that magnetic flux ropes can significantly affect magnetic connectivity to the source region and SEP propagation in the inner heliosphere, and that simultaneous velocity dispersion from two distinct particle sources can be used to place direct constraints on the topology of the flux rope. Our results highlight the value of combining particle anisotropy, release time, source spectra, and magnetic structure diagnostics to unravel SEP transport in complex transient magnetic structures, and also present new challenges for the current SEP transport model.