Detecting in situ directional discontinuities in the Solar Wind at Mercury’s orbit

¹ INAF-Institute for Astrophysics and Space Planetology, Via del Fosso del Cavaliere, 00133 Roma, Italy
² Department of Physics, University of Rome Tor-Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy
³ Istituto Nazionale di Geofisica e Vulcanologia, Rome, Italy
⁴ Space Research Institute, Austrian Academy of Sciences, Graz, Austria
⁵ Institut für Geophysik und Extraterrestrische Physik, Technische Universität, Braunschweig, Germany
⁶ Max Planck Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Italian Space Agency, Rome, Italy

^⋆ Corresponding author: pier.dibartolomeo@inaf.it

Received: 17 February 2025
Accepted: 13 June 2025

Abstract

Context. Since at closer distances to the Sun the solar wind is highly structured, the analysis of directional discontinuities (DDs) provides further insights into the physical properties of its embedded structures.

Aims. By combining magnetic field and PICAM ion sensor observations, we investigated the occurrence rate and nature of DDs observed by BepiColombo close to Mercury’s orbit.

Methods. We used the magnetic field attitude gradient method to detect discontinuities combined with minimum variance analysis to classify the boundaries of the structures.

Results. During the selected period between 5 and 16 October 2021, 960 DDs were identified. The majority (83%) were rotational discontinuities (RDs) or either discontinuities (EDs). Low compressibility (C_B < 0.03) conditions yielded a higher fraction of RDs and EDs (94%). PICAM observations revealed ion energy enhancements and particle deflections correlated with magnetic field reversals, particularly during structures characterised by low compressibility.

Conclusions. The results suggest that between 5 and 16 October 2021, BepiColombo observed a series of DDs compatible with the boundaries of magnetic switchbacks, characterised by low compressibility, which is expected for Alfvénic structures. The increase in energy and ion differential energy flux, despite the unfavourable aberration angle, and ion flux spikes strongly correlated with magnetic field reversals, could be explained by field-aligned particles that do not reverse their pitch angle.