Characterization of the solar wind context during the third Mercury flyby of BepiColombo

¹ Institut de Recherche en Astrophysique et Planétologie (IRAP), CNRS-UPS-CNES, Toulouse, France
² Laboratoire de Physique des Plasmas (LPP), CNRS-Observatoire de Paris Sorbonne Université – Université Paris Saclay-Ecole polytechnique – Institut Polytechnique de Paris, 91120 Palaiseau, France
³ Institut für Weltraumforschung (IWF), Graz, Austria
⁴ European Space Astronomy Center (ESAC), Madrid, Spain
⁵ Universidad de Alcalá, Space Research Group (SRG-UAH), Plaza de San Diego s/n, 28801 Alcalá de Henares, Madrid, Spain
⁶ Swedish Institute of Space Physics, Uppsala, Sweden
⁷ Institut für Geophysik und extraterrestrische Physik, Technische Universität Braunschweig, Braunschweig, Germany
⁸ Istituto Nazionale di Astrofisica, Roma, Italy
⁹ Institut Supérieur de l’Aéronautique et de l’Espace (ISAE-SUPAERO), Université de Toulouse, Toulouse, France
¹⁰ Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Sagamihara, Japan
¹¹ Department of Climate and Space Science, University of Michigan, MI, USA
¹² Physics Department, University of California, Berkeley, Berkeley, CA, USA
¹³ Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA

^★ Corresponding author: mathias.rojo@irap.omp.eu

Received: 23 January 2025
Accepted: 11 April 2025

Abstract

Context. The interaction of the solar wind (SW) with the coupled magnetosphere-exosphere-surface of Mercury is complex. Charged particles released by the SW can precipitate along planetary magnetic field lines on specific areas of the surface of the planet. The processes responsible for the particle precipitation strongly depend on the orientation of the interplanetary magnetic field (IMF) upstream of Mercury.

Aims. During the third Mercury flyby (MFB3) by BepiColombo, the properties of the SW inferred from BepiColombo observations of a highly compressed magnetosphere corresponded to those of a very dense plasma embedded in a slow SW. The Mercury Electron Analyzer (MEA) measured continuous high-energy electron fluxes in the nightside dawn sector of the compressed magnetosphere. In order to constrain further studies related to the origin of these populations, we aim to firmly confirm the initial inferences and detail the SW properties throughout MFB3.

Methods. We took advantage of a close radial alignment between Parker Solar Probe (PSP) and Mercury. We monitored the activity of the Sun using SOHO coronagraphs and we used a potential field source surface model to estimate the location of the magnetic footpoints of PSP and BepiColombo on the photosphere of the Sun. We propagated the plasma parameters and the IMF measured by PSP at BepiColombo, to check if the plasma impacted Mercury.

Results. We show that during MFB3, PSP and BepiColombo connected magnetically to the same region at the solar surface. The slow SW perturbation first measured at PSP propagated to Mercury and BepiColombo, as was confirmed by similarly elevated plasma densities measured at PSP and BepiColombo. The IMF orientation stayed southward during the whole MFB3.

Conclusions. Our results provide strong constraints for future studies of the magnetospheric structure and dynamics during MFB3, including tail reconnection, electron and ion energization, and subsequent plasma precipitation onto the surface of Mercury.