Electron density revealing the boundaries of Mercury’s magnetosphere via serendipitous measurements by SORBET during BepiColombo first and second Mercury swing-bys

Abstract

Aims. We aim to establish the boundaries of Mercury’s magnetosphere through a comparison of the drops and rises of the electron density revealed by PWI/SORBET, during BepiColombo’s first and second swing-bys of Mercury carried out on 1 October 2021 and 23 June 2022, with global 3D magnetohydrodynamic simulations. Methods. SORBET was switched on during both swing-bys and its radio spectra were re-analysed using a new method based on the theory of the quasi-thermal-noise spectroscopy and adapted to measurements registered with a non-deployed antenna (as planned for the entire cruise phase). In parallel, magnetohydrodynamical (MHD) global simulations of Mercury’s magnetosphere were run under different solar wind conditions. Profiles of the electron density obtained from SORBET data were compared with three MHD simulations, using different values for solar wind sonic Mach numbers and plasma β. Results. Three drops and rises of electron density are clearly identified with the boundaries of the magnetosphere (bow shock, magnetopause, and boundary of a region dominated by closed magnetic field lines) on the inbound part of the first Mercury swing-by. Conclusions. On the inbound part of the first swing-by, a good match is found between the SORBET data and the MHD simulations, revealing the quick reorganisation of the Mercury’s magnetosphere in a variable solar wind. This study also highlights the essential role of the electron density in the future detection of Mercury’s magnetosphere boundaries once BepiColombo will orbit the planet from December 2025.