[P-PS04] Mercury Science and Exploration

[PPS04-02] Summary of the Mercury Flyby Observations by MPPE on BepiColombo/Mio

*Yoshifumi Saito¹, Yuki Harada², Sae Aizawa³, Lina Hadid³, Dominique Delcourt³, Nicolas Andre⁴, Masafumi Hirahara⁵, Stas Barabash⁶, Kazushi Asamura¹, Yoshifumi Futaana⁶, Takeshi Takashima¹, Shoichiro Yokota⁷ (1.Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 2.Kyoto University, 3.Laboratoire de Physique des Plasmas, 4.Institut de Recherche en Astrophysique et Planétologie, 5.Nagoya University, 6.The Swedish Institute of Space Physics, 7.Osaka University)

Keywords:Mercury, Satellite Observation, Plasma, Magnetosphere, Solar Wind, Flyby

BepiColombo Mio that was launched on 20 October 2018 by Ariane 5 from Kourou, French Guiana will arrive at Mercury in November 2026, after nearly 8 years’ journey. Before arriving at Mercury in November 2026, BepiColombo experiences six Mercury flybys. The first, second, third, fourth and sixth Mercury flybys were on 1 October 2021, 23 June 2022, 19 June 2023, 4 September 2024, and 8 January 2025, respectively. In the 1st, 2nd and 3rd flybys, the closest approach was at an altitude of about 200 km (240km for the third flyby) from Mercury’s surface, and BepiColombo approached Mercury’s magnetosphere from the dusk-side tail, crossed the low-altitude regions in the dawn-side southern (northern for the third flyby) hemisphere, and exited the magnetosphere in the dayside southern hemisphere. The 4th and the 6th flybys were different from the previous three flybys. During the 4th flyby, BepiColombo approached Mercury’s magnetosphere from the dusk-side northern part of the magnetotail, crossed the low-altitude regions in the dawn-side equatorial region with the closest approach altitude of about 165km and exited the magnetosphere in the dayside southern hemisphere. During the 6th flyby, BepiColombo approached Mercury’s magnetosphere from the southern part of the magnetotail, crossed the low-altitude regions almost along Meridians passing through noon and midnight with the closest approach altitude of about 295km and exited the magnetosphere in the dayside northern hemisphere. During the Mercury flybys except for the 5th Mercury flyby, many of the onboard instruments on BepiColombo were turned on and successfully made observations of the Mercury magnetosphere. The 5th Mercury flyby was on 1 December 2024. The closest approach distance was much larger than the other five flybys and the instruments that use high voltage were not operated during the 5th Mercury flyby.

The Mercury Plasma/Particle Experiment (MPPE) is a comprehensive instrument package on BepiColombo/Mio spacecraft for plasma, high-energy particle and energetic neutral atom measurements. It consists of 7 sensors: two Mercury Electron Analyzers (MEA1 and MEA2), Mercury Ion Analyzer (MIA), Mass Spectrum Analyzer (MSA), High Energy Particle instrument for electron (HEP-ele), High Energy Particle instrument for ion (HEP-ion), and Energetic Neutrals Analyzer (ENA). Although the MOSIF (MMO Sunshield and Interface Structure) blocked most of the MPPE sensor's field of view until arrival at Mercury, all the MPPE sensors except HEP-ion were turned on to observe Mercury's magnetosphere during the Mercury flybys. During the 1st Mercury flyby, MEA (Mercury Electron Analyzers), MIA (Mercury Ion Analyzer), HEP-ele (High Energy Particle instrument for electron) and ENA (Energetic Neutrals Analyzer) succeeded in observing energy spectra of low energy electrons and ions, high energy electrons and energetic neutrals in the Mercury magnetosphere. In particular, MEA discovered periodic precipitations of keV electrons with velocity dispersion in the midnight to dawn sectors. During the 2nd Mercury flyby, MEA, MIA, MSA (Mass Spectrum Analyzer), HEP-ele and ENA made observation of the Mercury's magnetosphere. In particular, ENA detected natural energetic neutrals from Mercury's magnetosphere for the first time. MEA discovered inverted-V like electron populations and MSA obtained mass identified ion energy spectra during part of the flyby passage in the magnetosphere. During the 3rd Mercury flyby, MSA succeeded in observing mass identified ion energy spectra during the whole period of the flyby. MIA and MSA energy spectra showed an existence of ions with energy higher than 10keV/q in the midnight to dawn sectors near closest approach. During the 4th and 6th Mercury flybys, all the MPPE sensors except for HEP-ion were turned on and successfully made observation of the Mercury’s magnetosphere. The detail of the data obtained during the 4th and 6th Mercury flybys are under investigation now. The observation with full performance of MPPE will start after Mio's arrival at Mercury in November 2026.