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Accueil > A propos du LPP > Communication > Actualités archivées > 2023 > BepiColombo/Mio detects the first high-frequency electromagnetic waves in Mercury’s plasma environment

BepiColombo/Mio detects the first high-frequency electromagnetic waves in Mercury’s plasma environment

Toutes les versions de cet article : [English] [français]

Context :

Mercury is the closest planet to the Sun among the planets of the solar system, it is strongly influenced by the solar wind, a flow of plasma emitted by the Sun at high speeds (several hundred km/s). The first explorations of Mercury, carried out by the NASA/Mariner 10 space probe in 1974 and 1975, revealed that Mercury has a magnetic field, and therefore a magnetosphere, similar to that of Earth. In the 2000s, the NASA/MESSENGER satellite provided a detailed image of the Mercury’s magnetic field and magnetosphere and revealed that Mercury’s magnetic dipole is shifted northward from the center of the planet by about 0.2 RM (RM is Mercury’s radius of 2,439.7 km). The third exploration of Mercury is currently being carried out by the two probes of the BepiColombo mission launched on October 20, 2018 from Kourou (Guyana), namely the JAXA’s Mio magnetospheric orbiter (Fig. 1) and the ESA’s Mercury Planetary Orbiter (MPO). Both satellites are currently on their journey to Mercury ; their insertion into final orbit around the planet is planned for December 2025. By then, the satellites made and will make several flybys of Earth, Venus and Mercury in order to perform gravitational assistance maneuvers.

Figure 1

Unlike Mariner 10 and MESSENGER, Mio is equipped with a suit of “Waves” instruments (the PWI consortium, Plasma and Waves Instruments) designed specifically to study for the first time electric and magnetic waves and their role in the acceleration of charged particles (electrons, protons, heavier ions) in Mercury’s magnetosphere.

Figure 2

Results :

The present study, recently published in Nature Astronomy, was carried out by a Franco-Japanese team using data taken by the Mio search-coil magnetometers Fig. 2) built jointly between the LPP (DBSC instrument) and the University of Kanazawa (SC-LF instrument). The data was acquired during two flybys of Mercury that took place on October 1, 2021 and June 23, 2022, when the Mio spacecraft approached the planet at an altitude of approximately 200 km.
The stowed configuration of the BepiColombo satellites during the cruise phase is not optimal for measuring electromagnetic waves due to potential interference emitted by the satellite itself. However, the Mio spacecraft was specifically designed with the aim of minimizing its electromagnetic noise floor. This has allowed the SC-LF and DBSC instruments to make the first observations of electromagnetic waves around Mercury without being contaminated by the satellite interference. This revealed the local generation of high-frequency waves called “chorus” (Fig. 3), already measured in the Earth’s magnetosphere. The existence of these waves in the magnetosphere of Mercury, now confirmed, was predicted (frequency range, intensity, etc.) since the 2000s. What surprised the researchers who conducted this study was the “spatial localization" of these waves in an extremely limited region on the day side of Mercury’s magnetosphere. This location seems to be strongly correlated with the curvature of the magnetic field lines due to the dynamic pressure of the solar wind, the deformation being stronger on the night side than on the day side. Indeed, a theoretical and numerical study of the nonlinear growth rate of these waves showed that in the day sector energy is transferred more efficiently from electrons to electromagnetic fields along the magnetic field lines, thus creating conditions favoring the generation of “chorus” waves. This study should help better understanding of various energetic phenomena in the Hermian environment (aurora, precipitation of particles on the surface of the planet, etc.) and their impact on the large-scale dynamics of the magnetosphere. More complete studies will be conducted from December 2025 on, when the instruments will be fully deployed after BepiColombo’s insertion into its final orbit around Mercury.

Figure 3

Article :
Ozaki, M., Yagitani, S., Kasaba, Y., Kasahara Y., Matsuda, S., Omura, Y., Hikishima, M., Sahraoui, F., Mirioni, L., Chanteur, G., Kurita, S., Nakazawa, S., Murakami, G., Whistler-mode waves in Mercury’s magnetosphere observed by BepiColombo/Mio. Nature Astronomy (2023).

JAXA press release :

Contact at LPP :
Fouad Sahraoui (fouad.sahraoui and Laurent Mirioni

Dans la même rubrique :

CNRS Ecole Polytechnique Sorbonne Université Université Paris-Saclay Observatoire de Paris
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Directeur de la publication : Anne Bourdon (Directrice)