水星磁気圏側面における惑星起源イオンの輸送および加速の研究

Tohoku University笠羽康正課

Solar-wind electron precipitation on weakly magnetized bodies: the planet Mercury

Mercury is the archetype of a weakly magnetized, airless, telluric body immersed in the solar wind. Due to the lack of any substantial atmosphere, the solar wind directly precipitates on Mercury's surface. Using a 3D fully-kinetic self-consistent plasma model, we show for the first time that solar-wind electron precipitation drives (i) efficient ionization of multiple species (H, He, O and Mn) in Mercury's neutral exosphere and (ii) emission of X-rays from the planet's surface. This is the first, independent evidence of X-ray auroras on Mercury using a numerical approach.Comment: Submitted to Physical Review Letter

Maps of solar wind plasma precipitation onto Mercury's surface: a geographical perspective

Mercury is the closest planet to the Sun, possesses a weak intrinsic magnetic field and has only a very tenuous atmosphere (exosphere). These three conditions result in a direct coupling between the plasma emitted from the Sun (namely the solar wind) and Mercury’s surface. The planet’s magnetic field leads to a non-trivial pattern of plasma precipitation onto the surface, that is expected to contribute to the alteration of the regolith over geological time scales. The goal of this work is to study the solar wind plasma precipitation onto the surface of Mercury from a geographical perspective, as opposed to the local-time-of-day approach of previous precipitation modeling studies. We employ solar wind precipitation maps for protons and electrons from two fully-kinetic numerical simulations of Mercury’s plasma environment. These maps are then integrated over two full Mercury orbits (176 Earth days). We found that the plasma precipitation pattern at the surface is most strongly affected by the upstream solar wind conditions, particularly by the interplanetary magnetic field direction, and less by Mercury’s 3:2 spin-orbit resonance. We also found that Mercury’s magnetic field is able to shield the surface from roughly 90% of the incoming solar wind flux. At the surface, protons have a broad energy distribution from below 500 eV to more than 1.5 keV; while electrons are mostly found in the range 0.1-10 keV. These results will help to better constrain space weathering and exosphere source processes at Mercury, as well as to interpret observations by the ongoing ESA/JAXA BepiColombo mission

BepiColombo Science Investigations During Cruise and Flybys at the Earth, Venus and Mercury

The dual spacecraft mission BepiColombo is the first joint mission between the European Space Agency (ESA) and the Japanese Aerospace Exploration Agency (JAXA) to explore the planet Mercury. BepiColombo was launched from Kourou (French Guiana) on October 20th, 2018, in its packed configuration including two spacecraft, a transfer module, and a sunshield. BepiColombo cruise trajectory is a long journey into the inner heliosphere, and it includes one flyby of the Earth (in April 2020), two of Venus (in October 2020 and August 2021), and six of Mercury (starting from 2021), before orbit insertion in December 2025. A big part of the mission instruments will be fully operational during the mission cruise phase, allowing unprecedented investigation of the different environments that will encounter during the 7-years long cruise. The present paper reviews all the planetary flybys and some interesting cruise configurations. Additional scientific research that will emerge in the coming years is also discussed, including the instruments that can contribute

Transport et énergisation des ions planétaires dans les flancs magnétosphériques du mercure

The transport and energization of planetary ions within Kelvin-Helmholtz (KH) vortices developing in the magnetospheric flanks of Mercury are investigated using both numerical methods and data analysis. Due to the presence of heavy ions of planetary origin (e.g., O^+, Na^+, and K^+) and the complicated field structure present during the KH vortex development, the scale of electric field variations can be comparable to that of the ion gyromotion. Therefore, ions may experience non-adiabatic energization as they drift across the magnetopause. In this study, we focus on the effects of the spatial/temporal variations of the electric field along the ion path. We show that the intensification, rather than the change in orientation, is responsible for large non-adiabatic energization of heavy ions of planetary origin. This energization systematically occurs for ions with low initial energies in the direction perpendicular to the magnetic field. The energy gain is of the order of the energy corresponding to the maximum ExB drift speed. It is also found that the ion transport across the magnetopause is controlled by the orientation of the magnetosheath electric field. Analyzing data from MESSENGER allow us to compare the observational facts with our numerical results. We find that the counts of Na^+-group detected by FIPS increase with the existence of KH waves, which is consistent with our numerical results. Although some differences in the energy distribution are expected in our numerical results, the data show no significant differences. This will be the subject of further studies using the newly developed BepiColombo instruments.Dans cette thèse, par des méthodes numériques et l’analyse de données, le transport et l’accélération d’ions suite aux tourbillons KH qui peuvent se développer dans les flancs de la magnétosphère de Mercure sont examinés. Ici, l’échelle des variations du champ électrique peut être comparable à celle du mouvement de giration des ions lorsque les ions lourds d’origine planétaire (e.g., O^+, Na^+, ou K^+) se combinent à la complexité des champs électromagnétiques issus du développement des tourbillon KH. Les ions peuvent donc être accélérés de façon non-adiabatique lors de leur passage à travers la magnétopause de Mercure. Nous nous concentrons sur les effets des variations spatiale/temporelle du champ électrique le long du trajet ionique. Nos résultats montrent que l’intensification du champ, plutôt que le changement de son orientation, est responsable de l’accélération non-adiabatique à grande échelle des ions. Cette accélération se produit systématiquement pour les ions ayant des faibles énergies initiales dans la direction perpendiculaire au champ magnétique. Le gain énergétique étant du même ordre que l’énergie correspondant à la vitesse maximale de dérive ExB. Le transport des ions est aussi contrôlé par l’orientation du champ électrique de la magnetogaine. Comparant les données de MESSENGER, nous pouvons conclure que le nombre d’ion du groupe Na^+ détectés par FIPS augmente avec la présence d’ondes KH. Bien que nos résultats numériques supposent certaines différences dans la distribution énergétique des ions, nous n’avons pas trouvé de telles disparités. Pour mieux étudier ces conclusions, des études plus approfondies basées sur BepiColombo s’avèrent nécessaires

Sodium Ion Dynamics in the Magnetospheric Flanks of Mercury

International audienceWe investigate the transport of planetary ions in the magnetospheric flanks of Mercury. In situ measurements from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging spacecraft show evidences of Kelvin-Helmholtz instability development in this region of space, due to the velocity shear between the downtail streaming flow of solar wind originating protons in the magnetosheath and the magnetospheric populations. Ions that originate from the planet exosphere and that gain access to this region of space may be transported across the magnetopause along meandering orbits. We examine this transport using single-particle trajectory calculations in model Magnetohydrodynamics simulations of the Kelvin-Helmholtz instability. We show that heavy ions of planetary origin such as Na may experience prominent nonadiabatic energization as they E × B drift across large-scale rolled up vortices. This energization is controlled by the characteristics of the electric field burst encountered along the particle path, the net energy change realized corresponding to the maximum E × B drift energy. This nonadiabatic energization also is responsible for prominent scattering of the particles toward the direction perpendicular to the magnetic field

Ion dynamics in the magnetospheric flanks of Mercury

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Escape of planetary ions from Mercurys magnetosphere under different solar wind conditions

International audienceThe combination of a weak intrinsic magnetic field and strong solar wind conditions at Mercury results in the formation of a relatively small magnetosphere compared to that of the Earth. Typical bowshock and magnetopause locations at Mercury are 2 and 1.4 Mercury radius from the center of the planet, respectively, and thus, Mercurys mini-magnetosphere is strongly compressed and may disappear when solar wind condition is extreme. Under these particular circumstances, the solar wind can directly interact with its exosphere and surface and planetary ion escape is expected to be enhanced. In this study, the production and escape of planetary ions from the exosphere have been investigated under different solar wind conditions using the global hybrid simulation LatHyS. In particular, we have focused on dynamic pressure and interplanetary magnetic field dependence. This study will provide a good indication on the ability of a mini-magnetosphere to protect the planetary exosphere from ion escape, to be compared to that of the larger magnetosphere of the Earth