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Accueil > Recherche > Plasmas Spatiaux > Thématiques scientifiques > Modelling of plasma environments of small planets

Modelling of plasma environments of small planets

 LPP team :

Chanteur, G., Delcourt, D. Collaboration avec R. Modolo (LATMOS)

 Selection of publications :

 Environment of Mercury

Mercury’s ionized environment is peculiar, as both spatial and temporal scales of this magnetosphere are much smaller than those of the Earth and its boundary conditions are radically different (absence of ionosphere, magnetopause at a very short distance from the surface of the planet). This raises many questions concerning, for example, the large-scale structure of the hermian magnetosphere, the development of a plasma layer and a ring current, the sources and sinks of the magnetospheric populations, or the dynamics of the magnetospheric tail, and the magnetosphere-exosphere-surface interactions . These issues are among the many scientific objectives of the BEPI COLOMBO mission ( "Corner Stone" of ESA’s scientific program), whose launch is now expected 2018 for an arrival at Mercure in late 2024. This mission leads to a close cooperation between ESA, responsible for the Mercury Planetary Orbiter (MPO, 3-axis stabilized satellite in a 400 km x 1600 km polar orbit) and the Japanese space agency JAXA, responsible for the Mercury Magnetospheric Orbiter (MMO, spin stabilized probe in a polar orbit of 400 km x 12 000 km).

After the first observations by Mariner-10 in 1974-1975, the measurements of the MESSENGER probe (NASA) provided a harvest of data on the ionized environment of Mercury, eg, confirmation of the intrinsic magnetic field measured by Mariner-10, presence of ions of planetary origin (O +, Na +, Ca + ...) in different regions of the magnetosphere, rapid reconfigurations of the environment and significant forcing of the magnetosphere by the solar wind.

 LPP involvment

Different models are developed at LPP to study the interaction of the solar wind with this magnetosphere, which use different types of codes (3D hybrids, 3D test particles). The surface of the planet and its exosphere play an important role in the production of charged particles via various mechanisms (screening by solar wind and UV radiation, meteorite bombardment, thermal desorption, etc.). The simulations using test particles suggest, for example, that ions of planetary origin can make a significant contribution to the close hermian environment, with density levels of up to about 1 ion / cm3 at 400 Km of altitude (pericentre of MMO) at perihelion. After transport and acceleration in the magnetosphere, some of these ions precipitate in localized regions of the planet’s surface, leading to regolith enrichment and a reinforced screening process. Due to the small spatial scales of this magnetosphere, simulations with test particles also reveal important centrifugal effects during the escape of planetary ions, leading to a significant parallel acceleration (a few hundred eV for Na + ions) and to a transport of material of planetary origin in the close tail of the magnetosphere. This work also suggests a significant role of the induced electric field, during rapid reconfigurations of the magnetic field lines (e.g., during magnetic dipolarisations), in the acceleration and heating of the ions.
A 3D hybrid code, originally developed to simulate the plasma environments of Mars and Titan, has been adapted to Mercury. This hybrid code has been used in particular to examine the measurements of MESSENGER which indicate that the intrinsic magnetic field of the planet can be assimilated to a dipole off-centered by about 480 km towards the geographic North along the axis of rotation of the planet. It appears from simulation results that this model of eccentric dipole is perhaps only a convenient representation of the multipolar development of a planet field still poorly described, especially in the southern hemisphere because of the orbit of MESSENGER. In the longer term, kinetic models will be used to study the wave-particle interactions, while taking into account the peculiarities of ionic populations, such as, for planetary ions, the expected non-gyrotropy of the velocity distributions.

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Image : Simulations à l’aide de particules-tests et d’un modèle MHD montrant les distributions d’ions Na+ obtenues à partir d’un même modèle d’exosphère et avec un champ magnétique interplanétaire orienté vers le sud (-5 nT), en considérant des conductivités différentes (panneaux du haut et panneaux du bas) de la surface de la planète (d’après Seki et al. [2013])

These different numerical simulations will be useful for the analysis of the measurements of the MMO probe which is more specifically dedicated to the study of the ionized environment of Mercury. In partnership with ISAS-JAXA, LPP participates in the payload of MMO via two instruments : (1) as Co-PI of the Mass Spectrum Analyzer (MSA), which will measure distribution functions 3D ions over large mass ranges (from 1 to 40 mass units) and energies (from a few eV/q to 38 keV/q) ; (2) as Co-Pi of the Wave experiment PWI for the measurement of the magnetic field component parallel to the spin axis in the 0.1 Hz to 0.64 MHz range via a new broadband search coil model (DB-SM).

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Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Dominique Fontaine (Directrice)