De l’exosphère à la magnétosphère des objets planétaires faiblement magnétisés : optimisation de modélisations parallélisées pour une application à Ganymède

Jupiter’s moon Ganymede is the biggest and most massive satellite of our solar system. Thisobject has been observed from the Earth, with the Hubble Space Telescope (HST), and through in situ measurements by Galileo spacecraft. Thanks to these observations, a very tenuous atmosphere, or exosphere, has been detected at Ganymede. It is mainly composed of atomic hydrogen, atomic oxygen, and molecular oxygen. Ganymede is the only moon of the solar system to have its own intrinsic magnetic field, which generates a minimagnetosphere interacting with the magnetospheric jovian plasma. This magnetosphere is embedded in the jovian magnetosphere. It is the only known case of interaction between two magnetospheres. Galileo is the only mission that has investigated the complex ionized environment of Ganymede. The next space mission dedicated to investigate the Jovian magnetosphere and its galilean satellite is an European mission from ESA : JUICE (Jupiter ICy moons Explorer). In the frame of this mission, and to prepare future observations at Ganymede, my thesis work has consisted in modeling the global neutral and ionized environment of Ganymede. The first part of my thesis work has been dedicated to the study of Ganymede’s exosphere with a 3D Monte-Carlo model. I have parallelized this model to improve its performance and to enrich the physics described by the model. Results have been compared to those of other models, and to HST and Galileo observations. The ionized environment, in particular the magnetosphere of Ganymede, has then been studied with a 3D parallel hybrid model,considering the observation conditions of Galileo. Results are globally consistent with the observations and with other models, but show the necessity to significantly improve the spatial resolution. Therefore, a significant part of my work has been dedicated to the development of a multi-grid approach in the hybrid model, to divide by 2 the spatial resolution at the vicinity of Ganymede. Finally, results obtained with the optimized model are compared to Galileo observations.Ganymède, une lune de Jupiter, est le plus grand et le plus massif des satellites de notre système solaire. Cet objet a été observé depuis la Terre, notamment grâce au télescope Hubble (HST), et in situ par la sonde Galileo. Grâce à ces observations, une atmosphère très ténue, ou exosphère,principalement composée d'hydrogène, d'oxygène et d'oxygène moléculaire, a été détectée au voisinage de Ganymède. Ganymède est l'unique lune du système solaire possédant son propre champ magnétique intrinsèque, qui, en interagissant avec le plasma magnétosphérique jovien, génère unemini-magnétosphère. Cette magnétosphère est imbriquée dans celle de Jupiter. C'est le seul cas connu d'interaction entre deux magnétosphères. Galileo est l'une des seules sondes spatiales ayant investigué l'environnement complexe de Ganymède. La prochaine mission spatiale qui étudiera ce satellite estune mission européenne de l'ESA : JUICE (JUpiter ICy moon Exploration). Dans le cadre de cette mission, et dans un but de mieux connaître ce satellite, mon travail de thèse a consisté à modéliser l'environnement global neutre et ionisé de Ganymède.La première partie de mon travail de thèse a été consacrée à l'étude de l'exosphère de Ganymède à l'aide d'un modèle 3D Monte-Carlo. J'ai parallélisé ce modèle afin d'améliorer ses performances et d'enrichir la physique décrite par le modèle. Les résultats sont comparés à ceux d'autres modèles, ainsi que les observations effectuées par le HST et Galileo. L'environnement ionisé, en particulier la magnétosphère de Ganymède, a été ensuite étudié à l'aide d'un modèle hybride parallèle 3D, notamment en se plaçant dans les conditions d'observations de Ganymède par Galileo. Les résultats sont globalement cohérents avec les observations, et concordent avec ceux d'autres modèles, maismontrent néanmoins une nécessité d'améliorer significativement la résolution spatiale du modèle. De ce fait, une partie significative de mon travail de thèse a été dédiée au développement et à l'implémentation d'une approche multi-grilles au sein du modèle hybride, pour améliorer la résolution spatiale d'un facteur 2 dans le voisinage proche du satellite. Enfin, les résultats obtenus avec ce modèle optimisé sont confrontés aux observations de Galileo

Hybrid multi-grids simulations of Ganymede's magnetosphere

International audienceThe Jovian satellite Ganymede is the biggest moon of our solar system. One of the main motivation of our interest for this moon is its own intrinsic magnetic field, which has been discovered during the Galileo mission (Kivelson et al. 1996). The magnetic field of Ganymede directly interacts with the corotating jovian plasma, leading to the formation of a mini-magnetosphere which is embedded in the giant magnetosphere of Jupiter. This is the only known case of interaction between two planetary magnetospheres.In the frame of the European space mission JUICE (Jupiter Icy moon Exploration), we investigate this unique interaction with a 3D parallel multi-species hybrid model. This model is based on the CAM-CL algorithm (Matthews 1994) and has been used to study the ionized environments of Titan, Mars and Mercury. In the hybrid formalism, ions are kinetically treated whereas electrons are considered as a zero-inertial fluid to ensure the quasi-neutrality of the plasma. The temporal evolution of the electromagnetic fields is calculated solving Maxwell's equations. The jovian magnetospheric plasma is described as being composed of oxygen and proton ions. The magnetic field of Ganymede, which includes dipolar and induced components (Kivelson et al, 2002), is distorted by its interaction with the Jovian plasma and formed the Alfvén wings. The planetary plasma is described as being composed of O+, with a scale height equal to 125 km. The description of the exosphere is provided by the 3D multi-species collisional exospheric/atmospheric model of Leblanc et al, (2015) and Turc et al. (2014). The ionization of this neutral exosphere by charge exchanges, by electronic impacts, and by reaction with solar photons contributes to the production of planetary plasma. In this model, calculations are performed on a cartesian simulation grid which is refined (down to ~120 km of spatial resolution) at Ganymede, using a multi-grids approach (Leclercq et al., submitted, 2015). Results are compared with Galileo observations obtained during the G1, G2 and G8 flybys

Propagation of Transient Perturbations into a Planet's Exosphere: Molecular Kinetic Simulations

The upper atmospheres of Mars and Titan, as well as those on many other planetary bodies, exhibit significant density variations vs. altitude that are interpreted as gravity waves. Such data is then used to extract vertical temperature profiles, even when such perturbations propagate through the transition region from a collision dominated regime and into a planet's exosphere. Since the temperature profile is critical for describing the upper atmospheric heating and evolution, we use molecular kinetic simulations to describe transient perturbations in a Mars-like upper atmosphere. We show that the standard methods for extracting the temperature profile can fail dramatically so that molecular kinetic simulations, calibrated to observed density profiles, are needed in this region of a planet's atmosphere

From exosphere to magnetosphere of planetary objects : optimization of parallelized modelisations for an application to Ganymede

Ganymède, une lune de Jupiter, est le plus grand et le plus massif des satellites de notre système solaire. Cet objet a été observé depuis la Terre, notamment grâce au télescope Hubble (HST), et in situ par la sonde Galileo. Grâce à ces observations, une atmosphère très ténue, ou exosphère,principalement composée d'hydrogène, d'oxygène et d'oxygène moléculaire, a été détectée au voisinage de Ganymède. Ganymède est l'unique lune du système solaire possédant son propre champ magnétique intrinsèque, qui, en interagissant avec le plasma magnétosphérique jovien, génère unemini-magnétosphère. Cette magnétosphère est imbriquée dans celle de Jupiter. C'est le seul cas connu d'interaction entre deux magnétosphères. Galileo est l'une des seules sondes spatiales ayant investigué l'environnement complexe de Ganymède. La prochaine mission spatiale qui étudiera ce satellite estune mission européenne de l'ESA : JUICE (JUpiter ICy moon Exploration). Dans le cadre de cette mission, et dans un but de mieux connaître ce satellite, mon travail de thèse a consisté à modéliser l'environnement global neutre et ionisé de Ganymède.La première partie de mon travail de thèse a été consacrée à l'étude de l'exosphère de Ganymède à l'aide d'un modèle 3D Monte-Carlo. J'ai parallélisé ce modèle afin d'améliorer ses performances et d'enrichir la physique décrite par le modèle. Les résultats sont comparés à ceux d'autres modèles, ainsi que les observations effectuées par le HST et Galileo. L'environnement ionisé, en particulier la magnétosphère de Ganymède, a été ensuite étudié à l'aide d'un modèle hybride parallèle 3D, notamment en se plaçant dans les conditions d'observations de Ganymède par Galileo. Les résultats sont globalement cohérents avec les observations, et concordent avec ceux d'autres modèles, maismontrent néanmoins une nécessité d'améliorer significativement la résolution spatiale du modèle. De ce fait, une partie significative de mon travail de thèse a été dédiée au développement et à l'implémentation d'une approche multi-grilles au sein du modèle hybride, pour améliorer la résolution spatiale d'un facteur 2 dans le voisinage proche du satellite. Enfin, les résultats obtenus avec ce modèle optimisé sont confrontés aux observations de Galileo.Jupiter’s moon Ganymede is the biggest and most massive satellite of our solar system. Thisobject has been observed from the Earth, with the Hubble Space Telescope (HST), and through in situ measurements by Galileo spacecraft. Thanks to these observations, a very tenuous atmosphere, or exosphere, has been detected at Ganymede. It is mainly composed of atomic hydrogen, atomic oxygen, and molecular oxygen. Ganymede is the only moon of the solar system to have its own intrinsic magnetic field, which generates a minimagnetosphere interacting with the magnetospheric jovian plasma. This magnetosphere is embedded in the jovian magnetosphere. It is the only known case of interaction between two magnetospheres. Galileo is the only mission that has investigated the complex ionized environment of Ganymede. The next space mission dedicated to investigate the Jovian magnetosphere and its galilean satellite is an European mission from ESA : JUICE (Jupiter ICy moons Explorer). In the frame of this mission, and to prepare future observations at Ganymede, my thesis work has consisted in modeling the global neutral and ionized environment of Ganymede. The first part of my thesis work has been dedicated to the study of Ganymede’s exosphere with a 3D Monte-Carlo model. I have parallelized this model to improve its performance and to enrich the physics described by the model. Results have been compared to those of other models, and to HST and Galileo observations. The ionized environment, in particular the magnetosphere of Ganymede, has then been studied with a 3D parallel hybrid model,considering the observation conditions of Galileo. Results are globally consistent with the observations and with other models, but show the necessity to significantly improve the spatial resolution. Therefore, a significant part of my work has been dedicated to the development of a multi-grid approach in the hybrid model, to divide by 2 the spatial resolution at the vicinity of Ganymede. Finally, results obtained with the optimized model are compared to Galileo observations

Ganymede's interaction with the jovian plasma from hybrid simulation

Ganymede is a unique object: it is the biggest moon of our solar system, and the only satellite which has its own intrinsic magnetic field leading to the formation of a small magnetosphere. The magnetosphere of Ganymede being embedded in the Jovian magnetosphere, the environment of the Galilean moon presents the only known case of interaction between two magnetospheres (Kivelson et al. 1996). This peculiar interaction has been investigated by means of a 3D parallel multi-species hybrid model based on a CAM-CL algorithm (Mathews et al. 1994). This generic model has been largely used for other magnetized or unmagnetized bodies such as Mars (Modolo et al. 2005; 2006 and 2012), Titan (Modolo et al. 2007, Modolo and Chauteur 2008) or Mercury (Richer et al. 2012). IIn this formalism, ions have a kinetic description whereas electrons are considered as an inertialess fluid which ensure the neutrality of the plasma and contribute to the total current and electronic pressure. Maxwell's equations are solved to compute the temporal evolution of electromagnetic field. The hybrid simulation describes the dynamics of the magnetospheric plasma, composed of O+ and H+ ions, and Ganymede's ionospheric plasma (W+, H2+, H+). Similarly to Paty and Winglee (2004), a density profile with a scale height of 125km of the ionospheric plasma is loaded and feeded during the simulation. Charge exchange leading to H2+ and H+ are also computed. To represent Ganymede's magnetosphere a magnetic dipole is implemented at initialization with dipolar moments values taken from Kivelson et al, 2002. This dipole is progressively distorted and lead to the formal of the mini-magnetopshere. Simulation results also emphasize the presence of Alfvén wings and are in good agreement with other simulation results (Jia et al, 2008, Paty et al, 2008). Hybrid simulations are performed on a uniform cartesian grid with a spatial resolution of about 200 km. Simulations results are presented and compared to magnetometer and particle observations obtained during G1 and G2 Galileo flybys

A 3D parallel model of Ganymede's exosphere

Ganymede is a unique object : it is the biggest moon of our solar system, and the only satellite which has its own intrinsic magnetic field. Its surface is covered by water ice and by regolith. Some previous observations suggest that below its surface may exist an ocean of liquid water. The atmosphere of the planet is poorly known but should be composed essentially of water, hydrogen and oxygen (Marconi et al., Icarus, 2007). These atmospheric particles mainly originate from the surface thanks to sublimation of water-ice and sputtering, a process driven by the magnetospheric Jovian particles impacting Ganymede surface and leading to ejection of atoms and molecules into Ganymede atmosphere. We developed a model of Ganymede's atmosphere based on a 3D Monte Carlo description of the fate of the ejected particles from the surface. This model has been parallelized allowing a much better statistical, spatial and temporal description of Ganymede's environment. This model includes the main sources of the neutral atmosphere and is able to calculate all its characteristics. It was successfully compared to the few known observations as well as to previous modeling. In this presentation, we will present the main characteristics of this model and what it tells us on Ganymede's atmosphere, in terms of spatial structure, composition, temporal variability and relations with both magnetosphere and surface

3D magnetospheric parallel hybrid multi-grid method applied to planet-plasma interactions

International audienceWe present a new method to exploit multiple refinement levels within a 3D parallel hybrid model, developed to study planet-plasma interactions. This model is based on the hybrid formalism: ions are kinetically treated whereas electrons are considered as a inertia-less fluid. Generally, ions are represented by numerical particles whose size equals the volume of the cells. Particles that leave a coarse grid subsequently entering a refined region are split into particles whose volume corresponds to the volume of the refined cells. The number of refined particles created from a coarse particle depends on the grid refinement rate. In order to conserve velocity distribution functions and to avoid calculations of average velocities, particles are not coalesced. Moreover, to ensure the constancy of particles' shape function sizes, the hybrid method is adapted to allow refined particles to move within a coarse region. Another innovation of this approach is the method developed to compute grid moments at interfaces between two refinement levels. Indeed, the hybrid method is adapted to accurately account for the special grid structure at the interfaces, avoiding any overlapping grid considerations. Some fundamental test runs were performed to validate our approach (e.g quiet plasma flow, Alfven wave propagation). Lastly, we also show a planetary application of the model, simulating the interaction between Jupiter's moon Ganymede and the Jovian plasma

Numerical simulation of Ganymede's ionosphere

International audienceGanymede is one of the four Galilean moons that orbit around Jupiter and the key moon targeted by the JUpiter and ICy moons Explorer (JUICE) mission. Other than being the largest moon in the solar system, it is also the only one known to generate internally a magnetic field which is strong enough to overcome the background jovian field; thus, the moon carves out its own magnetosphere inside that of Jupiter. In addition, at Ganymede's orbit the jovian plasma is sub-Alfvénic and subsonic. The interaction of Ganymede's magnetosphere with its surroundings therefore differs from that of planetary magnetospheres resulting from the interaction with the super-Alfvénic and supersonic solar wind. All this makes Ganymede a peculiar celestial body to study. One of the main goals of the JUICE mission is to characterize Ganymede's exosphere, ionosphere, and magnetosphere as well as its interaction with the jovian surrounding in great details. Ahead of the arrival of JUICE at Jupiter, models have been developed to predict Ganymede's environment. Observational constraints are primarily given from Galileo and from Earth-based telescopes. They remain limited, especially in terms of the ionospheric number density and temperature. To address the currently poorly constrained ionospheric environment, we have developed a test particle model of Ganymede's plasma environment. The model is driven by the densities of neutral species from the exospheric model of Leblanc et al. (Icarus, 2016) and the electromagnetic field taken from the hybrid model of Leclercq et al. (PSS, 2016). The simulation follows the motion of millions of test particles in the environment of the moon and allows to generate maps of ion densities, bulk velocities, and temperatures. We will present simulation outcomes for different ions, including H+, O+, and O2+. We will also discuss how the results from the simulations are relevant to MHD and exospheric models and in interpreting plasma and particle data obtained by Galileo during its close flybys of Ganymede

Development of a Multi-Grids Approach into a Parallelized Hybrid Model to Describe Ganymede's Interaction with the Jovian Plasma

International audienceGanymede is the only satellite which has its own magnetosphere, which is embedded in the Jovian magnetosphere (Kivelson et al. 1996). This peculiar interaction has been investigated by means of a 3D parallel multi-species hybrid model based on a CAM-CL algorithm (Mathews et al. 1994). In this formalism, ions have a kinetic description whereas electrons are considered as an inertialess fluid which ensures the neutrality of the plasma and contributes to the total current and electronic pressure. Maxwell’s equations are solved to compute the temporal evolution of electromagnetic field. Hybrid simulations are performed on a uniform cartesian grid with a spatial resolution of about 240 km. Our results are globally consistent with other models and Galileo measurements. Nevertheless, our description of the magnetopause and the ionosphere is not satisfying enough due to the low spatial resolution. Indeed, we want to describe scale heights of 125 km in the ionosphere whereas the best spatial resolution that we are allowed to use is about 240 km. Therefore, in order to obtain more efficient and relevant results, it is necessary to improve the size of the grid. In this optic, we are introducing a multi-grids approach in order to refine the spatial resolution by a factor 2 (~120km) near Ganymede. The creation of a finer mesh in the simulation grid leads to make some peculiar computations at the interfaces between the two different grids, whether for the calculation of moments, such as charge density or current, or the computation of electromagnetic fields. Moreover, the parallelization of the code, based on domain decomposition methods, imposes us to take care of boundary conditions. In the hybrid model, macroparticules, which represent a kind of cloud of physical particles, have a volume equal to that of a grid cell. Then, the macroparticules entering into the higher spatial resolution region are splited into smaller macroparticules whose the volume corresponds to the volume of a cell of the finer mesh. The improvement of the spatial resolution in the hybrid model will also allow us to relevantly couple the results of this model with those of our 3D multi-species exospheric model (Turc et al. 2014), into a test-particle model that describes the ionosphere of Ganymede. Basic tests and validation results of the multi-grids approach are presented

Modelling Ganymede's neutral environment: A 3D test-particle simulation

International audienceIn this paper, we present a 3D parallelized test-particle model of Ganymede's neutral environment. The atmosphere sources are assumed to be the sputtering and the sublimation of water-ice, the former taking place in the polar regions and the latter near the subsolar point. It appears that Ganymede's atmosphere is deeply structured by these two processes, leading to a strong dichotomy between polar and subsolar regions. The densest part of the atmosphere is found in the vicinity of the subsolar point, where sublimated H2O is the predominant species near the surface. At higher latitudes and on the nightside, O2 prevails at low altitude, whereas the high altitudes are primarily populated by H2 everywhere in the atmosphere. An estimation of the number of collisions shows that the atmosphere is mostly collisionless, except a small region near the subsolar point. The O2 column density in our model is in good agreement with the observations. However, it appears that we underestimate the H density. This could suggest that the sublimation rates are significantly underestimated but not the sputtering. The escape rates of the different species are essentially lower than those previously obtained by Marconi (2007). The effects of varying surface emission fluxes are investigated in order to simulate Ganymede's passing into the shadow of Jupiter or in the plasma sheet. We estimate that the sublimated H2O peak in the subsolar region would disappear within one hour in the shadow of Jupiter. Likewise, the variation of the sputtering fluxes alters the atmospheric structure in a similar time scale