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

Kinetic plasma simulations of Mercury’s magnetosphere to prepare BepiColombo

Space plasmas permeate the Solar System, from the solar corona to the upper layers of planetary environments (e.g. magnetosphere and ionosphere). In the Solar System, only two telluric planets possess an intrinsic magnetic field, and therefore a magnetosphere, those are the Earth and Mercury. Differently from the Earth, Mercury has been rarely visited by exploratory space missions. Therefore, many of the properties of Mercury’s environment, and of its magnetosphere in particular, remain poorly investigated at present. This work contributes to the global understanding of Mercury’s plasma and planetary environment, in light of ongoing exploratory space missions. The ongoing ESA/JAXA BepiColombo mission provides in situ observations at Mercury with an advanced payload, able to observe the plasma dynamics –for the first time– down to electron kinetic scales. To interpret such observations, numerical models resolving electron kinetic scales are needed. In this work, I use two fully-kinetic models to study electron-scale processes in Mercury’s magnetosphere, both at local and global scales. I focus on the plasma processes at the origin of (i) electron acceleration by wave-particle interaction at the magnetopause, (ii) electron acceleration by magnetic reconnection in the magnetotail, and (iii) electron precipitation onto the surface of Mercury. The impact of these processes on Mercury’s magnetosphere-exosphere-surface coupling is also extensively studied. For this purpose, I develop and validate the first ab initio fully-kinetic global model of Mercury’s magnetosphere. In this PhD, I characterize the main processes that accelerate electrons in the magnetosphere of Mercury. First, electrons are accelerated by resonant wave-particle interaction with drift waves (generated by the lower-hybrid-drift instability) at the magnetopause. This process increases the parallel electron temperature up to a factor two, if the magnetopause width is of the order of the ion gyroradius. Second, electrons are accelerated by magnetic reconnection in the magnetotail. This process generates a flow of electrons with an energy of few keV directed towards the planet from the X-line in the tail. Such electrons populate the inner shells of the magnetosphere to form Mercury’s “partial ring current”. Third, a large fraction of the electrons in this “partial ring current” precipitates onto the surface of Mercury, thus driving plasma-exosphere and plasma-surface interactions. Magnetic reconnection in the tail is the main process accelerating electrons (up to few keV) in the magnetosphere of Mercury. These electrons, while being partially trapped in the nightside, precipitate onto the surface to drive (i) efficient ionization of exospheric H, He, O and Mn, (ii) a pattern of X-ray emissions more prominent at dawn consistent with MESSENGER/XRS observations, and (iii) differential space weathering of Mercury’s regolith. Finally, the findings of this work will be used to advance global models of Mercury’s coupled magnetosphere-exosphere-surface system and to interpret (ongoing) and to plan (future) observations by BepiColombo mission. The global model developed in this work for Mercury will also find applications to other bodies (such as the Moon, asteroids, Mars, and the Galilean Moons of Jupiter) in future works

Numerical simulations of lower-hybrid-drift instability and particle acceleration

Boundaries separating two plasmas with different properties are ubiquitous in space (e.g. the solar corona, planetary magnetospheres, astrophysical shocks and jets) as well as in laboratory plasmas (e.g. tokamak and thrusters). Such boundaries are often the location of plasma mixing and particle energization processes, thus affecting the whole plasma system from the local/small scales to the global/large scales. This work focuses on a specific kinetic instability that develops in such boundaries characterised by strong density gradients, namely the lower-hybrid-drift instability (hereafter LHDI), and its interplay with other fluid-scale instabilities 1 , targeting applications in space physics. The study has been carried out by means of analytical computation and numerical simulations. In particular, it aims at answering the following questions. (i) In which conditions is the LHDI the dominant mechanism in terms of particle acceleration and mixing processes? (ii) Do we expect to see the effect of such instability in future space missions on different solar objects? If so, with which intensity and efficiency? In this document, I shall present the analytical kinetic theory ( starting from the Vlasov equation) developed to study the linear growth of the LHDI, as well as innovative 3D full-kinetic simulations used to study the nonlinear stage of the LHDI and its interplay with other instabilities. This study finds that the LHDI acts as a major player in shaping any plasma boundary thinner than the ion skin depth. Nonetheless, we found that the LHDI electron acceleration is negligible for typical space parameters considered in this study. Indeed, the saturation of the LHDI, associated to the trapping of the ions in the wave potential, occurs at electric energy levels small compared to the typical electron thermal energy, which naturally leads to a slow and inefficient electron acceleration process. On longer time-scales, the LHDI, via a coupling with the drift-kink instability, forms large scale finger-like structures, which lead to strong cross-field plasma transport and a disruption of the initial configuration. In particular, the Kelvin-Helmholtz instability (KHI) is dominant only when strong velocity shear are realized, and it creates a diffuse layer, while for strong density gradients the LHDI dominates the layer dynamics, creating plasma intrusions. Large scale intrusions analog to the ones born in the LHDI nonlinear stage are created by the Rayleigh-Taylor instability (RTI), this coupled mechanism is studied for the first time in a curved layer, and we show its potential to create large scale finger-like structures extending far away from the layer. The results concerning electron acceleration are very general and valid for any plasma contexts able to trigger the LHDI. Here our study intends to address (i) ongoing space mission BepiColombo targeting Mercury’s highly kinetic magnetosphere and (ii) cometary plasma physics, e.g. past mission Rosetta, especially for the part of interplay LHDI-RTI

A large-scale instability competing with Kelvin-Helmholtz at Mercury's boundary layer

International audienceMagnetic reconnexion and Kelvin-Helmholtz (KH) instability are usually recognized as the two main mixing processes along magnetopauses. However, a recent work [Dargent et al., 2019] showed that in Mercury"s conditions, another instability can grow faster than the KH instability along the magnetopause. This instability seems to rely on gradients of density and/or magnetic field and develops large-scales finger-like structures that prevents the growth of the KH vortices. In this work, I will characterize this instability and try to identify it. In particular, I will look at the dependance of the growth rate of this instability to the different parameters of the plasma and compare it to the growth rate of the Kelvin-Helmholtz instability

Electron acceleration by the lower-hybrid-drift instability at Mercury: an extended quasilinear model

International audienceDensity inhomogeneities are ubiquitous in space and astrophysical plasmas, in particular at contact boundaries between different media. They often correspond to regions that exhibits strong dynamics on a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various plasma instabilities such as, for instance, the lower-hybrid-drift instability which in turn transfers energy to the particles through wave-particle interactions and eventually heats the plasma. Here, we address the role of this instability in the Hermean plasma environment were kinetic processes of this fashion are expected to be crucial in the plasma dynamics and have so far eluded the measurements of past missions (Mariner-X and MESSENGER) to Mercury. The goal of our work is to quantify the efficiency of the lower-hybrid-drift instability to accelerate and/or heat electrons parallel to the ambient magnetic field.To reach this goal, we combine two complementary methods: full-kinetic and quasilinear models.We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we develop an extended quasilinear model able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary: the magnetopause of Mercury, and we discuss our quantitative predictions of electron acceleration in support to future BepiColombo observations

Electron acceleration by the lower-hybrid-drift instability at Mercury: an extended quasilinear model

International audienceDensity inhomogeneities are ubiquitous in space and astrophysical plasmas, in particular at contact boundaries between different media. They often correspond to regions that exhibits strong dynamics on a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various plasma instabilities such as, for instance, the lower-hybrid-drift instability which in turn transfers energy to the particles through wave-particle interactions and eventually heats the plasma. Here, we address the role of this instability in the Hermean plasma environment were kinetic processes of this fashion are expected to be crucial in the plasma dynamics and have so far eluded the measurements of past missions (Mariner-X and MESSENGER) to Mercury. The goal of our work is to quantify the efficiency of the lower-hybrid-drift instability to accelerate and/or heat electrons parallel to the ambient magnetic field.To reach this goal, we combine two complementary methods: full-kinetic and quasilinear models.We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we develop an extended quasilinear model able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary: the magnetopause of Mercury, and we discuss our quantitative predictions of electron acceleration in support to future BepiColombo observations

Electron acceleration driven by the lower-hybrid-drift instability: An extended quasilinear model

International audienceContext. Density inhomogeneities are ubiquitous in space and astrophysical plasmas, particularly at contact boundaries between different media. They often correspond to regions that exhibit strong dynamics across a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various instabilities such as the lower-hybrid-drift instability, which, in turn, transfers energy to the particles through wave-particle interactions and eventually heats the plasma.Aims. Our study is aimed at quantifying the efficiency of the lower-hybrid-drift instability to accelerate or heat electrons parallel to the ambient magnetic field.Methods. We combine two complementary methods: full-kinetic and quasilinear models.Results. We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we have developed an extended quasilinear model that is able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, which is now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary, namely, the magnetopause of Mercury. Furthermore, we discuss our quantitative predictions of electron acceleration to support future BepiColombo observations

Electron acceleration by the lower-hybrid-drift instability at Mercury: an extended quasilinear model

International audienceDensity inhomogeneities are ubiquitous in space and astrophysical plasmas, in particular at contact boundaries between different media. They often correspond to regions that exhibits strong dynamics on a wide range of spatial and temporal scales. Indeed, density inhomogeneities are a source of free energy that can drive various plasma instabilities such as, for instance, the lower-hybrid-drift instability which in turn transfers energy to the particles through wave-particle interactions and eventually heats the plasma. Here, we address the role of this instability in the Hermean plasma environment were kinetic processes of this fashion are expected to be crucial in the plasma dynamics and have so far eluded the measurements of past missions (Mariner-X and MESSENGER) to Mercury. The goal of our work is to quantify the efficiency of the lower-hybrid-drift instability to accelerate and/or heat electrons parallel to the ambient magnetic field.To reach this goal, we combine two complementary methods: full-kinetic and quasilinear models.We report self-consistent evidence of electron acceleration driven by the development of the lower-hybrid-drift instability using 3D-3V full-kinetic numerical simulations. The efficiency of the observed acceleration cannot be explained by standard quasilinear theory. For this reason, we develop an extended quasilinear model able to quantitatively predict the interaction between lower-hybrid fluctuations and electrons on long time scales, now in agreement with full-kinetic simulations results. Finally, we apply this new, extended quasilinear model to a specific inhomogeneous space plasma boundary: the magnetopause of Mercury, and we discuss our quantitative predictions of electron acceleration in support to future BepiColombo observations

Full-kinetic global simulations of the plasma environment at Mercury: a model from planetary to electrons scales to support BepiColombo

International audienceMercury is the only telluric planet of the solar system, other than Earth, with an intrinsic magnetic field. Thus, the Hermean surface is shielded from the impinging solar wind via the presence of an "Earth-like" magnetosphere. However, this cavity is twenty times smaller than its alike at the Earth. The relatively small extension of the Hermean magnetosphere enables us to model it using global full-kinetic simulation with the aid of modern supercomputers. Such modeling is crucial to interpret, and prepare, the future observations of the ongoing joint ESA-JAXA mission BepiColombo.The model used in this work is based on three-dimensional, implicit full-PIC simulations of the interaction between the solar wind and Mercury's magnetosphere (i.e. at 0.3-0.47 AU). This model includes self-consistently the ion and electron physics down to kinetic electron scales. On top of that, we show comparisons between in-situ observations by Mariner-X and BepiColombo space missions. This comparison allows us (i) to validate our model and (ii) to gain insights into the electron dynamics in the Hermean environment, thought to be governed by kinetic-scale processes.First, we validate our model through a qualitative comparison between three-dimensional outcomes of our global simulations and the ones of reduced fluid/hybrid simulations (in the context of the SHOTS collaboration). Moreover, comparison with in-situ Mariner-X observations during its first Mercury flyby complete the validation of our model. Second, we study the global dynamics of electrons showing regions where strongest particle acceleration/energization occurs, giving quantitative estimate of electron temperature anisotropy in the Hermean environment. Such results are used to interpret past, and plan future, BepiColombo in-situ observations