Context. 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.
Context. The planet Mercury possesses a small but highly dynamic magnetosphere in which the role and dynamics of electrons are still largely unknown. Aims. We aim to model the global dynamics of solar-wind electrons impinging on Mercury's magnetosphere. Particular relevance is given to local acceleration processes and the global circulation patterns. Methods. The goals of this work are pursued by means of three-dimensional, fully kinetic particle-in-cell simulations modeling the interaction of the solar wind with the Hermean magnetosphere. This method allows a self-consistent representation of the plasma dynamics from the large planetary scale down to the electron kinetic scale. We carried out numerical simulations using two different solar-wind conditions: purely northward or purely southward interplanetary magnetic field direction. Results. We find a high plasma current (of the order of few µA/m 2 ) flowing at the magnetospheric boundaries (bow shock and magnetopause) dominated by electrons. This current is driven by the small-scale electron physics resolved in our model. Furthermore, we observe strong electron acceleration up to tens of keV as a consequence of magnetic reconnection when the interplanetary magnetic field is directed southward. Such energetic electrons are partially trapped in the dipolar magnetic field of the planet mainly at nightside. Finally, by studying the distribution of electrons in our simulations along Mariner10 and BepiColombo first-Mercury-flyby trajectories, we propose that both spacecraft observed this energetic quasi-trapped electron population around closest approach.
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