The dynamic response of Mercury’s magnetosphere to solarwind forcing: consequences for the acceleration of chargedparticles in a telluric planetary environment

Most global simulations of Mercury’s magnetosphere assume a steady solar wind. However, the planet's extremely small magnetosphere responds on characteristic timescales of only a few minutes, making it inherently sensitive to solar-wind variability. Such rapid temporal forcing is expected to generate a highly dynamic magnetospheric environment, favorable to the development of plasma instabilities. These processes can facilitate electron entry into the magnetosphere, their subsequent trapping and acceleration and, in some cases, their direct interaction with Mercury’s exosphere and surface.

To investigate these effects, we perform global magnetohydrodynamic (MHD) simulations of Mercury’s magnetospheric response to both steady and time-dependent solar-wind conditions using the spherical code PLANET_MAG_AMRVAC. These simulations are designed to support the upcoming science phase of the BepiColombo mission, scheduled to begin in less than a year, by predicting magnetospheric and exospheric observables accessible to instruments onboard both Mio and MPO under a broad range of solar-wind conditions.

As a first step, the model was tested through comparisons with electron density measurements obtained by the MEA1 instrument onboard Mio during Mercury’s three firsts flybys. The simulations include a planetary plasma source driven by the photoionization of exospheric neutrals, allowing for a more realistic representation of plasma populations in Mercury’s near-planet environment. In parallel, the code was used to predict the quasi-thermal noise spectra measurable by the SORBET instrument during flybys, when the antennas cannot be fully deployed during the cruise phase.

Building on this foundation, the focus of this work is the impact of transient solar wind structures -- for now magnetic holes or vortices -- on Mercury’s magnetosphere. Particular attention is paid to their transmission through the bow shock, their evolution within the magnetosheath, and the conditions under which they can penetrate into the magnetosphere.

To address these questions, we first adopt a global MHD approach to capture the large-scale dynamics and overall morphology of these events. We then aim to confront these results with kinetic or particle-in-cell (PIC) simulations in order to explore the associated small-scale physics beyond the MHD framework. Finally, we outline ongoing and future work involving the injection of test particles, treated within the guiding-center approximation, in selected regions of interest. This approach will allow us to investigate particle transport, acceleration, and loss processes in Mercury’s magnetosphere under different solar-wind disturbance scenarios.