Magnetospheric electric field effects on ion precipitation at Mercury: BepiColombo/MIPA observations

The precipitation of charged particles onto Mercury's surface is a key process in the Hermean system, as it is one of the main loss mechanisms for magnetospheric particles as well as results in surface sputtering, an important source of charged and neutral particles in the Hermean magnetosphere. These processes strongly depend on the properties of the precipitating particles, in particular their energy, which controls not only how they interact with the surface but also where precipitation occurs.

In this study, we use single-particle tracing numerical simulations to investigate the effect of the global magnetospheric electric field on precipitating particle properties. Ion measurements made by the Miniature Ion Precipitation Analyzer (MIPA) instrument onboard BepiColombo during its second Mercury flyby are used as inputs for our model.

Using these observed ion distributions, we show that the inclusion of an electric field in the simulations generates a dawn-dusk asymmetry in both the energies of the precipitated ions and the precipitating flux. Furthermore, we demonstrated that the presence of an electric field allows for increased dayside ion precipitation, created by low-energy ions transported towards the dayside by the E x B drift. This mechanism produces low-latitude and dayside precipitation regardless of upstream conditions, e.g. solar wind dynamic pressure or interplanetary magnetic field (IMF) orientation, which contrasts with results from hybrid simulations.

The charged particles traced in this work will eventually interact with Mercury's surface, inducing e.g. the release of Energetic Neutral Atom (ENA) stripped from the surface which could be monitored by the ENA instrument onboard BepiColombo. This study therefore illustrates that the combined analysis of ion and ENA measurements together with particle tracing simulations provides a powerful framework to better understand the plasma circulation in Mercury's magnetosphere.