Modelling Mercury's magnetosphere and plasma entry through the dayside magnetopause

Owing to the next space mission Messenger (NASA) and BepiColombo (ESA/JAXA), there is a renewed interest in modelling the Mercury's environment. The geometry of the Mercury's magnetosphere, as well as its response to the solar wind conditions, is one of the major issues. The weak magnetic field of the planet and the increasing weight of the IMF BX component at Mercury's orbit, introduce critical differences with respect to the Earth's case, such as a strong north-south asymmetry and a significant solar wind precipitation into the dayside magnetosphere even for non-negative IMF BZ. With the aim of analysing the interaction between the solar wind and Mercury's magnetosphere, we have developed an empirical-analytical magnetospheric model starting from the Toffoletto-Hill TH93 code. Our model has been tuned to reproduce the key features of the Mariner 10 magnetic data, and to mimic the magnetic field topology obtained by the self-consistent hybrid simulation developed by Kallio and Janhunen [Solar wind and magnetospheric ion impact on Mercury's magnetosphere. Geophys. Res. Lett. 30, 1877, doi: 10.1029/2003GL017842]. The new model has then been used to study the effect of the magnetic reconnection on the magnetosheath plasma entry through the open areas of the dayside magnetosphere (cusps), which are expected to be one of the main sources of charged particles circulating inside the magnetosphere. We show that, depending on the Alfvén speeds on both sides of the magnetopause discontinuity, the reconnection process would be able to accelerate solar wind protons up to few tens of keV: part of these ions can hit the surface and then trigger, via ion-sputtering, the refilling of the planetary exosphere. Finally, we show that non-adiabatic effects are expected to develop in the cusp regions as the energy gained by injected particles increases. The extent of these non-adiabatic regions is shown to be also modulated by upstream IMF condition.