Solar energetic particle event at Mercury

Mercuryʹs magnetosphere only partially protects its surface from solar particles. When a solar energetic particle (SEP) event encounters Mercury, a significant flux of energetic particles will reach Mercuryʹs surface which can change Mercuryʹs exosphere. In this paper, we describe some of the consequences of the encounter of a SEP event with Mercury before and few hours after the shock associated with such an event reaches Mercury. Energetic ions and electrons with energy between 10 keV and 10 MeV are followed across a model of Mercuryʹs magnetosphere (J. Geophys. Res. 103 (1998) 9113). The effects of such an encounter on the ion and neutral exospheres are estimated for one particular gradual proton event reported by Reames et al. (Astrophys. J. 483 (1997a) 515) and scaled to Mercuryʹs orbit. After the arrival of these SEP at Mercury, a population of quasi-trapped energetic ions and electrons is expected close to Mercury which is stable for hours after their arrival at Mercury. A significant dawn/dusk charge separation is observed. A fraction of the initial energetic particles (∼10%) impact the surface with a spatial distribution that exhibits north/south and dawn/dusk asymmetries. The flux of particles impacting the surface and the ability of a quasi-trapped population to be maintained near Mercury are highly dependent on the Bz sign of the interplanetary magnetic field. The impacting SEP can eject a non-uniform distribution of sodium atoms into Mercuryʹs exosphere and can be the origin of several exospheric features observed during last decade.