Energetic neutral atom imaging of Mercuryʹs magnetosphere 2. Distribution of energetic charged particles in a compact magnetosphere

In order to investigate the feasibility of energetic neutral atom (ENA) imaging of Mercuryʹs magnetosphere interacting with the tenuous exosphere, we developed a simple model simulating dynamics of hot proton plasma near Mercury. We did not consider specific acceleration mechanisms but parameterised the proton source by its location, size and the distribution function of the injected protons. The characteristics of the source are free parameters of the model and can be easily varied. To model the magnetic field in the magnetosphere we used the Tsyganenkoʹs model scaled for Mercuryʹs conditions. The dynamics of energetic protons 1 keV<E<50 keV are calculated in the drift approximation taking into account the convective electric field produced by the solar wind interaction with the magnetosphere. This enabled to establish the particle steady-state distribution. As for more energetic protons, E>50 keV, the large Larmour radius results in their almost immediate loss via intercepting the Hermean magnetopause. The model was tested for 10, 30, and 50 keV protons injected from a source of the size 1Rm×1Rm located at 3Rm, 2.5Rm, 2Rm, and 1.5Rm in the tail (Rm is the Hermean radius). This simple model turned out to be rather useful and flexible, and can be used to study proton dynamics in Mercuryʹs magnetosphere.