Energetic neutral atom imaging at Mercury Original Research Article

The paper reviews recent developments in the simulation of energetic neutral atom (ENA) imaging near Mercury. ENAs are produced in a wide energy range, from a few eV and up to 50 keV by: (1) charge-exchange of ions injected from the tail to the inner magnetosphere; (2) charge-exchange of freshly ionized exospheric ions accelerated by the magnetospheric electric field; (3) sputtering from the planetary surface by precipitating energetic ions. Using a scaled Tsyganenko 89 magnetic field model and assuming a convection electric field of 10 mV/m, we follow the trajectories of ions injected from the tail and Na+ photoions from the exosphere to obtain their global distribution in the magnetosphere and precipitation maps. That allows us to estimate the ENA fluxes in each energy range and understand the general morphology of ENA images. The high variability of the Hermean magnetosphere gives rise to pulsating ENA emissions (ENA “flashes”), that can be used to study the global dynamics of the magnetosphere. Sputtered ENAs display the particle fluxes impinging on the surface in a very similar way as Earthʹs upper atmosphere visualizes precipitating electrons displaying auroras. One can thus speak in terms of an “ENA aurora” on Mercury. Sputtering results in the production of very low energy neutral atoms (VLENA) with large masses (K, Na, Ca) and energy less than 10 eV. The VLENA trajectories experience gravitation bending making it possible to image regions behind the limb of the planet. We conclude the review with a summary of the Mercury ENA environment characteristics and implications for ENA instrumentation.