Energetic Neutral Atom imaging at Mercury by MPPE/ENA on Mio: Instrument status and preliminary results from the initial operations

Mercury does not possess an atmosphere. The magnetospheric plasma is thus interacting with the surface directly. The solar wind may directly access the surface when its dynamic pressure is high. The existence of the surface governs the plasma dynamics at Mercury. For example, the surface behaves as the sink of the space plasma (magnetospheric ions, electrons, and solar wind plasma). In addition, the surface is also the source of plasma (sputtered ions, reflected ions, photoelectrons) and neutral atoms (sputtered constituents, reflected atoms at various energies). The ongoing physics is a complex, interdisciplinary solar wind–magnetosphere–surface–exosphere coupling. The energetic neutral atom (ENA) imaging, a powerful diagnostic tool to image the plasma dynamics in a remote-sensing way, is highly suitable for characterizing the coupling. The surface is the source of ENAs via the sputtering and scattering (reflecting) processes. Therefore, the ENA measurements provide information on the plasma characteristics at the surface (at 0 km altitude) on a global scale, which is very difficult to obtain via in-situ plasma measurements. The information is highly unique by itself to characterize the plasma–surface interaction at Mercury. In addition, the ENA data includes information about physics operated on the precipitating ions between the spacecraft and the surface. This will allow complementary investigation of the plasma dynamics by combining with the in-situ plasma measurements. The other relevant source of ENAs is charge-exchange of energetic ions with the Mercury’s exosphere. The exosphere is tenuous, but still sufficiently dense to make charge-exchange collisions happen and then generate ENAs. Due to the small size of the Mercury’s magnetosphere, the dayside magnetosphere is easily filled with ions injected from the tail, which makes magnetosphere more variable than Earth’s. ENA images on the day side expect to visualize the shape of the magnetosphere. In this presentation, we will give a short review of sciences that ENA imaging at Mercury can address. The major science cases include: Precipitating ions at the surface. Imaging of open/close field regions: How much are the ions precipitating on the surface? What is the difference in flux, energy, and constituents between the open and close field lines? What are the temporal and spatial variations of ion precipitation? Physics below the spacecraft altitude. Measuring the potential drops between the spacecraft and the surface: What are the characteristic energies of ions at the surface? How much does the field-aligned potential exist? What are the temporal and spatial variations of the potential drop? Shape and variability of the magnetosphere: What is the shape of the dayside magnetosphere? How variable is the magnetosphere? During the cruise phase of BepiColombo, the field-of-view of the MPPE/ENA instrument is partially blocked by the MOSIF, the sunshield structure that will be discarded upon MPO and Mio separation, and a few side pixels are looking towards space. The instrument has been operated during the Earth, Venus and Mercury flybys as well as interplanetary cruising phase. We will present the instrument status as well as preliminary flight data taken during flybys.