Abstract | 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 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). 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.
This presentation will review the sciences that we aim to tackle by ENA imaging at Mercury. From the experience of the observations at the Earth's Moon, we place the sciences into a comparative context. The ENA sciences 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?
Global ENA imaging:
Does the solar wind reach the dayside surface globally? Under what conditions does it happen?
We will also present the first-light data of the MPPE/ENA instrument onboard the BepiColombo/MMO during the Earth, Venus, and the first Mercury flybys and interplanetary cruising.
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