Energetic Neutral Atom (ENA) Emission Characteristics at the Moon and Mercury From 3D Regolith Simulations of Solar Wind Reflection

Abstract

The reflection of solar wind protons as energetic neutral atoms (ENAs) from the lunar surface has regularly been used to study the plasma‐surface interaction at the Moon. However, there still exists a fundamental lack of knowledge of the scattering process. ENA emission from the surface is expected to similarly occur at Mercury and will be studied by BepiColombo. Understanding this solar wind backscattering will allow studies of both Mercury's plasma environment as well as properties of the hermean surface itself. Here, we expand on previous simulation studies of the solar‐wind‐regolith interaction with 3D grains in SDTrimSP‐3D to compare the predicted scattering energies and angles to ENA measurements from the Moon by the Chandrayaan‐1 and IBEX missions. The simulations reproduce a backward emission toward the Sun, which can be connected to the geometry of the regolith grain stacking. In contrast, the ENA energy distribution and its Maxwellian shape is mostly connected to the solar wind velocity. Our simulations also correctly describe a lunar ENA albedo between 10% and 20% and support its decrease with solar wind velocity. We further expand our studies to illustrate how BepiColombo will be able to observe ENAs at Mercury using hybrid simulations of Mercury's magnetosphere as an input for the complex surface precipitation patterns. We demonstrate that the variable ion precipitation will directly influence ENA emission from the surface. The orbits of BepiColombo's Mercury Planetary Orbiter and Mercury Magnetospheric Orbiter/Mio spacecraft are shown to be suitable to observe ENA emission patterns both on a local and a global scale.

Plain Language Summary

The Sun emits a continuous stream of charged particles, the solar wind. Recently, it was observed that these solar wind particles get reflected from the Moon in a similar way as it is the case for light from the Sun. The reflection is the result of a large number of collisions between the solar wind particles and atoms at the Moon's surface. This is expected to occur similarly at the planet Mercury. To better understand this process, we present a theoretical model of the reflection process. Our simulations take into account all the collisions with atoms, as well as the geometry of the lunar soil as a large number of loosely stacked grains. The model achieves very good agreement with most of the observations from the Moon, showing that sunward scattering and the scattering probability can be explained by the porous regolith. Consequently, we also sketch how particle scattering would occur at Mercury, where the solar wind only reaches the surface at select locations due to the planet's magnetic field. Our results confirm the variability of the scattered particles around Mercury and that the BepiColombo mission is well‐suited to observe them for imaging solar wind impacts onto the surface.

Key Points

• We model the solar wind proton reflection from lunar regolith grains to study energetic neutral atom (ENA) emission characteristics

• ENA emission angles are successfully connected to the regolith geometry, while ENA energies are mostly determined by the solar wind energy

• Expanding the model to Mercury, we predict ENA fluxes that will be observed by BepiColombo