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Modeling Energetic Neutral Atom Emission from Solar Wind Backscattering – Lessons from the Moon for Mercury

Presentation #116.09 in the session Mercury (Poster + Lightning Talk)

Published onOct 23, 2023
Modeling Energetic Neutral Atom Emission from Solar Wind Backscattering – Lessons from the Moon for Mercury

Observations by several spacecraft have shown that about 10 – 20% of precipitating solar wind protons are backscattered from the lunar regolith, mostly as energetic neutral atoms (ENAs) [1,2]. The scattering process is influenced by properties of the impacting particles as well as the surface and thus, ENA studies represent an opportunity to probe both. This will especially be of interest for Mercury, where knowledge of magnetospheric ion fluxes impacting the surface as well as of regolith properties is limited. BepiColombo will study Mercury’s ENA environment with instruments onboard both the MPO and MMO spacecraft [3,4]. However, a good model of the backscattering process is still needed to correctly deduce features of the magnetosphere and the surface from these ENA measurements.

To address this, we apply a regolith grain stacking implementation in the ion-surface interaction code SDTrimSP-3D to model the scattering of solar wind protons from planetary regolith [5,6]. Comparing to lunar ENA observations from Chandrayaan-1 and IBEX, the model reproduces all major ENA characteristics [7], such as the total backscattering probability, preferential backwards scattering, and a broad energy loss. Using our model, the reflection probability is shown to significantly depend on the regolith porosity at the surface, which we can constrain to the high value of about 85% ± 15% [6]. Modeling ENA fluxes from the surface of Mercury shows that the orbits of BepiColombo’s modules will allow ENA studies on both local and global scales. We will discuss further applications and challenges that will need to be considered for future observations at Mercury.

References:

[1] D.J. McComas et al. (2009), Geophys. Res. Lett. 36.12.

[2] M. Wieser et al. (2009), Planet. Space Sci. 57, 2132-2134.

[3] S. Orsini et al. (2021), Space Sci. Rev. 217, 1-107.

[4] Y. Saito et al. (2021), Space Sci. Rev. 217,70.

[5] U. Von Toussaint, A. Mutzke, A. Manhard (2017) Phys. Scr., 2017, 014056.

[6] P.S. Szabo et al. (2022), Geophys. Res. Lett. 49, e2022GL101232.

[7] P.S. Szabo et al. (2023), submitted to JGR: Planets.

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