Scattering characteristics and imaging of energetic neutral atoms from the Moon in the terrestrial magnetosheath

Abstract

We study hydrogen energetic neutral atom (ENA) emissions from the lunar surface, when the Moon is inside the terrestrial magnetosheath. The ENAs are generated by neutralization and backscattering of incident protons of solar wind origin. First, we model the effect of the increased ion temperature in the magnetosheath (>10 times larger than that in the undisturbed solar wind) on the ENA scattering characteristics. Then, we apply these models to ENA measurements by Chandrayaan‐1 and simultaneous ion measurements by Kaguya at the Moon, in the magnetosheath. We produce maps of the ENA scattering fraction, covering a region at the lunar near‐side that includes mare and highland surfaces and several lunar magnetic anomalies. We see clear signatures of plasma shielding by the magnetic anomalies. The maps are made at different lunar local times, and the results indicate an extended influence and altered morphology of the magnetic anomalies at shallower incidence angles of the magnetosheath protons. The scattering fraction from the unmagnetized regions remains consistent with that in the undisturbed solar wind (10%–20%). Moreover, the observed ENA energy spectra are well reproduced by our temperature‐dependent model. We conclude that the ENA scattering process is unchanged in the magnetosheath. Similarly to the undisturbed solar wind case, it is only magnetic anomalies that provide contrast in the ENA maps, not any selenomorphological features such as mare and highland regions.

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

“…The padding is added to allow a certain uncertainty in the back tracing, as well as an uncertainty in the effective area‐of‐influence of the LMAs relative to their crustal location (the area shielded by an LMA may, e.g., be shifted downstream relative to the solar wind flow; c.f. Deca et al, ; Lue et al, ; Zimmerman et al, ). The resulting regions are comparable to maps of LMA‐reflected H + observations (Lue et al, ; Poppe et al, , ).…”

“…By using empirical models for the H ENA scattering properties (e.g., Futaana et al, ; Schaufelberger et al, ), it is possible to remotely measure the precipitating plasma flux (Futaana et al, ) and flow speed (Futaana et al, ). Such measurements are of special interest wherever the plasma precipitation is modified at altitudes too low to directly measure with spacecraft, such as at the lunar magnetic anomalies (LMAs; Futaana et al, ; Harada et al, ; Lue et al, ; Saito et al, ; Wieser et al, ; Vorburger et al, , , ).…”

“…By using empirical models for the H ENA scattering properties (e.g., Futaana et al, 2012;Schaufelberger et al, 2011), it is possible to remotely measure the precipitating plasma flux (Futaana et al, 2006) and flow speed . Such measurements are of special interest wherever the plasma precipitation is modified at altitudes too low to directly measure with spacecraft, such as at the lunar magnetic anomalies (LMAs; Futaana et al, 2013;Harada et al, 2014;Lue et al, 2016;Saito et al, 2012;Wieser et al, 2010;Vorburger et al, 2012Vorburger et al, , 2013Vorburger et al, , 2015. Scattered H + (just like H + reflected from LMAs, e.g., Lue et al, 2011;Poppe et al, 2017;Saito et al, 2010) are important for the lunar plasma environment.…”

Volatiles and Refractories in Surface-Bounded Exospheres in the Inner Solar System

“…Therefore, it is possible to trace detected ENAs back to their place of origin (much more easily than for ions) and to build a map of locations where ENAs are reflected (e.g. Vorburger et al 2015;Lue et al 2016; see Fig. 17).…”

The Plasma Environment Surrounding the Reiner Gamma Magnetic Anomaly

“…may be highly non-adiabatic (Chu et al, 2021;Howes et al, 2015;Kallio et al, 2012;Lue et al, 2016;Poppe et al, 2012;Saito et al, 2012). Driven by electron dynamics and the presence of an associated ambipolar electric field above primarily horizontal magnetic field geometries, a mini-magnetosphere may form and a significant fraction of the ion population population may be deflected away from the surface (Bamford et al, 2012;Deca et al, 2014Deca et al, , 2015Hemingway & Garrick-Bethell, 2012;Kurata et al, 2005;Lin et al, 1998;Poppe et al, 2016;Usui et al, 2017;Zimmerman et al, 2015).…”