Scattering function for energetic neutral hydrogen atoms off the lunar surface

Abstract

The Sub‐keV Atom Reflecting Analyzer instrument on board the lunar orbiter Chandrayaan‐1 provided a large number of measurements of lunar energetic neutral atoms (ENAs). These ENAs were formerly solar wind ions, which were neutralized and backscattered from the lunar surface. The angles under which the ENAs are scattered strongly depend on the solar wind ions' incidence angle, which corresponds to the solar zenith angle (SZA). Our large dataset provides us with a complete coverage of the SZA and almost complete coverage of the scattering angles. When combining all available measurements, four distinct features are discernible with SZA increase: amplitude decrease, less azimuthal uniformity, bigger ratio of sunward versus anti‐sunward flux and shallower scattering. We analyzed more than 290′000 measurements and derived a mathematical description of the features and their dependencies on the SZA.

Energetic neutral atom observations of magnetic anomalies on the lunar surface

“…To determine an anomaly's shielding efficiency (the percentage by which a magnetic anomaly is able to reduce the backscattered ENA flux), we first had to fit our normalized ENA flux with the angular scattering function of solar wind protons backscattered as ENAs presented in Schaufelberger et al [2011] to obtain a reference curve for comparison. The scattering function comprises of four ad hoc functions, each of which describes the variation of one of the following observed features: (1) amplitude, (2) azimuthal uniformity, (3) ratio of sunward versus antisunward flux, and (4) mean polar scattering.…”

“…The normalized ENA flux and the scattering function are linked to each other by the sub‐solar reflection ratio [ Schaufelberger et al , 2011], for which the global average value has been recently evaluated to be 0.19 − 0.03 + 0.02 [ Futaana et al , 2012]. Since we are interested in the local variation of the ENA flux we fitted the amplitude of the scattering function to the data, which is the only fit parameter in this procedure.…”

“…We then compared our ENA flux to the total magnetic field strength measured by the Lunar Prospector magnetometer at an altitude of 30 km and presented by Richmond and Hood [2008]. Figure 2a shows the integral neutral hydrogen flux from a surface element derived from the CENA measurements on 17 June 2009 using the angular scattering function [ Schaufelberger et al , 2011] and applying a normalization with the solar wind proton flux. Figure 2b shows a gray scale albedo map provided by the Clementine spacecraft for context, and Figure 2c the magnetic field strength measured by the Lunar Prospector magnetometer at an altitude of 30 km [ Richmond and Hood , 2008].…”

Emission of energetic neutral atoms from water ice under Ganymede surface-like conditions

“…The relative difference between Y ENA and the total sputtering yield is larger for impinging ion energies below a few keV because fewer energetic neutrals with energies above E min are generated ( Wurz and Lammer, 2003;Wurz et al, 2007 ). Additionally, at low impinging ion energy, the obtained Y ENA is more uncertain due to the assumption of the energetic neutral flux being emitted according to a cos 4/3 θ angular distribution; e.g., for protons impinging on lunar regolith, a tri-modal angular energetic neutral atom flux distribution with an enhancement in both the specular reflection and impinging direction is observed ( Schaufelberger et al, 2011 ). In our data, Y ENA exceeds the model predicted total sputtering yield slightly for impinging hydrogen ions above 20 keV, which reflects the uncertainty in the scaling of the model to our application.…”

“…5 : incident oxygen ions with 3 keV energy create recoil hydrogen ENAs. Their upper energy limit is well modeled by a single elastic collision of the O + projectile with a hydrogen atom on the surface ( Niehus et al, 1993 ). In general, collisional sputtering dominates at low energies, with total yields that are fairly independent of incident energy ( Johnson, 1998 ).…”