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. 2018 Jul;123(7):5289-5299.
doi: 10.1029/2018ja025486. Epub 2018 Jun 26.

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

Affiliations

ARTEMIS Observations of Solar Wind Proton Scattering off the Lunar Surface

C Lue et al. J Geophys Res Space Phys. 2018 Jul.

Abstract

We study the scattering of solar wind protons off the lunar surface, using ion observations collected over 6 years by the ARTEMIS satellites at the Moon. We show the average scattered proton energy spectra, directional scattering distributions, and scattering efficiency, for different solar wind incidence angles and impact speeds. We find that the protons have a scattering distribution that is similar to existing empirical models for scattered hydrogen energetic neutral atoms, with a peak in the backward direction (toward the Sun). We provide a revised model for the scattered proton energy spectrum. We evaluate the positive to neutral charge state ratio by comparing the proton spectrum with existing models for scattered hydrogen. The positive to neutral ratio increases with increasing exit speed from the surface but decreases with increasing impact speed. Combined, these counteracting effects result in a scattering efficiency that decreases from ~0.5% at 300 km/s solar wind speed to ~0.3% at 600 km/s solar wind speed.

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Figures

Figure 1.
Figure 1.
Analysis example of ARTEMIS iESA observations of scattered protons from the Moon for a single data package from 26 November 2011 10:16:06. After back-tracing the proton trajectories from the spacecraft, those trajectories with a <300-km trace path to a point on the unmagnetized lunar surface are recorded. Panel (a) shows these traced ion trajectories between the spacecraft and the surface source points. Panel (b) shows the source points on the lunar surface. The white regions in panels (a) and (b) show the locations of the lunar magnetic anomaly (LMA) regions. The LMA regions are avoided in this study. The coordinate system in panels (a) and (b) is selenocentric solar ecliptic (SSE). Note that the LMA regions rotate in this frame according to the lunar phase, and that the shown example is near New Moon. Panel (c) shows the proton energy spectrum (given in differential flux, jde) as measured at the spacecraft and as inferred at the surface point according to the tracing. Solid lines in panel c indicate the standard error of the mean based on measurement variance and dashed lines indicate the Poisson error estimate (accounting for background count reduction), visible only where it is the greater error estimate. Panel (d) shows the definition of the scattering angles (azimuth, elevation, SZA) used herein, where SZA is the solar-zenith angle. Panel (e) shows the inferred scattering directions and directional fluxes (jd) at the surface for the measured particles.
Figure 2.
Figure 2.
Scattered proton energy spectra of inferred differential flux (jde) at the traced source point on the lunar surface, averaged over all data used in this study, separated into parameter bins with respect to solar wind speed (vsw) and solar-zenith angle (SZA). The error bars are defined as in Figure 1. The dark shaded regions represent energies above the mean solar wind incidence energy, and the dotted vertical lines represent 50% of the latter value.
Figure 3.
Figure 3.
Proton scattering distributions in directional flux (jd) illustrated in the same way as in Figure 1e, averaged over all data used in this study, separated into parameter bins with respect to solar wind speed (vsw) and solar-zenith angle (SZA).
Figure 4.
Figure 4.
Proton scattering rate versus solar wind speed (vsw) and solar-zenith angle (SZA), with a vsw-bin width of 50 km/s, estimated in two different ways (see Section 4). The error bars shown for Method 1 represent the standard error of the mean based on measurement variance. The horizontal lines show the results from Method 1, averaged over the nine vsw bins, where the dotted horizontal lines represent the standard-error of the mean.
Figure 5.
Figure 5.
Scattered proton energy spectra as in Figures 1 and 2, for a single solar zenith angle bin of 0°–75° and five solar wind speed (vsw) bins. Also plotted are the empirical models for neutral hydrogen from Futaana et al. (2012; F2012), for protons from Lue et al. (2014; L2014) and the L2014 model refitted to the ARTEMIS observations.
Figure 6.
Figure 6.
Analysis of the fitted spectrum model results and parameter results. Panel (a) shows the model results for the charge-state fraction as a function of exit speed and impact speed, calculated from the scattered H+ spectrum divided by the scattered HENA energy spectrum, given by the spectrum model from Futaana et al. (2012). Panel (b) shows the model results (spectrum fit) for the H+ scattering efficiency, compared with trapezoidal integration of the observations (ARTEMIS) and the model from Lue et al. (2014; L2014). Panel (c) shows the fitted model parameters kTENA and vc, compared with the model for kTENA (vsw) from Futaana et al. (2012; F2012). A curve corresponding to kTENA = 0.12·Esw (where Esw is the solar wind energy) is also plotted for reference.
Figure 7.
Figure 7.
Directional scattering distributions as in Figures 1 and 3, for three solar zenith angle bins. The columns respectively show the empirical model for neutral hydrogen from Vorburger et al. (2013; V2013), the observed proton scattering distribution (including observations for solar wind speeds of 225–675 km/s), the V2013 model refitted to the ARTEMIS observations, and the resulting ratio between the proton and hydrogen models.

References

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