ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies

Abstract

Despite their small scales, lunar crustal magnetic fields are routinely associated with observations of reflected and/or backstreaming populations of solar wind protons. Solar wind proton reflection locally reduces the rate of space weathering of the lunar regolith, depresses local sputtering rates of neutrals into the lunar exosphere, and can trigger electromagnetic waves and small-scale collisionless shocks in the near-lunar space plasma environment. Thus, knowledge of both the magnitude and scattering function of solar wind protons from magnetic anomalies is crucial in understanding a wide variety of planetary phenomena at the Moon. We have compiled 5.5 years of ARTEMIS (Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun) observations of reflected protons at the Moon and used a Liouville tracing method to ascertain each proton's reflection location and scattering angles. We find that solar wind proton reflection is largely correlated with crustal magnetic field strength, with anomalies such as South Pole/Aitken Basin (SPA), Mare Marginis, and Gerasimovich reflecting on average 5-12% of the solar wind flux while the unmagnetized surface reflects between 0.1 and 1% in charged form. We present the scattering function of solar wind protons off of the SPA anomaly, showing that the scattering transitions from isotropic at low solar zenith angles to strongly forward scattering at solar zenith angles near 90°. Such scattering is consistent with simulations that have suggested electrostatic fields as the primary mechanism for solar wind proton reflection from crustal magnetic anomalies.

Figure 1.

(a–h) ARTEMIS observations of solar wind protons reflected from the South Pole-Aitken Basin anomaly on 2 July 2014 as described in the text.

Figure 2.

(a–g) A comparison of solar wind and nonsolar wind populations from both ARTEMIS probes on 2 July 2014.

Figure 3.

Backtracing results for ARTEMIS P1 observations on 2 July 2014 (Figure 1): (a) The surface crustal magnetic field strength [Purucker and Nicholas, 2010], (b) the number of observations traced back to each 5 × 5° latitude and longitude bin, and (c) the reflected proton flux relative to the incident solar wind flux. Red labels in Figure 3a denote prominent magnetic anomalies: Mare Marginis (MM), South Pole/Aitken Basin (SPA), Gerasimovich (GER), and Reiner Gamma (RG). In Figures 3b and 3c, contours denote the modeled lunar surface crustal magnetic field strengths of 10, 50, and 100 nT [Purucker and Nicholas, 2010].

Figure 4.

Backtracing results for 1602 compiled ARTEMIS observations: (a) The total number of observations per spatial bin, (b) the percentage of the reflected proton velocity distribution, $f_{\mathrm{sc}}^{\left(\varphi ,\theta \right)}$ (v), in each spatial bin, and (c) the average reflected solar wind proton flux. Data for Figure 4c are provided in the supporting information.

Figure 5.

The angular scattering functions for the South/Pole/Aitken Basin magnetic anomaly for different cuts in solar zenith angle. (a, d, and g) The number of observations per bin. (b, e, and h) The two-dimensional scattering function; (c, f, and i) the reduced, one-dimensional angular scattering function as a function of the azimuthal angle. The red dashed line in Figures 5f and 5i show the empirical fit, as discussed in the text.

Figure 6.

A cartoon of proton scattering from lunar magnetic anomalies for (top, middle, and bottom) different incident solar zenith angles.