2017 ARTEMIS SCIENCE NUGGETS
ARTEMIS Observations of Proton Reflection from Lunar Crustal Magnetic Fields
by A. R. Poppe,
Space Sciences Laboratory, U.C. Berkeley
Introduction
While the Moon lacks a global, dipolar magnetic field like the Earth, it does possess a collection of smaller and somewhat weaker regions of crustal magnetic fields, termed 'magnetic anomalies'. First discovered in the Apollo era, these magnetic anomalies have fascinated scientists, both in terms of their as-of-yet unexplained origin and their complex interactions with the space plasma environment at the Moon. Observations taken in the last decade by a fleet of international spacecraft revealed that crustal magnetic fields are associated with fluxes of reflected solar wind protons. Such observations were at first surprising, given that theories at the time predicted these anomalies should be too weak to turn around the solar wind. Since then, continued observations [e.g., Lue et al., 2011] and concurrent plasma simulations [e.g., Poppe et al., 2012; Jarvinen et al., 2014; Fatemi et al., 2015] have suggested a new and complex mode of interaction whereby crustal magnetic fields induce for the formation of strong electrostatic fields when subjected to high energy proton fluxes, such as the solar wind. These electrostatic fields then efficiently reflect part of the solar wind before it can impact the lunar surface. In this study, we combined more than five years of observations of reflected solar wind protons as observed by the twin ARTEMIS spacecraft. We generated global maps of the fraction of solar wind proton reflection and studied the directions in which protons are scattered as they interact with the crustal magnetic and electrostatic fields.
ARTEMIS Observations
Figure 1 shows a typical ARTEMIS observation of solar wind protons reflected from lunar crustal magnetic anomalies. Panels 1(a) and (b) show the lunar position in geocentric coordinates and the ARTEMIS probe positions in selenocentric coordinates, respectively. During this observation, the Moon was in the solar wind and ARTEMIS P1 transited across the dayside lunar surface while ARTEMIS P2 remained away from the Moon. Panels 1(c) and (d) show the ion energy spectrum observed by P1 and P2 respectively. Both P1 and P2 observe the solar wind proton beam near 400 eV while P1 observes an additional flux of protons both above and below the solar wind energy. These additional protons have been reflected from crustal magnetic anomalies on the Moon and scattered downstream into the solar wind, where P1 observes them. Panels 1(e-h) show the interplanetary magnetic field, convection electric field, P1 and P2 altitudes, and the P1 solar zenith angle, respectively, for reference.
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| Figure 1. A typical observation of solar wind protons reflected from lunar crustal magnetic anomalies as observed by ARTEMIS. |
Global Mapping of Proton Reflection
By collecting all observations of reflected protons similar to that shown in Figure 1 over the 5.5 years of the ARTEMIS mission to date, we can construct a global map of proton reflection from the Moon. To achieve this, we used a backwards Liouville tracing technique, whereby the trajectory of each measurement of ion flux at the ARTEMIS probes is integrated backwards in time until the trajectory either strikes the Moon or leaves the lunar vicinity entirely. Those trajectories that struck the Moon are indicative of protons that scattered from the lunar surface or crustal magnetic fields. In total, we traced over 1 billion individual trajectories of reflected ion flux near the Moon, yielding a robust dataset. This dataset can then be straightforwardly integrated to calculate the map of reflected flux at each location on the Moon, shown in Figure 2. Black contours in Figure 2 show the strength of the lunar surface crustal magnetic fields at 10, 20, and 50 nT levels while the color shows the fraction of reflected proton flux relative to the incoming solar wind flux. Inspection of Figure 2 shows a high (although not perfect) degree of correlation between surface crustal magnetic fields and elevated levels of proton reflection. The region of strongest reflection of solar wind protons, ~12%, is near 180 degrees longitude coincident with the relatively large and strong South Polar/Aitken Basin magnetic anomalies. Other regions of magnetism on the lunar surface are typically associated with solar wind protons reflection between 1 and 10%. Interestingly, not all regions of crustal magnetic field are associated with strong reflection; this could be due to the particular geometry of any individual crustal field, which may affect the manner in which electrostatic fields are built up and in turn, solar wind protons reflected.
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| Figure 2. The global map of the percentage of reflected solar wind proton flux as compiled from more than five years of ARTEMIS observations at the Moon. Black contours denote surface crustal magnetic field strengths of 10, 20, and 50 nT, respectively, while colors denote the fraction of reflected solar wind flux. |
Conclusion
Using over 5 years of ARTEMIS data at the Moon, we have constructed a global map of proton reflection from the lunar surface and crustal magnetic fields. This effort extends mapping of proton reflection from the Moon by previous missions such as the Chandrayaan-1 spacecraft [Lue et al., 2011]. Our proton reflection map shows that the strongest regions of lunar crustal fields can reflect more than 10% of the solar wind before it strikes the lunar surface, most likely via the development of electrostatic fields within the anomaly, as suggested by simulations [e.g., Poppe et al., 2012; Jarvinen et al., 2014; Fatemi et al., 2015]. Future work with the dataset of reflected protons will explore the variability in the fraction of reflected solar wind flux as a function of upstream solar wind conditions such as density, velocity, and pressure. Such changes in the upstream conditions may affect the microphysics involved in reflecting solar wind protons from crustal fields and thus, knowledge of such changes may reveal the fundamental ways in which these fields interact with ambient plasma.
References
Poppe, A. R., J. S. Halekas, C. Lue, and S. Fatemi, ARTEMIS observations of the solar wind proton scattering function from lunar crustal magnetic anomalies, J. Geophys. Res.: Planets, 122, 2017Additional References
Lue, C., et al., Strong influence of lunar crustal fields on the solar wind flow, Geophys. Res. Lett., 38(L03202), 2011Poppe, A.R., et al., Particle-in-cell simulations of the solar wind interaction with lunar crustal magnetic anomalies: Magnetic cusp regions, J. Geophys. Res., 117, 2012
Jarvinen, R., et al., On vertical electric fields at lunar magnetic anomalies, Geophys. Res. Lett., 41, 2014
Fatemi, S., et al., Solar wind plasma interaction with Gerasimovich lunar magnetic anomaly, J. Geophys. Res.: Space Physics, 120, 2015
Biographical Note
Andrew Poppe is an Assistant Research Scientist in the Space Physics Research Group at the Space Sciences Laboratory / UC Berkeley. His research includes analysis of ARTEMIS data regarding solar wind and terrestrial magnetotail interactions with the Moon, as well as modeling and data comparison of interplanetary and planetary dust dynamics. See http://research.ssl.berkeley.edu/~poppe/ for more information.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu