2011 ARTEMIS SCIENCE NUGGETS
Observations of Lunar Surface Charging from Long Distances by ARTEMIS in the Geomagnetic Tail
by J.S. Halekas, UC Berkeley Space Sciences Laboratory
Introduction
ARTEMIS-P2, during an early lunar encounter, passed earthward from the Moon in the terrestrial magnetotail. Though the spacecraft trajectory only brought it within 8,000 km away from the dayside lunar surface, magnetic field lines connected the two points, allowing charged particles to flow from the Moon to the spacecraft. Using a technique pioneered for Lunar Prospector data analysis, we were able to use the measured electron distributions at the spacecraft to remotely sense lunar surface charging.
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| Figure 1. Cartoon of the magnetic field geometry during the lunar flyby (not to scale). |
Results
During magnetically connected intervals, ARTEMIS made the first accurate and quantitative remote measurements of lunar surface charging, from an observation point almost one hundred times more distant than previous remote measurements of surface potentials. ARTEMIS also measured the incident plasma populations responsible for charging the lunar surface. The large variability of these sources in the Earth’s magnetotail leads to highly variable charging currents to the surface. ARTEMIS measurements provide surprising evidence for negative dayside surface potentials (distribution of measurements shown in Figure 2), likely indicative of non-monotonic sheath potentials above the sunlit surface, in the plasma sheet and - for the first time - in the tail lobe. These non-monotonic potentials, and the resulting accelerated outward-going beams of lunar photoelectrons, may help maintain quasi-neutrality along magnetic field lines connected to the Moon.
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| Figure 2. Top Panel. Inferred magnitude of negative surface potential vs. electron temperature , colored by the solar zenith angle at the point of magnetic connection to the surface. Bottom Panel. Ratio between magnitude of negative surface panel and electron temperature, as a function of solar zenith angle. Red points in top panel, corresponding to points at SZA=100 in bottom panel, indicate surface potential values inferred from earthward-going electron beams measured at times when a straight line trace of the measured field vector does not intersect the Moon, likely indicating field line curvature. |
Conclusion
Simple probe theory states that the dayside lunar surface should charge positive due to photoemission. However, these new ARTEMIS data provide convincing evidence that the space charge built up by photoelectron emission actually warps the potential structure above the lunar surface, allowing the surface to charge negative even in sunlight. This produces beams of photoelectrons accelerated from the surface, which can have significant effects on the ambient plasma, perhaps even reaching all the way to the Earth.
Reference
J.S. Halekas, G.T. Delory, W.M. Farrell, V. Angelopoulos, J.P. McFadden, J.W. Bonnell, M.O. Fillingim, F. Plaschke, First remote measurements of lunar surface charging from ARTEMIS: Evidence for non-monotonic sheath potentials above the dayside surface, J. Geophys. Res., 116, A07103, doi:10.1029/2011JA016542, 2011.Biographical Note
Jasper Halekas is a Research Scientist at the U.C. Berkeley Space Sciences Laboratory. He has been studying the Moon since he was a graduate student, working primarily with Lunar Prospector data. He is currently working on hardware for the MAVEN and Solar Probe Plus missions, and spending as much time on ARTEMIS data analsis as he can.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong@igpp.ucla.edu