Lunar Prospector observations of the electrostatic potential of the lunar surface and its response to incident currents

Abstract

[1] We present an analysis of Lunar Prospector Electron Reflectometer data from selected time periods using newly developed methods to correct for spacecraft potential and self-consistently utilizing the entire measured electron distribution to remotely sense the lunar surface electrostatic potential with respect to the ambient plasma. These new techniques enable the first quantitative measurements of lunar surface potentials from orbit. Knowledge of the spacecraft potential also allows accurate characterization of the downward-going electron fluxes that contribute to lunar surface charging, allowing us to determine how the lunar surface potential reacts to changing ambient plasma conditions. On the lunar night side, in shadow, we observe lunar surface potentials of $À100 V in the terrestrial magnetotail lobes and potentials of $À200 V to $À1 kV in the plasma sheet. In the lunar wake, we find potentials of $À200 V near the edges but smaller potentials in the central wake, where electron temperatures increase and secondary emission may reduce the magnitude of the negative surface potential. During solar energetic particle events, we see nightside lunar surface potentials as large as $À4 kV. On the other hand, on the lunar day side, in sunlight, we generally find potentials smaller than our measurement threshold of $20 V, except in the plasma sheet, where we still observe negative potentials of several hundred volts at times, even in sunlight. The presence of significant negative charging in sunlight at these times, given the measured incident electron currents, implies either photocurrents from lunar regolith in situ two orders of magnitude lower than those measured in the laboratory or nonmonotonic near-surface potential variation with altitude. The functional dependence of the lunar surface potential on electron temperature in shadow implies somewhat smaller secondary emission yields from lunar regolith in situ than previously measured in the laboratory.These new techniques open the door for future studies of the variation of lunar surface charging as a function of temporal and spatial variations in input currents and as a function of location and material characteristics of the surface as well as comparisons to the increasingly sophisticated theoretical predictions now available.

Mercury exosphere I. Global circulation model of its sodium component

“…Strong surface potentials have been recently reported at the Moon on the nightside (Halekas et al 2008; and are likely at Mercury also. Since it is well established that charging affects the mobility of sodium in glas s(e.g., Miotello and Mazzoldi 1982), then depletion or enhancement can occur in the exospheric surface layer.…”

“…SWS ejection efficiency may also strongly decrease with increasing surface temperature since SWS acts through both ion momentum transfer and ESD. Surface charging can be large at Mercury with surface potentials larger than 100 V by analogy with lunar observations, (Halekas et al 2008), and, therefore, could also significantly impact the SWS efficiency. However, without better knowledge of this mechanism, we cannot specifically test its importance.…”

“…They suggested it was due to enhanced diffusion of the Na in the regolith due to plasma ion bombardment. As a matter of fact, negative surface charging in the shadow at the Moon surface was recently measured to be 2 to 10 times higher in the plasma sheet than in the plasma lobes of Earth magnetosphere (Halekas et al 2008). Using all the available data on the Moon sodium exosphere, Sarantos et al (2008;) correlated the exospheric measurements with the available electron measurements at the Moon.…”

ARTEMIS Science Objectives

“…Using measurements of the angular distribution of reflected electrons and accelerated secondary electrons from the surface, Lunar Prospector provided estimates of negative nightside lunar surface potentials of ∼−100 V or less in the wake and magnetospheric tail lobes (Halekas et al 2002a(Halekas et al , 2008c, and occasionally as high as −2-4 kV in the magnetospheric plasmasheet (Halekas et al 2005(Halekas et al , 2008c and during SEP events (Halekas et al 2007). However, all Lunar Prospector measurements were fundamentally handicapped by a lack of spacecraft potential measurements and the spectrometer's rather coarse electron energy resolution.…”

Influence of Back Electrostatic Field on the Collection Efficiency of an Electrostatic Lunar Dust Collector

“…Due to the rarefied atmosphere of the moon and absence of a strong magnetic field, the lunar surface is not shielded from high energetic solar radiation and solar winds (Abbas et al, 2007;Colwell et al, 2009;Calle et al, 2011). While photoemissive radiation (e.g., UV and X-ray) on lunar dayside accumulate positive charges, impingement of electrons on lunar nightside leads to negative charge accumulation on lunar grains (Walch et al, 1995;Halekas et al, 2002;Halekas et al, 2008;Dove et al, 2010). The like-charged particles create a local electric field near the surface, which lifts the particles off from the lunar surface because of interparticle repelling forces (Stubbs et al, 2006;Colwell et al, 2009).…”