Lunar Surface Charging Research Articles

Simulations on Levitation and Spatial Distribution of Charged Dust on the Moon Surface

The levitation of charged dust, which may cause serious hazards to astronauts and lunar rovers, has been one of the most significant challenges in lunar exploration. Here we simulate lunar sheath potentials in different solar wind conditions and solar zenith angles (SZAs) on the lunar surface by the particle-in-cell method. The simulated potentials exhibit two types of distributions as a function of height, depending on the SZAs. For SZA ∼ 0°–70°, the nonmonotonic distribution with positive surface potential dominates in the photoelectron sheath. For SZA >∼81°, the monotonic distribution with negative surface potential is observed in the plasma sheath. With the calculated potentials and the assumption that the dust radius distribution exponentially decreases, we further investigate spatial distributions of the dust levitated above the surface. It is found that number density of the levitating lunar dust is enhanced at the terminator (SZA ∼ 81°) in the plasma sheath. In the photoelectron sheath it gradually decreases as the SZA increases from 0° to 70°. Further calculations of the potential and the derived electrostatic field suggest that the dust spatial distributions can be influenced by the bulk velocity, number density, and temperature of the solar wind. Those findings deepen our understanding of lunar surface charging and the mechanism of lunar dust levitation, which can provide technical support for lunar explorations.

Characteristics of Lunar Surface Electrons Inferred From ARTEMIS Observations: 1. Backscattered Electrons

AbstractLunar surface charging is a scientifically and practically important topic at the Moon that is largely determined by the electron currents near the surface. Among those electron populations, lunar photoelectrons (PHE) and backscattered electrons (BSE) produced by incident electrons that make up the high‐energy tail of lunar emitted electrons are not well characterized yet. Recently, Xu et al. (2021, https://doi.org/10.1029/2020je006790) reported oxygen Auger electron observations at the Moon by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun spacecraft, which provides a unique feature to identify lunar photoelectrons. We utilize this feature to isolate cases of emitted electrons dominated by BSE over PHE. With selected BSE cases, we characterize how the backscattering coefficient η varies with primary electron energy, which decreases with increasing energy. Our results also reveal η to be dependent on the magnetic dip angle, as a fraction of BSE re‐impact the surface in a magnetized environment. The characterization of the backscattering coefficient not only gives insights into the lunar surface properties and lunar surface charging but could also be potentially applied to other airless bodies.

Modeling Photoelectron and Auger Electron Emission From the Sunlit Lunar Surface: A Comparison With ARTEMIS Observations

AbstractDue to the lack of a dense atmosphere, the Moon directly interacts with ambient plasmas and solar radiation, leading to lunar surface charging. Solar X‐rays drive the emission of photoelectrons and Auger electrons from the lunar surface to space. The Auger electrons have characteristic energies intrinsic to the photo‐emitting atoms and were recently identified at the Moon by Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) observations. In this study, we developed a numerical model of the energy spectrum of lunar photoelectrons and Auger electrons, thereby comparing the predicted and observed energy spectra. By adjusting a scaling factor, the model well reproduces the ARTEMIS observations obtained in the solar wind, where the energy spectra are minimally affected by surface charging. Meanwhile, the energy spectra obtained in the geomagnetic tail can be significantly altered by lunar surface potentials. We show that it is difficult to determine a unique combination of the scaling factor and the lunar surface potential with the ARTEMIS energy resolution because of a strong parameter degeneracy. Nevertheless, for a fixed scaling factor, a strong correlation is identified between the lunar surface potentials inferred from the shifts of the energy spectra and those from the upward photoelectron beam energies, providing a proof of concept for the use of the photo‐emitted electrons as a new remote sensing tool of the lunar surface potential. We advocate for future observations of lunar electrons with a high energy resolution.

Lunar surface potential and electric field**A contributed paper from the International Symposium on Lunar and Planetary Science (ISLPS) on 2018 June 12–15 at Macau University of Science and Technology.

The Moon has no significant atmosphere, thus its surface is exposed to solar ultraviolet radiation and the solar wind. Photoemission and collection of the solar wind electrons and ions may result in lunar surface charging. On the dayside, the surface potential is mainly determined by photoelectrons, modulated by the solar wind; while the nightside surface potential is a function of the plasma distribution in the lunar wake. Taking the plasma observations in the lunar environment as inputs, the global potential distribution is calculated according to the plasma sheath theory, assuming Maxwellian distributions for the surface emitted photoelectrons and the solar wind electrons. Results show that the lunar surface potential and sheath scale length change versus the solar zenith angle, which implies that the electric field has a horizontal component in addition to the vertical one. By differentiating the potential vertically and horizontally, we obtain the global electric field. It is found that the vertical electric field component is strongest at the subsolar point, which has a magnitude of 1 V m−1. The horizontal component is much weaker, and mainly appears near the terminator and on the nightside, with a magnitude of several mV m−1. The horizontal electric field component on the nightside is rotationally symmetric around the wake axis and is strongly determined by the plasma parameters in the lunar wake.

Surface charging of a crater near lunar terminator

Past lunar missions have shown the presence of dust particles in the lunar exosphere. These particles originate from lunar surface and are due to the charging of lunar surface by the solar wind and solar UV flux. Near the lunar terminator region, the low conductivity of the surface and small scale variations in surface topology could cause the surface to charge to different surface potentials. This paper simulates the variation of surface potential for a crater located in the lunar terminator regions using Spacecraft Plasma Interaction Software (SPIS). SPIS employs particle in cell method to simulate the motion of solar wind particles and photoelectrons. Lunar crater has been found to create mini-wake which affects both electron and ion density and causes small scale potential differences. Simulation results show potential difference of 300 V between sunlit area and shadowed area which creates suitable condition for dust levitation to occur.

Laboratory Measurement of Lunar Regolith Simulant Surface Charging in a Localized Plasma Wake

Laboratory experiments are carried out to measure the charging of Johnson Space Center Number OneA lunar simulant surface in a localized plasma wake similar to that at the lunar terminator. The results show that the floating potential of a regolith surface in a plasma wake may be significantly different from that of a solid dielectric surface because ion collection on a regolith surface is sensitively influenced by the surface collection area of dust grains. The results also suggest that at the same negative charging potential, an isolated dust grain charges 50 times more than a dust grain on the regolith surface.

Laboratory investigation of lunar surface electric potentials in magnetic anomaly regions

AbstractTo gain insight into lunar surface charging in the magnetic anomaly regions, we present the results of laboratory experiments with a flowing plasma engulfing a magnetic dipole field above an insulating surface. When the dipole moment is perpendicular to the surface, large positive potentials (close to ion flow energies in eV) are measured on the surface in the dipole lobe regions, charged by the unmagnetized ions while the electrons are magnetically excluded. The potential decreases exponentially with distance from the surface on the ion (flow) Debye length scale. The surface potentials become much smaller when the dipole moment is parallel to the surface, likely due to collisionality. We discuss the implications of our laboratory results for the lunar surface charging in the magnetic anomaly regions, suggesting that the surface potential may be much higher than the generally expected several volts positive due to photoemission.

Analysis of data obtained by the solar wind ion detector onboard the Chang’E-1 Lunar orbiter

The interaction between lunar plasma and spacecraft may cause surface charging/discharging effects, and degrade the performance of spacecraft. The charging potential is a key factor for discharging process. In order to evaluate charging/discharging effects, it is necessary to obtain the Lunar plasma environment. SWIDA/B on Chang’E-1 are the two scientific instruments of solar wind ion detector which could explore the plasma environment in the 200 km Lunar orbit, thereby deducing the solar wind bulk speed, density and temperature. In this paper we select the data in June 2008 derived from SWIDA sensor. First, we figure out the solar wind ion differential flux and energy spectrum, and then calculate the solar wind parameters such as velocity (300.00–600.00 km/s), plasma density (1–10 cm-3) and plasma temperature (1–20 eV). Finally, we adopt an equivalent circuit model to compute the Lunar spacecraft surface charging voltage which is -7–-70 V.

Dependence of lunar surface charging on solar wind plasma conditions and solar irradiation

The surface of the Moon is electrically charged by exposure to solar radiation on its dayside, as well as by the continuous flux of charged particles from the various plasma environments that surround it. An electric potential develops between the lunar surface and ambient plasma, which manifests itself in a near-surface plasma sheath with a scale height of order the Debye length. This study investigates surface charging on the lunar dayside and near-terminator regions in the solar wind, for which the dominant current sources are usually from the photoemission of electrons Jp, and the collection of plasma electrons Je, and ions Ji. These currents are dependent on the following six parameters: plasma concentration n0, electron temperature Te, ion temperature Ti, bulk flow velocity V, photoemission current at normal incidence JP0, and photoelectron temperature Tp. Using a numerical model, derived from a set of eleven basic assumptions, the influence of these six parameters on surface charging – characterized by the equilibrium surface potential, Debye length, and surface electric field – is investigated as a function of solar zenith angle. Overall, Te is the most important parameter, especially near the terminator, while JP0 and Tp dominate over most of the dayside. In contrast, V and Ti are found to be the least effective parameters. Typically, lunar surface charging in the solar wind can be reduced to a two-current problem: on the dayside in sunlight, Jp+Je=0, since ∣Jp∣⪢∣Je∣⪢∣Ji∣, while near the terminator in shadow, Je+Ji=0. However, situations can arise that result in a truly three-current problem with some important consequences; e.g., very cold Te and/or very fast V can result in ∣Jp∣⪢∣Je∣≈∣Ji∣ on the dayside. The influence of surface charging pervades the environments of the Moon and other airless bodies, and the investigation presented here provides insights into the physical processes involved, as well as being useful for interpreting and understanding more complicated simulations.

Electric potentials in magnetic dipole fields normal and oblique to a surface in plasma: Understanding the solar wind interaction with lunar magnetic anomalies

We experimentally investigated the solar wind interaction with moderate‐strength lunar magnetic anomalies in which the electrons are magnetized but the ions remain unmagnetized. Previously, we studied the plasma sheaths above an insulating surface in a magnetic dipole field oriented parallel to the surface. In this paper, when the dipole field is oriented normal to the surface, the surface potential largely rises, and a potential bump forms in the sheath in the magnetic cusp region due to a significant magnetic mirror reflection of the electrons. It is also found that the electrons are shielded from the central dipole wings and diverted into the side of the wings. When the dipole field obliquely intersects the surface, an asymmetric potential distribution develops. Our experimental results indicate that lunar surface charging can be greatly modified in the magnetic anomaly regions, creating extreme local electrical environments.

Surface charging of Saturn's plasma‐absorbing moons

Recent results from the study of the interaction of Earth's moon with the solar wind have highlighted lunar surface charging as an important physical process not only for the local interaction of the surface and the impinging plasma but also for charged dust ejection and transport. Excluding Earth's moon, however, such studies have been limited to very few solar system bodies. Here we assess the importance of surface charging for the Saturnian plasma‐absorbing moons. In this initial study, we focus on the various moons' trailing hemispheres and their variable charging patterns as a function of their magnetospheric local time. With only a few exceptions, our results indicate negative potentials for all moons, with the most extreme values predicted for Rhea. When the leading hemisphere is partially sunlit, the surface potential profile can be quite complex with many different transition regions. We also find that electrostatic acceleration of dust is at least equally as (if not more) important as it is for Earth's moon for the submicron grains, but it is probably not sufficient to explain the detection of the larger micron‐sized grains in the vicinity of the large moons of the outer planets. Regarding Saturn's asteroid‐sized moons, we predict that electrostatic forces can accelerate grains above the escape velocity and populate the Saturnian system and/or contribute to dust transport across those moons' surfaces. On the basis of these results, we also propose several methods by which Cassini's instruments could be used to directly observe the effects of surface charging.

Lunar surface charging during solar energetic particle events: Measurement and prediction

We analyzed lunar surface charging during solar energetic particle (SEP) events, utilizing Lunar Prospector measurements of surface potentials and electron fluxes, and upstream energetic particle data. Outside of the magnetosphere, we find a nearly one‐to‐one correspondence between extreme negative lunar surface charging and large solar proton events. Using new techniques to correct for spacecraft potential, we present the first quantitative measurements of lunar charging during SEP events, during which we find that the nightside surface reaches potentials of up to −4.5 kV, with negative potentials of a kilovolt or larger often observed. These potentials are far higher than typical nightside potentials of a few hundred volts negative and may increase the risk of electrostatic discharge and/or dust effects, introducing an additional hazard to the already dangerous radiation environment. For eight of eleven event periods, surface potentials correlate with electron temperature and with the ratio of energetic electron flux to both energetic proton flux and total electron flux. For these eight events, charging models taking into account both thermal/suprathermal and energetic particle fluxes, as well as secondary emission, can successfully predict surface potentials. However, during the other three events, surface potentials do not correlate with the same measurable quantities, and charging models cannot reproduce measured potentials. In order to develop reliable and accurate models for lunar surface charging during SEP events, we will need better measurements of ion and energetic particle behavior in the lunar environment, secondary electron emission from lunar materials, and lunar surface potentials.

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

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 ∼−100 V in the terrestrial magnetotail lobes and potentials of ∼−200 V to ∼−1 kV in the plasma sheet. In the lunar wake, we find potentials of ∼−200 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 ∼−4 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.

Modelling long-term trends in lunar exposure to the Earth's plasmasheet

Abstract. This paper shows how the exposure of the Moon to the Earth's plasmasheet is subject to decadal variations due to lunar precession. The latter is a key property of the Moon's apparent orbit around the Earth – the nodes of that orbit precess around the ecliptic, completing one revolution every 18.6 years. This precession is responsible for a number of astronomical phenomena, e.g. the year to year drift of solar and lunar eclipse periods. It also controls the ecliptic latitude at which the Moon crosses the magnetotail and thus the number and duration of lunar encounters with the plasmasheet. This paper presents a detailed model of those encounters and applies it to the period 1960 to 2030. This shows that the total lunar exposure to the plasmasheet will vary from 10 h per month at a minimum of the eighteen-year cycle rising to 40 h per month at the maximum. These variations could have a profound impact on the accumulation of charge due plasmasheet electrons impacting the lunar surface. Thus we should expect the level of lunar surface charging to vary over the eighteen-year cycle. The literature contains reports that support this: several observations made during the cycle maximum of 1994–2000 are attributed to bombardment and charging of the lunar surface by plasmasheet electrons. Thus we conclude that lunar surface charging will vary markedly over an eighteen-year cycle driven by lunar precession. It is important to interpret lunar environment measurements in the context of this cycle and to allow for the cycle when designing equipment for deployment on the lunar surface. This is particularly important in respect of developing plans for robotic exploration on the lunar surface during the next cycle maximum of 2012–2019.

Extreme lunar surface charging during solar energetic particle events

We analyzed a series of solar energetic particle events in late April and early May of 1998, during which lunar surface potentials reached values as large as ∼−4.5 kV (the largest recorded by Lunar Prospector). The two largest surface charging events during this time period correspond to energetic particle injections, when the electron flux between 50 keV and 5 MeV exceeded the proton flux over the same energy range. We searched the entire Lunar Prospector data set for other large negative surface charging events, and found that they occur almost exclusively during magnetotail crossings (when the Moon encounters the plasmasheet) and solar energetic particle events. Lunar surface charging (and its effect on the lunar dust environment) during inherently unpredictable space weather events represents a significant hazard for exploration.