In-situ ionospheric observations near lunar south pole by the Langmuir Probe on Chandrayaan3 Lander

The near surface lunar plasma environment is modulated by important components like the photoelectron sheath, solar wind, lunar surface potential, etc. In situ measurements of lunar near surface plasma are not available as of now. Previous lunar missions which explored the near surface environment have arrived at estimates of lunar photo electron densities mainly from lunar sample returns. The Chandrayaan-2 lunar mission affords a unique opportunity to explore the near surface lunar plasma environment from the lunar lander platform. A Langmuir probe is developed indigenously for probing the tenuous lunar near surface plasma environment from the top deck of the lunar lander. The probe is designed to cater to a wide dynamic range of 10/cc to 10,000/cc. The probe behaviour is characterized in the ambient room conditions using a current source. The sensitivity of the probe to incoming ionized species is also characterized in a vacuum chamber. The Langmuir probe response is characterized such that the input current to the probe is correctly deciphered during the mission duration. The calibration of the present Langmuir probe is carried out using a standard calibrated Langmuir probe. The details of the theoretical simulations of the expected currents, the characterization and calibration activities are presented and discussed.

The analytical study of the two-stream instability (TSI) generation is carried out in the lunar ionosphere. The solar wind is considered an electron beam, which interacts with the lunar ionosphere, generated due to the photoionization of the lunar neutrals by the extreme ultraviolet component of the solar radiation. In this interaction process, the lunar electrons constitute the background plasma as the ion population is considerably low in the lunar plasma environment. In the present study along with the non-energetic (“cold”) electrons, which are in the majority, a fraction of energetic electrons (“hot”) of the total lunar electron population are also considered and the fraction of energetic electrons is taken in the range of 1%–25% of the total lunar electron count. The particle-in-cell simulations suggest that the presence of energetic electrons in the lunar plasma environment hastens the electron bunching during the interaction with the incoming solar wind electrons during the TSI. The energetic electrons in the lunar plasma environment are capable of triggering non-linear phenomena, such as the generation of lunar plasma waves. The inclusion of hot electrons in the lunar plasma ambiance changes the scenario for the TSI to occur in the lunar ionosphere, and the analysis shows that it modifies the TSI dispersion relation and can have a significant impact on the growth and decay of the TSI and its threshold for generation in a lunar plasma environment.

The Rashid-1 rover, which was part of the Emirates Lunar Mission (ELM) program, was a small rover aimed to be operated for one lunar day on the lunar surface. As part of its scientific instrumentation, Rashid-1 carried a Langmuir probe experiment (LNG) in order to provide the first extensive, high-resolution in situ measurements of the bulk parameters of the lunar dayside thermal plasma at different altitudes above the lunar surface. The LNG was comprised of four probes, mounted at different locations and heights above the lunar surface on the Rashid-1 rover. This way, the LNG was intended to derive an altitude profile of the two plasma parameters electron density and electron temperature above the lunar surface. The design of the instrument and a description of the data analysis technique, calibration, and validation are provided in this paper. Due to the short separation between the probes and the rover body (in terms of Debye length), the measurements of the LNG were expected to be influenced by the presence of the rover and its sheath. This was addressed through numerical modeling, which is described and preliminary results are presented. Unfortunately, the landing in the Atlas crater of the lunar lander carrying Rashid-1 to the surface was not successful – however, this description of the instrument design and the data analysis techniques are still useful for future explorations of the lunar plasma environment.

The Rashid-1 rover, which was part of the Emirates Lunar Mission (ELM) program, was a small rover aimed to be operated for one lunar day on the lunar surface. As part of its scientific instrumentation, Rashid-1 carried a Langmuir probe experiment (LNG) in order to provide the first extensive, high-resolution in situ measurements of the bulk parameters of the lunar dayside thermal plasma at different altitudes above the lunar surface. The LNG was comprised of four probes, mounted at different locations and heights above the lunar surface on the Rashid-1 rover. This way, the LNG was intended to derive an altitude profile of the two plasma parameters electron density and electron temperature above the lunar surface. The design of the instrument and a description of the data analysis technique, calibration, and validation are provided in this paper. Due to the short separation between the probes and the rover body (in terms of Debye length), the measurements of the LNG were expected to be influenced by the presence of the rover and its sheath. This was addressed through numerical modeling, which is described and preliminary results are presented. Unfortunately, the landing in the Atlas crater of the lunar lander carrying Rashid-1 to the surface was not successful – however, this description of the instrument design and the data analysis techniques are still useful for future explorations of the lunar plasma environment.

Despite lacking a global magnetic field, the Moon features localized magnetized regions called lunar magnetic anomalies [1]. Their interaction with the solar wind results in significant proton reflection and deflection [2], creating unique structures often referred to as lunar mini-magnetospheres [3, 4]. Previous studies have shown that the largest magnetic anomaly, the South Pole-Aitken (SPA) cluster, induces global-scale perturbations in the near-surface lunar plasma environment on both the dayside [5, 6] and nightside [7]. However, its influence on the plasma environment in south polar regions remains unknown.In this study, we produce new composite images of backscattered energetic neutral hydrogen derived from Sub-KeV Atom Reflecting Analyzer (SARA) [8] data. These images reveal that plasma perturbations generated by the SPA cluster can extend to lunar south-polar regions, depending on local time and upstream solar wind conditions. These perturbations affect solar wind proton precipitation patterns, either decreasing or enhancing impinging proton fluxes depending on whether the south pole lies downstream or outside of the SPA anomaly. Based on these observations, we develop an empirical model of solar wind compression by the SPA cluster to evaluate its impact on ion instrument measurements at the south pole.Understanding the complex interactions between the plasma, dust, and electromagnetic environments is an important asset to ensure safe and sustainable human presence on the Moon. We will discuss the role of the SPA cluster in these interactions, which will establish preliminary measurement requirements for in-situ plasma instruments in polar regions.

While the Moon lacks a global intrinsic magnetic field, its crust features small-scale magnetized regions known as lunar magnetic anomalies [1]. Their interaction with the solar wind causes significant proton reflection and deflection [2] and creates unique structures known as lunar mini-magnetospheres [3, 4]. Previous studies have shown that lunar magnetic anomalies induce global-scale perturbations in the near-surface lunar plasma environment on both the dayside [5] and nightside [6]. In particular, simulations suggest that the largest lunar magnetic anomaly, the South Pole-Aitken (SPA) cluster, causes large-scale solar wind compressions and interplanetary magnetic field enhancements due to the interaction between protons reflected by SPA and the solar wind that can reach south-polar regions when the SPA cluster is at local noon [7]. However, the influence of these solar wind compressions on the plasma environment at the lunar South Pole remains unknown.In this study, we produce new composite images of backscattered energetic neutral hydrogen derived from Sub-KeV Atom Reflecting Analyzer (SARA) [8] measurements. These images show that the plasma perturbations generated by the SPA cluster can extend to lunar south-polar regions, depending on local time and upstream solar wind conditions. These perturbations affect solar wind proton precipitation patterns, either decreasing or enhancing impinging proton fluxes depending on whether the south pole lies downstream or outside of the SPA anomaly. Based on these observations, we develop an empirical model of solar wind compression by the SPA cluster to evaluate its impact on ion instrument measurements at the south pole.Understanding the complex interactions between the plasma, dust, and electromagnetic environments is an important asset to ensure safe and sustainable human presence on the Moon. We will discuss the role of the SPA cluster in these interactions, which will establish preliminary measurement requirements for in-situ plasma instruments in polar regions.

China seismo-electromagnetic satellite (CSES) is launched to detect the electromagnetic environment in space for the study of seismic early warning. Langmuir probe is one of the payloads of the CSES satellite, and it is the first time that the Langmuir probe technique has been used in the Chinese satellite. The use of the Langmuir probe is to measure the space plasma parameters, such as electron density (Ne), electron temperature (Te), and to identify the instantaneous change of the space plasma. The Langmuir probe payload is composed of three parts, i.e., two sensors, two rods, and one electronics box. The sensor is installed at the top of the rod to extend out of the satellite surface, and is parallel to the direction of the satellite orbit. The electronics box is installed inside the satellite which includes the sweep voltage circuit, sensor signal circuit, DPU control and processing circuit, the satellite interface circuit, power supply circuit, etc. The sensor is spherical. Its upper hemisphere is a collecting electrode, and its lower hemisphere is a protective electrode. The same sweep voltage is applied to the upper hemisphere and the lower one which can eliminate the terminal effect of the connecting point between the traditional spherical structure and the rod. The diameters of the two sensors are respectively 50 and 10 mm, and the surface areas of the two sensors are respectively 1/2000 and 1/13000 times the satellite surface area. The stability of the satellite ground potential is not affected by the sweep voltages on the sensors. In addition, TiN material is coated on the sensor surface to ensure a uniform surface work function, and to prevent the space atomic oxygen erosion. The decontamination function is designed for the Langmuir probe to eliminate the possible pollution on the orbit. A positive 100 V voltage is applied to the sensor to accelerate electrons to bombard the sensor surface, thereby removing the contamination from the sensor surface. The advantage of the electron bombardment effect is that the TiN film is not damaged, meanwhile the positive 100 V voltage has high reliability and safety on orbit. The decontamination function has been proved to be effective by the test in Italy National Institute for Astrophysics-Institute for Space Astrophysics and Planetology (INAF-IAPS). The plasma environment calibration test of the Langmuir probe is carried out in INAF-IAPS. We measure the electron density and temperature at three different distances from the plasma source, and compare the results with the measured results of the INAF-IAPS reference Langmuir probe. Results show that the test data of our Langmuir probe are consistent with the INAF-IAPS reference data. Our Langmuir probe design is proved to be feasible to achieve the missions of the satellite.

The Moon–solar wind interaction results in the formation of a complicated lunar space plasma environment. Here, we investigate the solar wind turbulence around the Moon using the magnetic field observed by the dual-probe mission Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon’s Interaction with the Sun (ARTEMIS). Structure functions in a time range on kinetic scales are computed to measure the scaling index ζ, the spatial distribution of which reveals the global aspects of the lunar space plasma and shows the dependence on the local instability. On the lunar nightside, in the plasma void, the dominating magnetic pressure over the thermal pressure restrains the turbulence, and a quiet zone is built with , where is the scaling index in the ambient solar wind. Downstream in the lunar wake, ζ is elevated gradually and goes above at a radial distance of (lunar radius), which implies that the plasma refilling process in the lunar wake begins to generate the local turbulence. On the dayside around the subsolar point, ζ is enhanced at a low altitude of ∼200 km, where the solar wind turbulence might be strengthened due to interaction with the lunar source plasma. The largest scaling indices lie around the day–night terminator with , and the observed dawn–dusk asymmetry could be an effect of the magnetic field not being parallel with the solar wind. The correspondence between the enhanced scaling index and the local instability also raises new questions about the description of solar wind turbulence.

Whistler‐mode waves are commonly observed within the lunar environment, while their variations during Interplanetary (IP) shocks are not fully understood yet. In this paper, we analyze two IP shock events observed by Acceleration, Reconnection, Turbulence and Electrodynamics of the Moons Interaction with the Sun (ARTEMIS) satellites while the Moon was exposed to the solar wind. In the first event, ARTEMIS detected whistler‐mode wave intensification, accompanied by sharply increased hot electron flux and anisotropy across the shock ramp. The potential reflection or backscattering of electrons by the lunar crustal magnetic field is found to be favorable for whistler‐mode wave intensification. In the second event, a magnetic field line rotation around the shock region was observed and correlated with whistler‐mode wave intensification. The wave growth rates calculated using linear theory agree well with the observed wave spectra. Our study highlights the significance of magnetic field variations and anisotropic hot electron distributions in generating whistler‐mode waves in the lunar plasma environment following IP shock arrivals.

Unlike the Earth, the Moon lacks is not protected from the atmosphere and global magnetic field, and will be directly exposed to complex radiation environments such as high-energy cosmic rays, solar wind, and the Earth’s magnetotail plasma. The surface of the Moon is covered with a thick layer of lunar soil, and the particles in the soil with a diameter between 30 nm–20 μm are called lunar dust. In the complex environments such as solar wind or magnetotail plasma, lunar dust carries an electric charge and becomes charged lunar dust. Charged lunar dust is prone to migration under the action of the electric field on the lunar surface. Charged migrated lunar dust is easy to adhere to the surface of instruments and equipment, resulting in visual impairment, astronauts’ movement disorders, equipment mechanical blockage, sealing failure, and material wear, which affects the lunar exploration mission. As an important lunar exploration landing site, the lunar south pole receives special solar radiation and produces a special dust plasma environment due to its special location. In order to provide an environmental reference for lunar south pole exploration, it is necessary to explore the characteristics of the dust plasma environment in the lunar south pole and its impact. In view of the lunar south pole environment, The Spacecraft Plasma Interactions Software (SPIS) software developed by the European Space Agency is used to carry out modelling and simulation in this work. Through the simulation, the logarithmic distribution of the lunar dust space density in a range of 0–200 m at the lunar south pole, the potential distribution near the lunar surface, and the spatial distribution characteristics of plasma electrons and ions are obtained. The obtained lunar dust space density and lunar surface potential are similar to the previous theoretical derivation and field detection data, so the simulation results have high reliability. The spatial potential distribution and the spatial density distribution of electrons and ions in the lunar environment with and without lunar dust are compared. Finally, the conclusions can be drawn as follows. The space potential increases with altitude increasing. The potential at 0–10 m near the lunar south pole is about –40 V, and the space potential at 100 m is about –20 V. The density of lunar dust in an altitude range below 10 m is 10<sup>7.22</sup> m<sup>–3</sup>–10<sup>4.66</sup> m<sup>–3</sup>. The electron density in the dust plasma near the lunar surface is 10<sup>5.47</sup> m<sup>–3</sup>, and the ion density is 10<sup>6.07</sup> m<sup>–3</sup>, and both increase with altitude increasing. Charged lunar dust affects the spatial distribution of lunar dust, mainly through affecting the distribution of the space electric field, which leads to difference in electron distribution, but has little effect on ions.

Langmuir Probes (LP) are well-proven simple instruments allowing to estimate the electron density (Ne) and temperature (Te) of a plasma. They are also used to estimate the electric potential of satellites to the benefit of other instruments and technical systems. On the Swarm satellites the LPs are part of the Electric Field Instruments (EFI) featuring thermal ion imagers (TII) measuring 3-d ion distributions. The main task of the Langmuir probes is to provide measurements of spacecraft potentials influencing the ions before they enter the TIIs. In addition also electron density (Ne) and temperature (Te) are estimated from EFI LP data. The design of the Swarm LP includes a standard current sampling under sweeps of the bias voltage, and also, for most of the time, a novel ripple technique yielding derivatives of the current-voltage characteristics at three points in a rapid cycle. In normal mode the time resolution of the Ne and Te measurements so becomes only 0.5 s. We show first Ne and Te estimates from the EFI LPs obtained. The data feature very low instrumental noise thanks to the ripple technique. The LP data are compared with observations by incoherent scatter radars, namely EISCAT UHF, VHF, the ESR, and also Arecibo.

A Magnetic Pole Enhanced inductively coupled RF He- N2/ Ar plasma is characterized using a Langmuir probe and optical emission spectroscopy (OES) techniques. The effect of helium mixing on electron density (ne) and temperature (Te), electron energy probability functions (EEPFs), [N] atomic density, and N2 dissociation is investigated. A Langmuir probe and a zero slope method based on trace rare gas-optical emission spectroscopy (TRG-OES) are employed to measure the electron temperature. It is noted that the electron temperature shows an increasing trend for both methods. However, the temperature measured by a zero slope method Te(Z·S) approaches the temperature measured by a Langmuir probe; Te(L·P) at 56% and above helium concentration in the discharge. “Advance actinometry” is employed to monitor the variation in [N] atomic density with helium concentration and gas pressure. It is noted that [N] atomic density increases at 56% and above helium in the discharge, which is consistent with the trend of electron temperature and EEPFs. A drastic enhancement in N2 dissociation fraction D1 determined by “advance actinometry” is noted at 56% and above helium concentration in the mixture due to modifications in different population and depopulation mechanisms. However, it is also noted that the dissociation fraction D2 determined by intensity ratio method increases linearly with helium addition.

In this study we calibrate and validate in situ ionospheric electron density (Ne) and temperature (Te) measured with Langmuir probes (LPs) on the three Swarm satellites orbiting the Earth in circular, nearly polar orbits at ~500 km altitude. We assess the accuracy and reliability of the LP data (December 2013 to June 2016) by using nearly coincident measurements from low‐ and middle‐latitude incoherent scatter radars (ISRs), low‐latitude ionosondes, and Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC) satellites, covering all latitudes. The comparison results for plasma frequency ( ) for each Swarm satellite are consistent across these three, principally different measurement techniques. It shows that the Swarm LPs systematically underestimate plasma frequency by about 10% (0.5–0.6 MHz). The correlation coefficients are high (≥0.97), indicating accurate relative variation in the Swarm LP densities. The comparison of Te from high‐gain LPs and those from ISRs reveals that all three satellites overestimate it by 300–400 K but exhibit high correlations (0.92–0.97) against the validation data. The low‐gain LP Te data show larger overestimation (~700 K) and lower correlation (0.86–0.90). The adjustment of the Swarm LP data based on Swarm‐ISR comparison results removes the systematic biases in the Swarm data and gives plasma frequencies and high‐ and low‐gain electron temperatures that are precise within about 0.4 MHz (8%), 150–230 K, and 260–360 K, respectively. We demonstrate that the applied correction significantly improves the agreement between (1) the plasma densities from Swarm, and from ionosondes and COSMIC, and (2) the Te from Swarm LPs and International Reference Ionosphere 2016.

<p>This study investigates the lunar plasma environment when embedded within Earth's magnetotail. We use data from 10 years of tail crossings by the Acceleration, Reconnection, Turbulence, and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft in orbit around the Moon. We separate the plasma environments by magnetosheath-like, magnetotail lobe-like, and plasma sheet-like conditions. Our findings highlight that the lobe-like plasma is associated with low densities and a strong magnetic field, while the plasma sheet is characterized by higher densities and a weaker magnetic field. These regions are flanked by the fast, predominantly tailward flows of the terrestrial magnetosheath. During a single lunar crossing, however, the magnetotail displays a wide range of variability, with transient features—including reconnection events—intermixed between periods of lobe-like or sheet-like conditions. We compare and contrast the Moon's local magnetotail plasma to the environments near various outer-planet moons. In doing so, we find that properties of the ambient lunar plasma are, at times, unique to the terrestrial magnetotail, while at others, may resemble those near the Jovian, Saturnian, and Neptunian moons. These findings highlight the complementary role of the ARTEMIS mission in providing a deeper understanding of the plasma interactions of the outer-planet moons.</p>

The Moon was the first extraterrestrial body to attract the attention of space pioneers. It has been about half a century since an active lunar exploration campaign was carried out. At that time, a series of Russian and American automatic landing vehicles and the American manned Apollo Program carried out an unprecedented program of lunar exploration in terms of its saturation and volume. Unique breakthrough data on the lunar regolith and plasma environment were obtained, a large number of experiments were carried out using automated and manned expeditions, and more than 300 kg of lunar regolith and rock samples were delivered to Earth for laboratory research. A wealth of experience has been accumulated by performing direct human activities on the lunar surface. At the same time, the most unexpected result of the studies was the detection of a glow above the surface, recorded by television cameras installed on several lunar landers. The interpretation of this phenomenon led to the conclusion that sunlight is scattered by dust particles levitating above the surface of the Moon. When the Apollo manned lunar exploration program was being prepared, this fact was already known, and it was taken into account when developing a program for astronauts’ extravehicular activities on the lunar surface, conducting scientific research, and ground tests. However, despite preparations for possible problems associated with lunar dust, according to American astronauts working on the lunar surface, the lunar dust factor turned out to be the most unpleasant in terms of the degree of impact on the lander and its systems, on the activities of astronauts on the surface, and on their health. Over the past decades, theoretical and experimental model studies have been carried out aimed at understanding the nature of the lunar horizon glow. It turned out that this phenomenon is associated with the complex effect of external factors on the lunar regolith, as a result of which there are a constant processing and grinding of the lunar regolith to particles of micron and even submicron sizes. Particles of lunar regolith that are less than a millimeter in size are commonly called lunar dust. As a result of the influence of external factors, the upper surface of the regolith acquires an electric charge, and a cloud of photoelectrons and a double layer are formed above the illuminated surface. Coulomb forces in the electric field of this layer, acting on microparticles of lunar dust, under certain conditions are capable of tearing microparticles from the surface of the regolith. These dust particles, near-surface plasma, and electrostatic fields form the near-surface dusty plasma exosphere of the Moon. The processes leading to the formation of regolith and microparticles on the Moon, their separation from the surface, and further dynamics above the surface include many external factors affecting the Moon and physical processes on the surface and near-surface dusty plasma exosphere. As a result of the research carried out, a lot has been understood, but many unsolved problems remain. Recently, since the space agencies of the leading space powers have been turning their attention to intensive research and subsequent exploration of the Moon, interest in the processes associated with the dynamics of lunar dust and its influence on landing vehicles and their engineering systems is increasing, and significant attention is being paid to reducing and mitigating the negative impact of lunar dust on the activities of astronauts and their health.