It is generally accepted that a fraction of H+ pickup ions originating from the Martian hydrogen (H) exosphere can be reflected and energized at the bow shock (BS), but the underlying reflection and acceleration mechanisms have not been explicitly identified. Using in situ observations from the Mars Atmosphere and Volatile EvolutioN ( MAVEN ) spacecraft, we present evidence for both shock surfing acceleration (SSA) and shock drift acceleration (SDA) of exospheric H+ pickup ions at the Martian quasi-perpendicular BS. On 12 August 2016, MAVEN detected a reflected H+ ion beam with energies of similar to 1.3-7.5 keV, propagating nearly along the solar wind convection electric field. The low-energy component is consistent with single reflections via SSA, while the high-energy component exceeds the maximum energy achievable from single reflections, indicating multiple reflections at the BS. These high-energy ions likely experienced either direct SDA or sequential acceleration involving SSA followed by SDA, depending on their incident normal velocities. The middle-energy component can be explained by either single or multiple reflections via SDA alone, or by a combination of SSA and subsequent SDA at the BS.

Context. Airless planetary bodies are directly exposed to solar wind ions, which can scatter or become implanted upon impact with the regolith-covered surface, while also sputtering surface atoms. Aims. We constructed a semi-analytical model for the scattering of ions of hundreds of electron volts (eV) and the sputtering of surface atoms, both resulting in the emission of negative ions from the lunar surface. Our model contains a novel description of the scattering process that is physics-based and constrained by observations. Methods. We used data from the Negative Ions at the Lunar Surface (NILS) instrument on the Chang'e-6 lander to update prior knowledge of ion scattering and sputtering from lunar regolith through Bayesian inference. Results. Our model shows a good agreement with the NILS data. We find that a precipitating solar wind proton has a roughly 22(-6.1)(+4.9) % chance of scattering from the lunar surface in any charge state and 8.1(-3.9)(+7.9) % chance of sputtering a surface hydrogen atom. The resulting ratio of scattered to sputtered hydrogen flux is eta(sc)/eta(sp) = 1.5(-1.1)(+1.5) for a proton speed of 300 km/s. We found a high probability (7-20%) that a hydrogen atom leaves the surface negatively charged. The angular emission distributions at near-grazing angles for both scattered and sputtered fluxes are controlled by surface roughness. Our model also indicates significant inelastic energy losses for hydrogen interacting with the regolith, suggesting a longer effective path length than previously assumed. Finally, we estimated a surface binding energy of 5.5 eV, consistent with the observations. Conclusions. Our model describes the scattering and sputtering of particles of any charge state from any homogeneous, multi-species surface. Using NILS data, we successfully applied the model to update our understanding of solar wind interacting with lunar regolith and the emission of negative hydrogen ions.

Abstract: Surface alteration by the exhaust plumes of lunar landers is a major concern when interpreting measurements acquired at the lunar surface [1–3]. For example, because the spectral and angular properties of lunar-emitted particles depend on regolith structure and mineralogy [4–8], lander-induced surface contamination is expected to affect particle emissions over several hundred meters around the landing site. However, the effects of lander-induced regolith disturbances on lunar particle emissions remain poorly constrained due to the lack of dedicated in situ investigations.Using energetic neutral atom (ENA) measurements from the Advanced Small Analyzer for Neutrals (ASAN) [9] onboard the Chang’e-4 rover Yutu-2, we investigate variations in the spectral and angular properties of backscattered hydrogen ENAs as a function of distance from the Chang’e-4 lander. Excluding periods when the precipitating solar wind was disturbed by the South Pole–Aitken magnetic anomaly [10], we obtain 30 energy spectra acquired at distances of 30–930 m from the Chang’e-4 lander over a 4.5-year interval beginning one months after landing. For each spectrum, we applied a Bayesian inference method to derive the hydrogen surface binding energy and angular scattering function using an empirical hydrogen ENA energy spectrum model [6].Our analysis shows that the surface binding energy inferred from ASAN observations is lower, at 5.0 ± 1.8 eV, within the first 150 m from the lander, compared to 6.2 ± 2.9 eV at larger distances. This transition distance of 150 m is comparable to the size of blast zones produced by previous lunar landers, suggesting that the same surface alteration processes responsible for the visible blast zone also modify the energy spectrum of lunar-emitted particles. In contrast, we find no significant dependence of the scattering function on distance from the lander. This result is contrary to previous expectations, namely that lander plume–induced disturbances in the micrometer-scale structure of the regolith inferred from photometric measurements would affect the ENA angular scattering function [1,4]. These results motivate future investigations of surface alteration using rover-borne particle instruments, particularly during the first lunar day, when transient contamination is expected [2] but not covered by the ASAN dataset.References:[1] Clegg-Watkins et al. (2016). Icarus, 273, 84–95. https://doi.org/10.1016/j.icarus.2015.12.010.[2] Farrell et al. (2022). Icarus 376: 114857. https://doi.org/10.1016/j.icarus.2021.114857.[3] Prem et al. (2020). JGR: Planets, 125(8), e2020JE006464. https://doi.org/10.1029/2020JE0064643[4] Kenmotsu et al. (2004). Journal of Plasma and Fusion Research, 80 (5), 406–9. https://doi.org/10.1585/jspf.80.406.[5] Szabo et al. (2023). JGR: Planets, 128 (9), e2023JE007911. https://doi.org/10.1029/2023JE007911.[6] Wieser et al. (2024). A&A, 684, A146. https://doi.org/10.1051/0004-6361/202348876.[7] Ricketts et al. (2025). The Planetary Science Journal, 6 (10), 244. https://doi.org/10.3847/PSJ/ae113e.[8] Canu-Blot et al. (2026). A&A, in press: https://doi.org/10.1051/0004-6361/202659499.[9] Wieser et al. (2020). Space Science Reviews, 216(4), 73. https://doi.org/10.1007/s11214-020-00691-w.[10] Maynadié et al. (2026). JGR: Space Physics, 131(4), e2026JA035258. https://doi.org/10.1029/2026JA035258.

When the upstream Alfvén Mach number is low, the magnetic field may play a dominant role in the solar-wind interaction with Mars, yet the resulting magnetospheric dynamics and associated energy conversion remain poorly constrained. Bulk proton acceleration reaching about 150% of the solar-wind speed is detected jointly by MAVEN, Tianwen-1, and MEX missions in the magnetosheath. The acceleration regions exhibit pronounced asymmetry between the two electric-field hemispheres, defined by the solar-wind electric field, while the shape of the adjacent magnetotail shows axial and hemispheric asymmetries. We demonstrate that the large-scale J×B force, with J denoting the electric current density and B the magnetic field, is the primary driver of acceleration and axial asymmetry. Mass loading of planetary ions introduces strong hemispheric asymmetries—features unique to unmagnetized bodies with atmospheres. Here, we reveal a pathway by which magnetic field converges and redistributes solar-wind energy, and infer that plasma acceleration under low upstream Alfvén Mach number operates not only at Earth but also at Mars and other bodies with magnetosheaths. Mars sometimes encounters low-Alfvén-Mach-number solar wind, allowing magnetic fields to control plasma flow. Here, the authors show that this accelerates magnetosheath protons and compresses the Martian magnetotail.

Apart from positive ions and electrons, negative ions are expected in various astrophysical environments. However, they have never been detected on the Moon until the Chang'E-6 mission. The NILS instrument onboard Chang'E-6 lander is the first dedicated instrument for detecting negative ions beyond Earth and has successfully obtained H- spectra on the lunar surface, providing an unprecedented opportunity to investigate their origin and distribution. Here, we present a positive correlation between the H- spectra and solar wind parameters, which provides direct evidence for the generation of negative ions from solar wind-surface interaction. Combined with Monte Carlo simulations, we predict a thin dayside H- layer and a long nightside H- tail, which can contribute to the lunar plasma environment, especially during an extreme solar wind density event. These findings greatly improve our understanding of the generation and distribution of negative ions on the Moon and other airless bodies.

Since November 2021,MINPA has continuously performed ion observations,accumulating a substantial dataset for scientific analysis.Although the instrument's background signals are generally weak,they can still interfere with the identification of ion structures and the estimation of plasma parameters under low-signal conditions.To improve data reliability,this study systematically investigates the statistical characteristics of the background signals.The analysis reveals several distinct features,including a uniform energy distribution,strong dependences on azimuthal angle and mass group,and pronounced temporal variability,with notable differences among detection channels.Based on these insights,a dynamic background removal method was developed by leveraging the statistical properties of the background and validated using representative observational events.The results demonstrate that the proposed method effectively suppresses background interference and enhances the extraction of genuine ion signals,notably improving the accuracy of key parameters such as bulk velocity and temperature during low-signal periods.This work establishes a solid foundation for the high-quality utilization of MINPA data and provides methodological support for future investigations of space plasma environments.

Context. Airless planetary bodies are directly exposed to solar wind ions, which can scatter or become implanted upon impact with the regolith-covered surface, while also sputtering surface atoms. Aims. We constructed a semi-analytical model for the scattering of ions of hundreds of electron volts (eV) and the sputtering of surface atoms, both resulting in the emission of negative ions from the lunar surface. Our model contains a novel description of the scattering process that is physics-based and constrained by observations. Methods. We used data from the Negative Ions at the Lunar Surface (NILS) instrument on the Chang’e-6 lander to update prior knowledge of ion scattering and sputtering from lunar regolith through Bayesian inference. Results. Our model shows a good agreement with the NILS data. We find that a precipitating solar wind proton has a roughly 22−6.1+4.9 % chance of scattering from the lunar surface in any charge state and 8.1−3.9+7.9 % chance of sputtering a surface hydrogen atom. The resulting ratio of scattered to sputtered hydrogen flux is ηsc/ηsp = 1.5−1.1+1.5 for a proton speed of 300 km/s. We found a high probability (7-20%) that a hydrogen atom leaves the surface negatively charged. The angular emission distributions at near-grazing angles for both scattered and sputtered fluxes are controlled by surface roughness. Our model also indicates significant inelastic energy losses for hydrogen interacting with the regolith, suggesting a longer effective path length than previously assumed. Finally, we estimated a surface binding energy of 5.5 eV, consistent with the observations. Conclusions. Our model describes the scattering and sputtering of particles of any charge state from any homogeneous, multi-species surface. Using NILS data, we successfully applied the model to update our understanding of solar wind interacting with lunar regolith and the emission of negative hydrogen ions.

A sizeable fraction of solar wind protons precipitating onto the lunar surface is backscattered from the lunar surface. Additionally the lunar surface is sputtered by the precipitating particle populations. Previous observations by spacecraft from orbit (Chandrayaan-1, Kaguya, IBEX) and from the lunar surface (Chang'e-4) show that up to 20% of the impinging solar wind protons are backscattered as energetic neutral atoms and about 0.1% to 1% are backscattered as protons. However, particles emitted from the surface can have any charge state. The recent discovery of negative ions by the Negative Ions at the Lunar Surface (NILS) instrument on Chang'e-6 allows for the first time to investigate the full charge state distribution of solar wind induced backscattered and sputtered particle populations from the lunar surface. We present and interpret new data obtained from the lunar surface and discuss the impact of the emitted particle populations on the lunar exo-ionosphere.

Atmospheric escape plays a critical role in shaping the long-term climate evolution of Mars. Among the various escape mechanisms, energetic neutral atoms (ENAs) generated through charge exchange between solar wind ions and exospheric neutrals serve as an important diagnostic for ion-neutral interactions and upper atmospheric loss. This study presents direct observations of hydrogen ENAs (H-ENAs) on the dayside of Mars by using the Mars Ion and Neutral Particle Analyzer (MINPA) onboard China's Tianwen-1 orbiter. By analyzing H-ENA data during a coronal mass ejection and a stream interaction region from December 29, 2021, to January 1, 2022, and comparing these data with MAVEN/SWIA (Mars Atmosphere and Volatile EvolutioN/Solar Wind Ion Analyzer) solar wind measurements, we examine the temporal evolution of H-ENA flux and the associated sputtered escape of atmospheric constituents. The observed H-ENA velocity is consistent with upstream solar wind ions, and the H-ENA-to-ion intensity ratio is used to infer variations in exospheric density, revealing a delayed response to enhanced solar wind activity. Penetrating H-ENA intensities reach up to 5.3 x 10(6) s(-1) cm(-2), with energy fluxes on the order of (0.5-8.1) x 10(-3) mW/m(2). The estimated oxygen sputtered escape rate driven by penetrating H-ENAs ranges from 5.5 x 10(23) s(-1) to 5.2 x 10(24) s(-1), comparable to or exceeding previous estimates based on penetrating ions. The findings highlight the need for low-altitude H-ENA observations to better quantify their atmospheric interactions and refine our understanding of nonthermal escape processes at Mars.

Solar wind has been regarded as an important source for the surficial water on the Moon. Here we investigate the global migration of solar wind-derived hydroxyl (OH) molecules with a Monte Carlo model, in which the shielding effects of both the Earth's magnetosphere and the lunar magnetic anomalies on the production rate of OH are considered. It is found that the OH surface concentration depends on the latitude, the local time, and the lunar phase. Specifically, the surface concentration can increase from nearly zero ppm near the subsolar point to about 1,000 ppm near the polar regions, and there is a local peak near the morning terminator that can result in a diurnal variation of about 100 ppm. Moreover, the OH abundances can be reduced by about 15% in the magnetotail when at low and mid latitudes. But when in the polar region, the OH abundance can keep increasing, resulting in an average deposition rate of . In addition, a south-north asymmetry can be caused by the magnetic anomalies, in which the OH abundance in the southern hemisphere is generally lower than that in the northern hemisphere, with maximum difference of about 31% near the 75 degrees latitude. In the exosphere, the OH number density shows a dawn-dusk asymmetry, with relatively higher number densities on the dawnside, and the maximum density can reach near the subsolar region. These results can greatly improve our understanding on the migration and distribution of OH molecules on the Moon.

The Light Ion Analyzer (LIA) instrument, part of the Solar-wind-Magnetosphere–Ionosphere-link- Explorer (SMILE) mission, is designed to measure the ion velocity distribution function within an energy range of 5 eV up to 25 keV. LIA provides in-situ measurements of the ion velocity distribution functions of the solar wind and magnetosheath, from which the moments can be derived on the ground, serving as an upstream input for the magnetosphere-ionosphere downstream responses. Two identical 2 π sr field-of-view LIA instruments are mounted on two opposite sides of the spacecraft platform, offering a combined 4 π sr instantaneous field-of-view. Each LIA consists of a top-hat electrostatic analyzer, electrostatic aperture deflectors, and a microchannel plate detector for analyzing the energy, direction, and flux of ions. Depending on the operation mode, the angular resolution ranges from 5.625° to 22.5° in elevation and from 7.5° to 30° in azimuth, and the time resolution spans from 0.25 to 2 seconds. This paper describes the design of the LIA, its performance, ground calibration, operation procedures, and resultant data products.

One of the detection objectives of the Chinese Asteroid Exploration mission is to investigate the space environment near the Main-belt Comet（MBC, Active Asteroid） 311P/PANSTARRS. This paper outlines the scientific objectives, measurement targets, and measurement requirements for the proposed Gas and Ion Analyzer（GIA）. The GIA is designed for in-situ mass spectrometry of neutral gases and low-energy ions, such as hydrogen,carbon, and oxygen, in the vicinity of 311P. Ion sampling techniques are essential for the GIA's Time-of-Flight（TOF） mass analysis capabilities. In this paper, we present an enhanced ion sampling technique through the development of an ion attraction model and an ion source model. The ion attraction model demonstrates that adjusting attraction grid voltage can enhance the detection efficiency of low-energy ions and mitigate the repulsive force of ions during sampling, which is influenced by the satellite's surface positive charging. The ion source model simulates the processes of gas ionization and ion multiplication. Simulation results indicate that the GIA can achieve a lower pressure limit below 10 -13 Pa and possess a dynamic range exceeding 10～9. These performances ensure the generation of ions with stable and consistent current, which is crucial for high-resolution and broad dynamic range mass spectrometer analysis. Preliminary testing experiments have verified GIA's capability to detect gas compositions such as H 2 O and N 2 . In-situ measurements near 311P using GIA are expected to significantly contribute to our understanding of asteroid activity mechanisms, the evolution of the atmospheric and ionized environments of main-belt comets, the interactions with solar wind, and the origin of Earth's water.

The "Taiji program in space" for space-based gravitational wave detection will open a window for observing gravitational waves in the mid-to-low-frequency range (0.1 mHz to 1 Hz). Taiji-1 satellite as the inaugural satellite, was launched on 31 August 2019, to validate several critical technologies. The radiofrequency ion micro-propulsion (RMP) as a primary propulsion system underwent in-orbit flight validation. All four thrusters successfully completed crucial performance tests. This paper will present the primary inorbit test results. It is found that the thrust range of the RMP is 5 mu N to 52 mu N. When a thrust of 50 mu N, the thrust noise is approximately 0.4 mu N/pHz (10 mHz-1 Hz), which is 2 to 4 times that of low-thrust (5 mu N). The thrust response time is about 20 ms. The characterization of RMP in orbit has reached the expectations of Taiji-1 satellite mission. The current in-orbit performance (thrust resolution, thrust noise and lifetime) of the RMP falls short of the goals set by the "Taiji program in space" and requires further enhancement.

The Tianwen-2 mission aims at exploring the near-Earth asteroid 2016HO3 and the main belt comet 311P by means of near-distance measuring and asteroid sampling and return. As one of main payloads of the Tianwen-2 mission, the Charged and Neutral Particle Analyzer (CANPA) will measure in-situ the plasmas and neutral atmosphere around the asteroid and the comet. The CANPA instrument is highly integrated and miniaturized, and composes four independent subsystems: the thermal ion analyzer (TIA), the thermal electron analyzer (TEA), the solar wind ion analyzer (SIA) and the gas and ion analyzer (GIA). The flight model has been calibrated by using the on-ground calibration system. The results show that the TIA can measure thermal ions at the energy range of 2.79 eV-32.03 keV with an energy resolution of 8.79%. The TEA can measure thermal electrons at the energy range of 1.07 eV-20.61 keV with an energy resolution of 14.29%. Both TIA and TEA have the field-of-view of 360 degrees x 90 degrees. The SIA can measure solar wind ions at the energy-per-charge range of 5 eV-5 keV with an energy resolution of 4.23%. The GIA can measure neutral gas and ions of the comet ionosphere at the mass range of 1-350 amu and the mass resolution is about 0.17 amu@1-50 amu, 0.52 amu@51-150 amu, and 1.22 amu@151-350 amu. The CANPA instrument can fulfill the detection requirements for the asteroid and the comet.

The solar wind can interact directly with the surface of airless bodies like the Moon. The interaction causes sputtering of surface materials and solar wind ions are also partially backscattered to space. Particles leaving the surface can have any charge-state. At the Moon, backscattered or sputtered positive ions1, 2, 3-4 and energetic neutral atoms5, 6, 7-8 have been observed, but all attempts to find negative ions in electron measurements have failed so far. Here we present measurements by Chang'E-6 from the lunar farside9 revealing the existence of a layer of negative ions close to the lunar surface. We found that about 2.5-0.8+1.2%\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$2.{5}_{-0.8}<^>{+1.2} \%$$\end{document} of the impinging solar wind protons charge exchange on the lunar regolith and are backscattered as negative hydrogen ions. The negative ion fraction is similar to the observed positive ion fraction1,3. We estimate a H- surface density of 0.18-0.03+0.04cm-3\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$0.1{8}_{-0.03}<^>{+0.04}\,{{{{\rm{cm}}}}}<^>{-3}$$\end{document}. On the dayside, the lifetime of negative hydrogen ions is short due to photodetachment10, confining them to a layer with a scale height of about 10 km. Such surface-bound layers or regions with negative ions should exist at any planetary object with a surface directly exposed to solar wind11,12, including low gravity bodies such as asteroids or comets.

The Light Ion Analyzer (LIA) instrument, part of the Solar-wind-Magnetosphere–Ionosphere-link- Explorer (SMILE) mission, is designed to measure the ion velocity distribution function within an energy range of 5 eV up to 25 keV. LIA provides in-situ measurements of the ion velocity distribution functions of the solar-wind and magnetosheath, from which the moments can be derived on ground, serving as an upstream input for the magnetosphere-ionosphere downstream responses. Two identical 2π sr field-of-view LIA instruments are mounted on two opposite sides of the spacecraft platform, offering a combined 4π sr instantaneous field-of-view. Each LIA consists of a top-hat electrostatic analyzer, electrostatic aperture deflectors, and a microchannel plate detector for analyzing the energy, direction, and flux of ions. Depending on operation mode, the angular resolution ranges from 22.5° to 5.625° in elevation and from 30° to 7.5° in azimuth, and the time resolution spans from 0.25 to 2 seconds. This paper describes the design of the LIA, its performance, ground calibration, operation procedures, and resultant data products.

The Martian magnetotail serves as an important channel for the escape of planetary ions, with abundant dynamic processes. After Tianwen-1 successfully entered the scientific orbit around Mars, the sun is becoming increasingly active. With the orbital apoapsis ~10,760 km, Tianwen-1 completed its first magnetotail phase from March to July 2022, providing a good opportunity to investigate the response of the Martian far magnetotail to interplanetary coronal mass ejections (ICMEs). We made a preliminary analysis of the dynamic tail under an ICME impact on 16 May 2022, with Tianwen-1 monitoring magnetotail and Mars Atmosphere and Volatile EvolutioN (MAVEN) providing upstream measurements. Based on MAVEN observations, the arrival of the ICME was determined to be around 05:10 UT on 16 May 2022. Subsequently, a significant increase in the energy levels of H+ and O+ ions was seen when Tianwen-1 entered the magnetotail about one and a half hours later. Tianwen-1 continuously detected a subset of O+ ions with energies exceeding 1 keV. Accordingly, the escape rate of O+ became ~6.2 times greater during this ICME, and the highest O+ enhancement happened between 1 keV and 3 keV. The disturbance lasted 39 hours before returning to a quiet level. Furthermore, we conducted a statistical analysis on the escape rate of O+ in the far magnetotail (attitude higher than 2 Mars radius) during 11 ICME events from March to July 2022. The ion loss rates substantially increased during ICME events, especially for O+ with energy above several keV. This observation suggests the presence of effective acceleration processes in the Martian tail under ICME conditions.

Interplanetary coronal mass ejections (ICMEs) are solar transients that have significant effects on the Martian space environment. The simultaneous spacecraft observations from Tianwen-1 and Mars Atmosphere and Volatile Evolution (MAVEN) are used to study the planetary ion escape for a dramatic ICME. MAVEN passes through the upstream solar wind, +E hemisphere, and -E hemisphere in one orbital period at 20:00 UT -24:00 UT on 2022 April 24. During this period, the interplanetary magnetic field (IMF) remained stable and dominated by the +Y component. In addition to the well-known “plume” escape channels located in the +E hemisphere, MAVEN also observed one ion escape channel in each hemisphere. The additional escape channel located in the +E hemisphere was easily identified as ionized atoms originating from the exosphere, which became significant during CME and was first reported. These ions are observed in both the solar wind and the magnetosheath, and the observed flux of these ions is strongest when MAVEN is very close to the upstream of the bow shock. In this event, ion density of this channel is up to 0.03 cm-3, which is 10 % ~ 30 % of the observed plume. The escape channel structure in the -E hemisphere is complex, and MAVEN has insufficient observation of this channel due to its orbital inclination. Tianwen-1 provided a powerful supplement based on the 1.5 hr observation of this structure, revealing many characteristics of this escape channel. The channel in the -E hemisphere also shows a narrow band in the energy spectrum, similar to the plume. Moreover, its density is between the ion densities of the two +E hemispherical channels. Interestingly, it is more likely to be observed near the magnetic pileup boundary rather than the entire -E hemisphere magnetosheath. These new channels reveal more details of Martian ion escape. The solar wind conditions similar to the early solar system during the ICMEs also help to study the early evolution of Mars.