Han-Wen Shen

Mars possesses a unique plasma environment formed by the interaction between the solar wind and the Martian induced magnetosphere. Ion escape from Mars has long been recognized as a key process governing the evolution of the Martian atmosphere and climate. Global hybrid simulations provide a powerful tool for investigating the large‐scale Martian plasma environment while retaining ion kinetic effects. However, many recent hybrid models simplify ionospheric ion production by prescribing ion emission from the ionosphere. In this study, we have developed a new three‐dimensional (3‐D) global hybrid simulation model, the general curvilinear plasma simulation code for Mars (GCPIC‐M), in which ionospheric ion production is self‐consistently determined through photochemical reactions. Under given upstream solar wind conditions, the simulated heavy ion density profiles in the ionosphere agree well with Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Using this model, we have quantified planetary ion escape and investigated its dependence on the interplanetary magnetic field (IMF) cone angle. The simulated escape rates are on the order of s ⁻¹ , consistent with typical values in observations. Moreover, the escape rate exhibits a nonmonotonic dependence on IMF cone angle, reaching a maximum of ∼7 × s ⁻¹ near ∼, where foreshock wave activity is enhanced. These results highlight the kinetic nature of Martian ion escape and the importance of IMF geometry. By self‐consistently linking ion production, transport, and atmospheric escape, GCPIC‐M provides a unified framework for investigating ion kinetic processes in the Martian plasma environment.

Disappearing solar wind events (DSWEs), intervals when the solar wind proton density <1 , represent one extreme of the Mars solar wind interaction. Using data from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission from February 2016 to May 2024, 154 orbits across 90 unique days were identified. During these events, the solar wind kinetic energy flux had a median value of ∼0.08 and never exceeded 0.37 , whereas normal times have a median value twice as large (∼0.16 ) and maximum values exceeding 8 . In contrast, electromagnetic energy fluxes and solar ionizing irradiance remained near normal levels. Observations of and density and fluxes from MAVEN's SupraThermal and Thermal Ion Composition (STATIC) instrument were analyzed. During DSWEs, ion density and ion flux distributions are significantly restructured, consistent with expansion of Mars' ionosphere and induced magnetosphere. Ion escape rates are significantly reduced during DSWEs compared to normal solar wind conditions, with falling to ∼75% and to ∼88% of normal values (9.45 and 1.30 ions/s). DSWE ion fluxes binned by solar wind kinetic and solar ionizing irradiance followed the same empirical relations as under normal conditions, whereas DSWE ion fluxes were depleted relative to the background trend for solar wind electromagnetic energy. Thus, the observed decrease in ion escape is likely attributable to the reduced solar wind kinetic energy flux. These results suggest that even under extreme solar wind density reductions, Mars' ion escape remains mostly governed by the same background drivers as during normal times.

Atmospheric ion escape driven by the solar wind is a key process controlling the long-term loss of the Martian atmosphere. Localized plasma clouds can carry substantial fluxes of planetary ions away from Mars, representing episodes of bulk escape. However, their origin has remained unclear due to the absence of simultaneous upstream measurements. Using joint observations from the MAVEN and Tianwen-1 missions, which provide real-time upstream monitoring, we present direct evidence that these plasma clouds are nonlinear wave packets generated by the Kelvin-Helmholtz instability (KHI). The spatial scale of KH waves is constrained for the first time via two-point measurements. Ion fluxes within plasma clouds are one to two orders of magnitude higher than those in typical steady-state escape channels. Our results indicate that KHI is an important process for solar wind coupling to planetary upper atmospheres and plays a crucial role in shaping atmospheric ion escape for unmagnetized planets.

Magnetic reconnection is a fundamental and ubiquitous process in astrophysical plasmas that converts magnetic energy into plasma kinetic and thermal energy. Throughout the heliosphere, the solar wind is permeated with current sheets (CSs), providing a natural laboratory for investigating this process. Using measurements from NASA’s Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft, we report the first direct observations of magnetic reconnection occurring within the solar wind CSs near Mars. Specifically, MAVEN observed the classic Petschek-type reconnection exhaust regions, evidenced by bifurcated magnetic field signatures and Alfvénic ion outflows. Notably, the observed exhaust region appears to be large scale, significantly exceeding the typical thickness of solar wind CSs near Mars. This suggests that magnetic reconnection may significantly broaden the CS. Our results underscore the ubiquity of magnetic reconnection across heliocentric distances and may provide new insights into the large-scale evolution of the solar wind and the development of turbulence within it.

Mars hosts a global induced magnetosphere formed through the interaction between the solar wind and its upper atmosphere. Understanding the structure of the induced magnetosphere is essential for elucidating the processes governing atmospheric ion escape. Using 9 years of measurements by NASA's Mars Atmosphere and Volatile EvolutioN (MAVEN) mission, we investigate the magnetic fields with different topologies and construct their global distributions. We find that the distribution of magnetic topologies is primarily controlled by the solar wind, particularly in the nightside magnetotail. Specifically, draped field lines preferentially occur in the +E hemisphere, where the solar wind convective electric field () points away from Mars. In contrast, open‐to‐day and closed‐to‐day field lines preferentially occur in the −E hemisphere, where points toward the planet. Open‐to‐night field lines exhibit a weaker preference for the +E hemisphere, but only near the terminator, and this asymmetry diminishes farther downtail. A comparison with Venus reveals strong similarities, suggesting that such hemispheric asymmetries are an intrinsic property of induced magnetospheres. Furthermore, we show that field lines connected to the dayside ionosphere alone cannot fully account for the formation of the cold, dense ion trail, indicating that draped field lines also play an important role.

Energy redistribution during plasma compression or expansion remains poorly understood, particularly within induced magnetospheric systems. The polytropic index serves as an important diagnostic of this process by characterizing the thermodynamic response of temperature and pressure to density variations. This study presents the first statistical determination of the ion polytropic index in the Martian magnetosheath, leveraging 9 yr of Mars Atmosphere and Volatile EvolutioN (MAVEN) observations. Derived indices largely fall within the subisothermal range, yielding a median value of 0.42. Such polytropic behavior signifies systematic ion cooling during local compression—a thermodynamic response that departs significantly from both the adiabatic condition and the near-isothermal trends typically observed in Earth’s magnetosheath. Beyond this characteristic, a well-defined spatial trend is also identified, with the index increasing radially from the bow shock toward the induced magnetosphere boundary. The polytropic index displays a clear anticorrelation with the amplitude of low-frequency magnetic field fluctuations, indicating a close association between wave activity and ion thermodynamics in the Martian magnetosheath. In summary, this study provides a pioneering quantitative characterization of ion polytropic behavior at Mars, offering a critical benchmark for comparative analyses between induced and intrinsic magnetospheric environments.

Magnetotail current sheet (CS) flapping is a universal plasma phenomenon observed at multiple planets, yet its triggering mechanisms remain poorly understood outside of Earth. At Mars, single‐spacecraft observations have also reported tail flapping, but the processes responsible for its onset have never been identified. In this study, we investigate the potential correlation between magnetic reconnection and magnetotail flapping using multipoint measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN) and Tianwen‐1 (TW‐1) missions. We analyze an example event in which MAVEN observed a reconnection‐associated CS crossing in the near tail while TW‐1 simultaneously detected CS flapping further downtail. A statistical survey of joint observations from November 2021 to February 2024 identifies that about two‐thirds of TW‐1 flapping events coincide with reconnection signatures observed by MAVEN. Multiple magnetic flux ropes were also detected before or during flapping intervals, similar to previous observations at Earth, suggesting that reconnection‐generated magnetic flux ropes may propagate tailward and drive plasma instabilities that trigger the tail flapping at Mars. These results provide the first multipoint evidence of a potential statistical correlation between magnetic reconnection and magnetotail flapping at Mars, enabling us to explore the potential triggering mechanism of magnetotail flapping. Our findings also offer new insights into Martian magnetotail dynamics and broaden the comparative understanding of this fundamental plasma process across planetary environments.

In this study, we investigated the role of interplanetary magnetic field (IMF) orientation in controlling the location and structure of the current sheet (CS) in the Martian magnetotail. Based on carefully selected cases as well as statistical studies using the magnetic field and plasma data of Mars Atmosphere and Volatile EvolutioN from 2014 October to 2020 February, our work shows that the IMF orientation can systematically influence the magnetotail CS structure of Mars. Our results reveal a systematic dawn–dusk ( Y -axis) asymmetry in CS positioning under different IMF orientations. Specifically, a sunward-directed IMF (cone angle < 60°) shifts the CS toward the dusk (+ Y ) hemisphere, while a tailward-directed IMF (cone angle > 120°) shifts it toward the dawn (− Y ) hemisphere. Under cross-flow IMF conditions (70° < cone angle < 110°), no significant CS displacement is observed. This pattern persists even after accounting for possible confounding influences such as Martian crustal magnetic fields and solar EUV intensity. Our findings suggest that the cone angle of the IMF can systematically control the CS structure and magnetic lobes in the magnetotail of Mars. This IMF-controlled CS asymmetry has implications for understanding planetary ion escape and magnetotail dynamics at unmagnetized bodies.

Magnetotail current sheet (CS) flapping is a universal plasma phenomenon observed at multiple planets, yet its triggering mechanisms remain poorly understood outside of Earth. At Mars, single-spacecraft observations have also reported tail flapping, but the processes responsible for its onset have never been identified. In this study, we investigate the potential link between magnetic reconnection and magnetotail flapping using multipoint measurements from Mars Atmosphere and Volatile EvolutioN (MAVEN) and Tianwen-1 (TW-1) missions. We analyze an example event in which MAVEN observed a reconnection-associated current sheet crossing in the near tail while TW-1 simultaneously detected CS flapping further downtail. A statistical survey of joint observations from November 2021 to February 2024 identifies that about two-thirds of TW-1 flapping events coincide with reconnection signatures observed by MAVEN. Multiple magnetic flux ropes were also detected before or during flapping intervals, similar to previous observations at Earth, suggesting that reconnection-generated magnetic flux ropes may propagate tailward and drive plasma instabilities that trigger the tail flapping at Mars. These results provide the first multipoint evidence of a potential correlation between magnetic reconnection and magnetotail flapping at Mars, enabling us to constrain the triggering mechanism of magnetotail flapping. Our findings also offer new insights into Martian magnetotail dynamics and broaden the comparative understanding of this fundamental plasma process across planetary environments.