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The current sheet is crucial in releasing magnetic free energy in cosmic plasmas via fast magnetic reconnection or wave excitation. This research investigates the characteristics of the Martian tail current sheet by analyzing data acquired from the Mars Atmosphere and Volatile EvolutioN spacecraft over 7 years. For the first time, we find that approximately 15% of the current sheet events display reconnection signatures, including Hall magnetic fields and fast proton flows. These reconnecting current sheets are detected more frequently on Mars than on Earth. The Martian tail current sheet is first demonstrated to be on the proton scale, which can explain the high reconnection occurrence rate. The research also reveals that the current sheets are thinner in regions closer to Mars and in the –E hemisphere. On average, reconnecting current sheets carry high fluxes of protons rather than oxygen ions. Plain Language Summary As a ubiquitous plasma configuration in various cosmic plasma environments, the current sheet is critical in facilitating the release of magnetic energy through explosive processes, such as fast magnetic reconnection. On Earth, magnetic reconnection in the tail current sheet can be responsible for triggering substorms and generating auroras. Recent observations have indicated that reconnection in the Martian tail enhances ion escape from the planet’s atmosphere. However, the frequency of reconnection events in the Martian tail and their involvement in ion loss is still not fully understood based on previous few research. In this study, we use data from the Mars Atmosphere and Volatile EvolutioN spacecraft to study the Martian tail current sheet. For the first time, we find that almost 15% of the current sheet events on Mars have reconnection signatures, happening more often than on Earth. The high occurrence rate of magnetic reconnection at the Martian tail current sheet can be attributed to its extremely thin structure, which is on the scale of protons. Magnetic reconnection may drive high fluxes of hydrogen ions and thus potentially impact the evolution of the Martian atmosphere. Key Points MAVEN recorded 880 current sheet events in the Martian tail in the past 7 years and about 15% of them show reconnection features The average thickness of the current sheet is on the proton scale and is thinner in the −E hemisphere The reconnecting tail current sheet carries a higher net tailward flux of hydrogen ions

Magnetic reconnection is a rapid process that releases magnetic free energy stored in current sheets (CSs), driving various explosive phenomena throughout the Universe. In planets within the solar system, reconnection is controlled by both the solar wind and the planet’s magnetic field and rotation. In this study, we perform a statistical analysis of the characteristics of CSs and reconnection events in the Martian magnetotail, considering varying solar wind conditions and local crustal field variations. The results show that higher solar wind dynamic pressure leads to a significantly increased probability of reconnection, primarily due to the enhanced cross-tail current density. In contrast, the orientation of interplanetary magnetic fields and local crustal anomalies predominantly affect the spatial distribution of CSs and reconnection events. Moreover, high solar wind dynamic pressure significantly enhances the average tailward flux of hydrogen and oxygen ions, likely attributable to the enhanced pickup of planetary ions.

Flux ropes (FRs), ubiquitous helical magnetic structures in solar system plasmas, are important to energy and particle transport. At Mars, where global intrinsic magnetic fields are absent, FRs form through magnetic reconnection (MR) and magnetospheric or ionospheric boundary wave instabilities (BWIs), but their role in ion escape remains controversial. Here, we first present the global distribution of MR‐ and BWI‐FRs from Martian ionosphere to magnetosheath, utilizing 4,012 FR events identified from 5‐year observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) satellite. We find that the global occurrence rate of FRs associated with BWIs is comparable with those from MR. Enhanced oxygen ion outflow fluxes and densities within most nightside BWI‐FRs suggest they predominantly originate from the dayside ionosphere/magnetosphere. These BWI‐FRs have sufficient magnetic field intensity to carry oxygen ions beyond escape energies, suggesting their potential role in facilitating global ion escape from Mars via magnetotail transport.

Electromagnetic ion cyclotron (EMIC) and mirror-mode (MM) waves are widely observed in terrestrial and other planetary magnetosheaths. Even though linear theory supports that EMIC and MM waves may grow at comparable rates under suitable plasma conditions, their coexistence is rarely reported within magnetosheaths, primarily due to their similar free energy source from anisotropic plasmas. Using MAVEN spacecraft data, we present the first direct in situ observations of mixed MM and EMIC waves in the Martian magnetosheath. Our observations reveal the concurrent presence of EMIC waves and MM waves, both generated in the Martian magnetosheath. Unlike in the Earth’s magnetosheath, where strong plasma compressions at the quasi-perpendicular bow shock can drive the growth of EMIC or MM waves, our results suggest that, in the Martian magnetosheath, the substantial ion anisotropy to generate EMIC and MM waves is provided by both upstream bow shock compressions and ubiquitous ion pickup processes of newborn ions. This study offers new insights into the role of ion pickup processes in the excitation and growth of EMIC and MM waves within planetary magnetosheaths, particularly in magnetosheaths where both shock heating and newborn ion pickup processes provide the prevailing anisotropic plasma environment, as seen in the magnetosheaths of Venus, Mars, and comets.

Magnetic reconnection between neighboring magnetic field loops, the so-called interloop reconnection, is a common process to drive flares in the solar atmosphere. However, there is no direct evidence that a similar but less explosive process can take place on planets. The strong crustal fields on Mars generate plenty of magnetic loops in the near-Mars regions, providing a unique environment to research the interloop reconnection on a planet. Here, we report magnetic reconnection events between crustal field loops in the Martian ionosphere observed by Mars Atmosphere and Volatile EvolutioN (MAVEN) for the first time. During the current layer crossing, MAVEN recorded the characteristic signals of collisionless magnetic reconnection, including the Hall magnetic field, Alfvénic outflow, and electron energization. This finding implies that the interloop reconnection in the Martian ionosphere could contribute to the localized energy deposition and particle energization, which provides the seed source for aurora in the Martian atmosphere.

The interplanetary magnetic field (IMF) is one of the primary factors influencing the Martian plasma environment. In this study, a multifluid magnetohydrodynamic model is adopted to investigate how variations in IMF affect planetary ion escape, particularly the tailward escape flux. Our results reveal that for nominal IMF direction ( 56° Parker spiral), as IMF intensity increases, the ion escape rate decreases considerably. This reduction is primarily due to the decrease in planetary ion density in the plume and the magnetotail, which is caused by the lower ion production rate through the charge exchange process under high IMF conditions. With high IMF conditions, the dynamo at the bow shock is significantly enhanced, leading to a more severe deceleration of solar wind protons and fewer protons entering the magnetosheath. Consequently, intensified electromagnetic fields create a stronger induced magnetosphere, which shields the Martian ionosphere and atmosphere. Although the enhanced loading process for planetary ions results in higher ion escape velocities, the overall ion escape fluxes decrease due to the significant reduction in planetary ion density.

Magnetic lineations on Mars, the remanent magnetization presents east–west-trending banded features, were suggested to be associated with seafloor spreading, dike intrusion, hot spot tracking, mantle convection, discrete source merging, etc. However, a missing link remains between the magnetization layer’s geometry, controlled by these processes, and the scale and magnitude of magnetic signals in the satellite orbit. Combining the magnetic field anomalies with the geometry of the crust, we find high correlations between the polarity of the radial field and the crustal thickness in the Martian southern highlands. Forward modeling is also used to reproduce the magnetic lineations based on the magnetization layer with a nonuniform thickness in a 25 km crust. The forward modeling results show lateral variation in the radial field ranges 10∼40 nT per degree, comparable to the result from the spherical harmonic model based on satellite data. Our study suggests the mantle plume upwelling causes the nonuniformity in the magnetization layer, producing strip-shaped magnetic anomalies with periodic polarity reversals.

The Martian dynamo evolution is critical for understanding Mars’s interior structure, thermal evolution, and climate change. It has been inferred to shut down at ∼4.1–4.0 Ga based on the magnetic signatures of large impact craters, but be present at ∼3.9 Ga and ∼3.7 Ga from the paleomagnetic studies and magnetic fields above volcanic units. Here, we investigate the magnetic signatures of the Iota crater, located inside the CT3-G area with a centrally strong magnetic anomaly. The Iota crater shows a weak central magnetic field with an inside-outside strength ratio of 0.39. Forward modeling is established to explore the relationship between the magnetic field signatures of craters and the magnetization caused by impact. The results show that the average magnetization of the retained materials beneath the Iota crater is about 20% of the maximum of the surroundings, indicating that the dynamo strength at that time became weak. The magnetic signatures of Iota and CT3-G reveal that the Martian dynamo decayed at ∼4.1 Ga, but did not stop completely.

Kelvin–Helmholtz (K-H) instability is a fundamental boundary instability between two fluids with different speeds, exchanging the mass, momentum, and energy across the boundary. Although the K-H instability has been suggested to play a critical role in atmospheric ion loss on Mars, the knowledge about its formation and evolution is still poor, due to the limitation of spacecraft missions and a dearth of dedicated simulation codes. In this study, we combine observations from the Mars Atmosphere and Volatile EvolutioN mission and global 3D kinetic simulations to investigate the solar wind–Mars interaction. For the first time, it is found that K-H waves prominently appear in the −E hemisphere, which is attributed to the stronger proton velocity shear therein associated with the asymmetric diamagnetic drift motion of protons. The K-H instability is mainly excited in the −E hemisphere and propagates downstream along the boundary, with the waves also able to be generated near the subsolar point. The K-H waves produce plasma clouds with a net oxygen ion escape rate of about 1.5 × 1024 s−1, contributing to almost half of the global loss on present-day Mars. This heavy ion escape pattern associated with K-H instability is cyclic and could occur on other nonmagnetized planets, potentially influencing planetary atmosphere evolution and habitability.

In the Martian induced magnetosphere, the motion of planetary ions is significantly controlled by the ambient electric fields, which can be decomposed into three components: the motional, Hall, and ambipolar electric fields. Each of them is dominant in different regions and provides the ion acceleration with a particular effectiveness. Therefore, it is necessary to characterize the global distribution of these electric field components. In this study, a global multifluid Hall-MHD model is applied, which considers the motional, Hall, and ambipolar electric fields in ion transport and magnetic induction equations to self-consistently investigate the morphology of the electric fields in the Martian space environment. Numerical results suggest that the motional electric field is dominant in the upstream of the bow shock and in the magnetosheath along the Z MSE direction, leading to the formation of the ion plume escape channel. At the bow shock, the ambipolar electric field points outward, to decelerate and deflect the solar wind plasma flow. In the magnetosheath region, the ambipolar and motional electric fields with inward direction tend to reaccelerate the solar wind ions. However, along the magnetic pileup boundary, the Hall electric field pointing outward prevents the solar wind ions from penetrating the Martian induced magnetosphere, which also prevails in the Martian magnetotail region, to accelerate the ions’ tailward escape. This is the first systematic investigation of the global distribution of electric fields, which is helpful to understand the processes of ion acceleration/deceleration and escape within the Mars–solar wind interaction.