David L. Mitchell

Magnetic flux ropes are twisted magnetic field lines that are commonly observed in space and astrophysical plasmas. At Mars, these structures are especially interesting because the planet does not have a global magnetic field like Earth, but instead has remanent magnetic fields preserved in its crust. In this study, we report a close conjunction observation by MAVEN and Tianwen-1 of a magnetic flux rope within the Martian ionosphere. The two spacecraft observed closely related magnetic signatures at near-identical locations and altitudes, allowing us to examine the spatial structure of the event. Our analysis suggests that the observed flux rope may have a loop-like geometry, with magnetic-field lines connected to the Martian dayside ionosphere at both ends. This structure may have formed or evolved through ionospheric instabilities under unusually low solar-wind dynamic pressure or internal reconnection between neighboring crustal magnetic fields. This event shows that Mars may produce a loop-like magnetic flux rope that resembles, in geometry, twisted magnetic loops seen in the solar atmosphere, providing a useful connection between planetary and solar plasma processes.

The interaction between the supermagnetosonic solar wind and the atmosphere of the weakly magnetized planet Mars forms an induced magnetosphere preceded by a magnetic pileup boundary (MPB) and bow shock (BS). In situ measurements have shown that these boundaries are permanent and well-defined structures that exhibit significant variability driven by external (Sun, solar wind) and internal (planetary) factors. In this work, we used MAVEN observations from 2014 to 2019 to analyze the coupled behavior of the BS and MPB, focusing on their spatial correlation and variability under varying conditions. We examine contiguous BS-MPB spacecraft crossings and the magnetosheath region between them, and we explore the effects of various plasma parameters on the relative position of the boundaries and the thickness of the magnetosheath. We demonstrate a statistical correlation between the BS and MPB positions across multiple timescales, including sustained local solar wind and planetary conditions, seasonal changes, and solar cycle variations. These results support the interpretation that the MPB plays a key role as an electromagnetic obstacle to the solar wind, effectively controlling the formation of the bow shock. We also confirm the limited size of the Martian magnetosheath, both in absolute terms and relative to fundamental plasma scales, which has important implications for understanding the microscopic plasma processes responsible for solar wind thermalization. While acknowledging the limitations of single-spacecraft measurements, this study provides a foundation for future multipoint probing of the Martian environment and the magnetosphere as a system.

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.

Solar extreme ultraviolet (EUV, 10–100 nm) and X‐ray (<10 nm) photons are the primary ionization source for the dayside Mars ionosphere. Their fluxes, especially at short wavelengths, increase significantly during solar flares. Among the series of solar flares and solar storms encountered by Earth and Mars in May 2024, one of the strongest solar flares occurred on 20 May 2024 when the Mars Atmospheric and Volatile EvolutioN (MAVEN) spacecraft was entering the Mars ionosphere. This event provides a unique opportunity to study the Mars ionospheric response to the onset and early phase of an extremely intense flare. This study utilizes two particle measurements onboard MAVEN to infer the flare timeline and a flare class of X12.8. The observed main ionospheric response is the increased density at ∼400 km altitude at the flare onset and at ∼300 km altitude near the flare peak.

This study uses a novel approach to infer the solar soft X‐ray (SXR) irradiance from Auger photoelectron fluxes. Solar SXR irradiance is highly variable, particularly during solar flares. Solar SXR photons at 2.5 nm produce oxygen Auger photoelectrons by photoionizing atmospheric constituents. More specifically, we derive the fit function between the Auger photoelectron fluxes observed by Mars Global Surveyor (MGS) and the 0.1–0.8‐nm solar irradiance observed by the Geostationary Operational Environmental Satellite (GOES) X‐ray Sensors (XRS). The fit function is applied to the MGS data to produce a proxy for the 0.1–0.8‐nm SXR irradiance and to classify 167 identified solar flare events from 1999 to 2006 at Mars, which is an additional vantage point away from Earth. These results are of particular interest for studying Mars's atmospheric response to solar flares and over‐the‐limb and far‐side solar activities.

The Martian magnetotail is a dynamic region where several processes contribute to plasma acceleration. Here, we analyze years of Mars Atmosphere and Volatile EvolutioN (MAVEN) data to evaluate the role of magnetic tension forces in driving plasma acceleration within current sheets in the tail. Based on magnetic field measurements, we identify 547 current sheet crossings that follow a Harris profile and find that the median observed current sheet density is 110 nA , with a typical sheet width of 100 km. We estimate a median normalized of 0.1, and force of N , capable of accelerating planetary ions within the sheets to 1 keV over 2 . We also analyze plasma energization signatures in nine high‐ case studies and find they can be explained by work done by , although observed ion differential streaming suggests additional forces may be present.

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.