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Mars once had a dense atmosphere enabling liquid water existing on its surface, however, much of that atmosphere has since escaped to space. We examine how incoming solar and solar wind energy fluxes drive escape of atomic and molecular oxygen ions (O+ and O2+ ${\mathrm{O}}_{2}^{+}$) at Mars. We use MAVEN data to evaluate ion escape from 1 February 2016 through 25 May 2022. We find that Martian O+ and O2+ ${\mathrm{O}}_{2}^{+}$ both have increased escape flux with increased solar wind kinetic energy flux and this relationship is generally logarithmic. Increased solar wind electromagnetic energy flux also corresponds to increased O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape flux, however, increased solar wind electromagnetic energy flux seems to first dampen ion escape until a threshold level is reached, at which point ion escape increases with increasing electromagnetic energy flux. Increased solar irradiance (both total and ionizing) does not obviously increase escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$. Our results suggest that the solar wind electromagnetic energy flux should be considered along with the kinetic energy flux as an important driver of ion escape, and that other parameters should be considered when evaluating solar irradiance's impact on O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape. Plain Language Summary: Mars was once like Earth with a dense atmosphere enabling liquid water to exist on its surface. However, in the billions of years since then, Mars has lost much of its atmosphere to space. We study how energy inputs from the Sun and from the solar wind can drive escape of the ionized constituents of water from Mars' atmosphere. Ion escape is one of several processes of atmospheric loss, and it is a particularly effective process for removing species heavier than hydrogen and helium from terrestrial atmospheres. We find that previously unconsidered energy fluxes may play an important role in driving ion escape. Key Points: Increased solar wind electromagnetic energy flux increases escape of O+ and O2+ ${\mathrm{O}}_{2}^{+}$O+ and O2+ ${\mathrm{O}}_{2}^{+}$ have increased escape rates with increased solar wind kinetic energyUnclear dependence on increased solar irradiance for O+ and O2+ ${\mathrm{O}}_{2}^{+}$ escape [ABSTRACT FROM AUTHOR]

ESA's Mars Express (MEX) may have observed twice solar wind protons backscattered by Phobos. However, these detections remain uncertain and call for an independent confirmation. Here, we analyze the proton measurements collected by the Suprathermal And Thermal Ion Composition experiment onboard NASA's Mars Atmosphere and Volatile EvolutioN mission (MAVEN) during the 15 closest encounters of MAVEN with Phobos (closest approach between 80 and 370 km). We use a model of magnetic connectivity to the Martian bow shock and an exospheric pickup ion model to identify the origin of the protons observed close to the Martian moon. We find no evidence of protons backscattered by Phobos. Instead, foreshock‐shock foot protons and Martian exospheric pickup protons are often observed near Phobos. These results put into new context not only the past MEX detections, but also the future observation attempts of the upcoming JAXA Martian Moons Exploration mission. Plain Language Summary: The surface a solar system body that does not have an atmosphere can be directly bombarded and processed by the small positively charged particles ejected by the Sun, known as solar wind ions. At the Earth Moon, previous work discovered that 0.1%–1% of the solar wind protons hitting the lunar surface are backscattered in space as charged particles instead of traveling through the planetary surface. The only other place where this fundamental process may have been detected so far is Phobos, a small rocky moon that orbits the red planet Mars. Indeed, ESA's satellite Mars Express (MEX) may have previously observed such protons at Phobos, but these detections remain uncertain. To advance on the question of solar wind proton backscattering at Phobos, we analyze in this article the proton measurements gathered by NASA's Mars Atmosphere and Volatile EvolutioN mission (MAVEN) mission close to Phobos. Models are employed to identify the origin of the protons observed by MAVEN and we find no evidence of protons backscattered by the Martian moon. This result puts into new context the past MEX detections and the future observation attempts of the upcoming JAXA Martian Moons Exploration mission. Key Points: Mars Atmosphere and Volatile EvolutioN mission (MAVEN) flew by Phobos 15 times at a distance lower than 370 kmWe find no evidence of solar wind protons backscattered by the surface of Phobos in the MAVEN proton measurementsForeshock—shock foot protons and exospheric pickup protons are often detected and can be misidentified as solar wind backscattered protons [ABSTRACT FROM AUTHOR]

The Martian interaction with the solar wind leads to the formation of a bow shock upstream of the planet. The shock dynamics appear complex, due to the combined influence of external and internal drivers. The extreme ultraviolet fluxes and magnetosonic Mach number are known major drivers of the shock location, while the influence of other possible drivers is less constrained or unknown such as crustal magnetic fields, solar wind dynamic pressure, or the Interplanetary Magnetic Field (IMF) intensity, and orientation. In this study, we compare the influence of the main drivers of the Martian shock location, based on several methods and published datasets from Mars Express (MEX) and Mars Atmosphere Volatile EvolutioN (MAVEN) missions. We include here the influence of the crustal fields, extreme ultraviolet fluxes, solar wind dynamic pressure, as well as (for MAVEN, thanks to magnetic field measurements) magnetosonic Mach number and Interplanetary Magnetic Field parameters (intensity and orientation angles). The bias due to the cross‐correlations among the possible drivers is investigated with a partial correlations analysis. Several model selection methods (Akaike Information Criterion and Least Absolute Shrinkage Selection Operator regression) are also used to rank the relative importance of the physical parameters. We conclude that the major drivers of the shock location are extreme ultraviolet fluxes and magnetosonic Mach number, while crustal fields and solar wind dynamic pressure are secondary drivers at a similar level. The IMF orientation also plays a significant role, with larger distances for perpendicular shocks rather than parallel shocks. Plain Language Summary: The Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) observations underlined the complex interaction between the solar wind plasma flowing out from the Sun and the Martian environment. The supersonic flow creates a bow shock (and several other plasma boundaries) upstream of the planet before it is deflected around the conductive ionized atmosphere of the planet that acts as an obstacle to the flow. Understanding the dynamics of the plasma boundaries that structure the interaction is of crucial importance to better understand the evolution of the Martian environment. In this study, we investigate in detail the complex influence of external and internal drivers of the Martian bow shock, including solar extreme ultraviolet fluxes, solar wind dynamic pressure, magnetosonic Mach number, crustal magnetic fields locked in the martian crust, as well as the Interplanetary Magnetic Field intensity and orientation. We use innovative methods to disentangle the influence of the various drivers that are correlated to each other. Key Points: The relative importance of the drivers of the Martian shock location is studied according to the AIC and LASSO model selection methodsThe Mach number and EUV fluxes are the primary drivers of the shock location, crustal fields, and solar wind pressure are secondary driversThe angle between the shock normal and the Interplanetary Magnetic Field appears as a significant driver of the shock location [ABSTRACT FROM AUTHOR]

Previous missions underlined the complex influence of the crustal magnetic fields on the Martian environment, including the plasma boundaries. Their influence on the bow shock is however poorly constrained, with most studies showing North/South differences attributed to the crustal fields, with various conclusions from little to strong variabilities. We analyze for the first time in detail the influence of crustal fields on the Martian shock location based on a multi‐mission analysis (MAVEN and MEX). We introduce the angular distance to the strongest crustal field region in the southern hemisphere that induces the largest influence (but not unique, with a minimum pressure threshold analyzed). Its impact is at large scale (>40–60° around), is modulated by the local time of the strongest source region (with no influence beyond terminator), and maximizes when the Interplanetary Magnetic field (IMF) is stable during the preceding hours. We introduce a technique, that is, partial correlations, to provide a coherent picture for both MAVEN/MEX due to existing cross correlations with Extreme UltraViolet (EUV). A composite parameter is proposed, that represents the combined influence of EUV, magnetosonic mach number (two major drivers) and crustal fields, the latter having an impact of hundreds of km. The influence of crustal fields on the shock appears seasonal and correlated with the Total Electronic Content, revealing a large scale coupling between the crustal fields, the ionosphere and the shock. The crustal field influence on the shock is thus significant and complex, with a coupling to both the ionosphere below and the IMF above. Plain Language Summary: The Mars Express (MEX) and Mars Atmosphere and Volatile Evolution (MAVEN) observations underlined the complex interaction between the solar wind plasma flowing out from the Sun and the martian environment. The supersonic flow creates a bow shock upstream of the planet, before it is deflected around the conductive ionized atmosphere of the planet that acts as an obstacle to the flow. Contrary to Earth, Mars lost its intrinsic global dynamo magnetic field that could protect efficiently the atmosphere from the incident solar wind sputtering, thus eroding the atmosphere with time. However, the planet possesses remnant crustal magnetic fields locked up in its crust, in particular in the southern hemisphere, that are known to play a major role in the martian interaction with the incident solar wind. We investigate here the influence of these crustal fields on the martian bow shock location, and show they are a significant driver according to MAVEN and MEX observations, despite being less important than the solar Extreme UltraViolet fluxes or magnetosonic mach number of the solar wind. We also show that that this influence varies with season, revealing a strong coupling between the crustal fields, the ionosphere and the bow shock location. Key Points: The influence of crustal magnetic fields on the Martian shock is significant based on the first multi‐mission studyThe strongest crustal field region has a major influence in a large angular range, when close to noon and when the interplanetary magnetic field is stableThe crustal field influence varies with season, showing a coupling between crustal fields, the ionosphere and the shock [ABSTRACT FROM AUTHOR]

Electron and magnetic field measurements from the Mars atmosphere and volatile environment (MAVEN) mission are utilized to study the interaction between Mars and the solar wind. Instruments like the solar wind electron analyzer (SWEA) aboard MAVEN measure properties of the electron environment over a broad range of electron energies. Measurements at low electron energies include contributions from spacecraft photoelectrons and secondary electrons that must be accounted for to accurately characterize the environment. We developed an algorithm to identify and remove secondary electron contamination to improve estimates of electron densities and temperature. We then compiled global maps of average electron density, temperature, and temperature anisotropy under different conditions, considering quasi-parallel and quasi-perpendicular bow shocks and upstream solar wind Alfven Mach number. Higher temperature anisotropy is observed for quasiperpendicular shock crossings, as expected. We find significant electron temperature anisotropy upstream of the bow shock for quasi-perpendicular shock crossings, suggesting a heating mechanism, such as that provided by electromagnetic waves. We analyzed the influence of hi and low Alfven Mach number conditions and found the electron plasma beta to be the only electron property significantly affected. We studied the relationship between the electron distribution function and the generation of instabilities and conclude that the upstream Alfven Mach number influences the stability of electron distributions in the Martian environment. Plain Language Summary Studying how the solar wind which flows from our sun interacts with Mars can give great insight into how Mars went from a warm, wet planet to a cold dry one over several billion years. An important part of this investigation is looking at the near-Mars space environment and how the solar wind traverses this environment to deposit energy into the upper Martian atmosphere. Electron properties often dictate the structure and variability of the planetary space environment. In this work, we utilize electron measurements from around Mars to understand how electron temperature and density affect this space environment. We also look at how unstable electron populations can transfer energy between particle populations and electromagnetic waves. However, instruments that measure electrons tend to have a significant amount of contamination in low energy measurement bins. Thus, a new algorithm is proposed to remove one source of contamination (secondary electrons from instrument and spacecraft surfaces), which leads to more accurate measurements of electron density and temperature. [ABSTRACT FROM AUTHOR]

Magnetic reconnection is often suggested to be implicated in a variety of dynamic phenomena observed in the complex magnetosphere of Mars. We present a first global survey of ion jets within current sheets, which are thought of as a primary product of magnetic reconnection, with Mars Atmosphere and Volatile EvolutioN (MAVEN) data obtained on the dayside and nightside in the Martian magnetosphere. We develop a fully automated algorithm to efficiently and reliably identify ion flow velocity deviations during current sheet crossings recorded by MAVEN. The obtained statistical results show that (i) sunward and anti‐sunward ion jets embedded within current sheets are commonly and widely observed in the dayside and nightside magnetosphere of Mars, (ii) measured magnetic field profiles and inferred magnetic topology during the jet crossings are generally consistent with those expected for reconnecting current sheets, (iii) the jet occurrence is apparently independent of upstream driver conditions, and (iv) most of the identified current sheets are thin with a roughly ion scale half‐thickness and are embedded in low beta plasma, suggesting that they are seemingly capable of reconnection based on the known onset conditions. The widespread distributions and common detection of ion jets, as well as the ubiquitous presence of thin, low‐beta current sheets, imply that magnetic reconnection could operate frequently in many locations around Mars, thereby playing an important role in the dynamics of the Martian magnetosphere for most, if not all, of the solar wind conditions in the present epoch. Key Points: Both sunward and anti‐sunward ion jets embedded within current sheets are widely and commonly observed in the near Mars spaceMagnetic field configuration and topology around the jets are generally consistent with those expected from magnetic reconnectionThe jet occurrence appears independent of upstream drivers, possibly resulting from ubiquitous formation of low‐beta, thin current sheets [ABSTRACT FROM AUTHOR]

In October 2014, the close encounter between Mars and comet Siding Spring produced a transient ionized layer in the upper atmosphere composed primarily of Mg+ and Fe+ ions. The layer was detected by instruments on three spacecraft, including the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) on Mars Express. Analyses of the MARSIS data indicated the transient layer persisted up to 19 h after the comet's closest approach. We report MARSIS observations that suggest the transient layer lasted at least 7 days—and potentially as long as 32 days—after closest approach. During this period, the transient layer was mostly confined to a narrow latitude range between 20°N and 60°N and a longitude range spanning 275°E to 95°E. Since this period coincided with a highly active Sun, we discuss how solar flares may have contributed to the layer's prolonged lifetime. Plain Language Summary: In October 2014, comet Siding Spring flew by Mars and produced a meteor shower that deposited a large amount of material in the planet's upper atmosphere. This material produced a layer of plasma in the Martian ionosphere, composed primarily of magnesium ions. Observations from multiple spacecraft detected this layer, and implied that it lasted 2 days after closest approach of the comet. We report new observations suggesting the layer lasted at least 7 days, and potentially up to 32 days after closest approach. Our results help to constrain models of metal ion chemistry in the Martian upper atmosphere. Key Points: The transient ionospheric layer at Mars caused by comet Siding Spring's flyby may have lasted at least 7 days and up to 32 daysAll detected transient layer measurements were located between 20°N and 60°N latitude, and most were confined to one longitudinal hemisphereWe discuss how solar flares may have contributed to the prolonged lifetime of the transient layer [ABSTRACT FROM AUTHOR]

The Martian plasma environment contains a complex magnetic topology, with contributions from the interplanetary magnetic field (IMF) and crustal magnetic fields. The topology can control how plasma is exchanged between the solar wind and the Martian ionosphere. Here we use 7 years of suprathermal electron pitch angle distributions recorded by the Mars Global Surveyor (MGS) spacecraft to determine the topology at 2 am and 400 km on the Martian nightside as a function of geographic location and external drivers. Observations show that topology statistically varies with IMF direction, solar wind pressure, solar extreme ultraviolet (EUV) flux, and season. Changes in topology with IMF direction and season are consistent with changes in the likelihood of magnetic reconnection between individual crustal fields and the draped IMF. Changes in topology at a given location with solar wind pressure and solar EUV flux are consistent with reconfiguration of topological structures in the plasma environment. Key Points: Magnetic topology on the nightside of Mars responds in systematic ways to external driversChanges in interplanetary magnetic field direction or in season are consistent with changes in topology via magnetic reconnectionChanges in solar wind ram pressure or in solar EUV flux are consistent with changes in the measured topology via reconfiguration [ABSTRACT FROM AUTHOR]

The ion composition boundary layer that separates solar wind from ionospheric plasma at Mars typically contains a smooth compositional transition. However, ~10% of boundary crossings display large nonmonotonic variations in density, indicating an instability. We find that most of these instabilities occur on the flanks or downstream of the planet, and many fall into two distinct classes, which occur in different locations, under different preferred conditions. Plume events, which occur where the solar wind electric field points outward, likely represent the motion of the escaping ion plume across the spacecraft. Snowplow events, which occur where the electric field points inward, likely represent the escape of discrete parcels of plasma, which could account for ~10% of the total ion loss from Mars. Snowplow events occur under conditions that favor the development of ionospheric irregularities, which could form a seed for the snowplow mechanism that detaches and accelerates ionospheric plasma downstream. Plain Language Summary: The solar wind, a hot ionized gas—or plasma—that flows from the Sun, interacts directly with plasma derived from the Martian atmosphere, across a boundary layer known as the ion composition boundary. Usually, this boundary layer has a stable structure and contains smoothly varying densities of planetary and solar wind ions. However, at some times, large nonmonotonic variations in density across this boundary layer suggest a rippled and/or unsteady boundary, indicating the presence of some instability. We investigate these unstable cases, and find that they fall into two distinct classes. Plume events likely represent a flapping motion of a discrete plume of escaping ions. Snowplow events likely represent the escape of discrete clouds of plasma from the boundary layer. These two types of events occur in different locations, under different conditions. These trends help reveal the coupling between the Martian upper atmosphere, the ionosphere, the magnetosphere, and the solar wind. Key Points: Instabilities occur near the boundary layer between ions of solar wind origin and ions of ionospheric origin at MarsWe identify two distinct classes of boundary layer instabilities, which have different characteristics, and occur in different locationsSnowplow events, a potentially significant ion loss mechanism, occur under conditions that favor an irregular ionospheric structure [ABSTRACT FROM AUTHOR]

Abstract: On 10 September 2017, some of the strongest solar activity occurred in association with active region 12673 (AR2673), including an X‐class solar flare and a fast coronal mass ejection. Although AR2673 was not centrally facing Mars, the activity impacted the local space weather conditions at Mars. We give an overview of observations obtained from the Mars Atmosphere and Volatile EvolutioN, Mars Science Laboratory, and Mars Express missions. Numerical results from the Wang‐Sheeley‐Arge (WSA)‐Enlil‐cone model together with Earth/L1 and STEREO‐A observations are also presented to provide some heliospheric context. We discuss the initial results on the space weather impacts at Mars, which include heating of the upper atmosphere by solar flare emissions, flare‐related enhancements of ion and neutral densities, solar energetic particles impacting the atmosphere and surface, bright emissions of a diffuse (global) aurora, deeply penetrating interplanetary magnetic fields over the Martian dayside, and enhanced atmospheric escape rates. [ABSTRACT FROM AUTHOR]

Magnetic holes (MHs) are pressure‐balanced structures characterized by distinct decreases in the interplanetary magnetic field strength in otherwise unperturbed solar wind. In this paper we present an analysis of MHs upstream of the Martian bow shock based on 3 months of observations by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Plasma properties within and around these structures as well as their shape characteristics are examined. We find an occurrence rate of around 2.1 events per day. About 51% of all events are of linear type with magnetic field rotation across the hole less than 10°. We observe linear MHs both as isolated events and as part of a train of MHs. The proton temperature anisotropy inside MHs increases, while alpha particles remain mostly isotropic. The average electron temperature inside MHs modestly increases with increasing hole depth. The duration of linear holes at 1.5 AU shows an increase compared to durations at smaller heliocentric distances, but the structures remain asymmetrical and ellipsoid. A case study of MHs accompanied by a population of heavy pickup ions is also discussed. Plain Language Summary: The solar wind, a supersonic flow of electrons and ions flowing radially outward from the Sun, carries a magnetic field. Magnetic holes are structures with reduced magnetic field strength that last between a few seconds to a few minutes. These structures have been observed at many places throughout the solar system. A common formation mechanism for magnetic holes is a wave mode instability that converts magnetic energy into ion thermal energy, known as the mirror mode. Mirror mode instabilities grow when the temperature of ions moving perpendicular to the magnetic field is higher than that of ions moving in the parallel direction. Mirror waves alter the magnetic field and the spatial structure of the plasma. In this paper, we use observations from the Mars Atmosphere and Volatile EvolutioN spacecraft to study magnetic holes in the space environment around Mars. Key Points: Linear and rotational magnetic holes are observed in the solar wind upstream of MarsProton temperature anisotropy inside the holes increases, while alpha particles remain mostly isotropicPickup ions create an environment favorable for generation of mirror mode structures upstream of the bow shock [ABSTRACT FROM AUTHOR]

We present a novel method to determine solar wind proxies from sheath measurements at Mars. Specifically, we develop an artificial neural network (ANN) to simultaneously infer seven solar wind proxies: ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude and its vector components, using spacecraft measurements of ion moments, magnetic field magnitude, magnetic field components in the sheath, and the solar extreme ultraviolet flux. The ANN was trained and tested using 3 years of data from the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. When compared with MAVEN spacecraft's in situ measured values of the solar wind parameters, we find that the ANN proxies for the solar wind ion density, ion speed, ion temperature, and interplanetary magnetic field magnitude have percentage differences of 50% or less for 84.4%, 99.9%, 86.8%, and 79.8% of the instances, respectively. For the cone angle and clock angle proxies, 69.1% and 53.3% of instances, respectively, have angle differences of 30∘ or less. Plain Language Summary: We introduce a new technique for determining solar wind parameter values upstream of Mars using spacecraft‐measured values of parameters in the plasma environment near Mars. This technique involves using an artificial neural network. Data in the solar wind and the plasma environment near Mars measured by the Mars Atmosphere and Volatile EvolutioN spacecraft, which has been orbiting Mars since 2014, were used to train an artificial neural network to simultaneously infer seven solar wind parameters: solar wind density, speed, temperature, and interplanetary magnetic field and its three components. Comparison of the neural network‐inferred values to the in situ measured values reveals that the artificial neural network can infer the solar wind density, speed, temperature, and the interplanetary magnetic field magnitude with high accuracies and the orientation of the magnetic field with moderate accuracies. Thus, this artificial neural network can be successfully used for inferring solar wind parameters at Mars. Since Mars lacks a dedicated solar wind monitor, unlike at Earth, this technique is useful for obtaining solar wind parameters during times when a Mars orbiter does not traverse through the solar wind upstream of Mars. Knowledge of the solar wind parameter values is essential for studying how the solar wind influences Mars' atmospheric escape. Key Points: An artificial neural network (ANN) is implemented to infer solar wind proxies from sheath measurements at MarsThe ANN can determine solar wind ion density, speed, temperature, and magnetic field magnitude with high accuraciesThe ANN can determine the magnetic field cone and the clock angles with moderate accuracies [ABSTRACT FROM AUTHOR]