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Understanding the interaction between Mars and the solar wind is crucial for comprehending the atmospheric evolution and climate change on Mars. To gain a comprehensive understanding of the Martian plasma environment, global numerical simulations are essential in addition to spacecraft observations. However, there are still discrepancies among different simulation models. This study investigates how these discrepancies stem from the considered physical processes and numerical implementations. We compare two global multispecies MHD models: the "Sun model" based on the BATS‐R‐US code and the "Sakata model" based on a newly developed multifluid model MAESTRO. By employing the same typical upstream conditions and the same neutral atmosphere for current Mars, along with similar numerical implementations such as inner boundary conditions, we obtain simulation results that exhibit unprecedented agreement between the two models. The dayside results are nearly identical, especially along the subsolar line, indicating the robustness of MHD models to predict dayside interaction under given upstream conditions and ionosphere assumptions. The escape rates of planetary ions are also in good agreement. However, discrepancies remain in the terminator and nightside regions. Detailed numerical implementations, including inner boundary conditions, magnetic field divergence control methods, and radial resolutions, are shown to influence certain aspects of the results greatly, such as magnetotail configuration and ion diffusion. Key Points: The same physical assumptions are employed for two different global multispecies MHD modelsThe results for the dayside interaction and planetary ion escape are in unprecedented good agreementDetails of the numerical implementations influence the results, especially on the nightside [ABSTRACT FROM AUTHOR]

Mars lacks a global intrinsic dipole field but possesses localized crustal fields, making it unique in the solar system. Recent observations revealed that electrons can be injected into the crustal fields, and exhibit drift-dispersed bursts due to the magnetic drift motion, which are characterized by increases or decreases in energy over time in the energy spectrum. However, the source of the drift-dispersed electrons and the mechanism of their injection into the crustal fields remains unclear. Here, by leveraging data from the Mars Atmosphere and Volatile EvolutioN mission, along with test-particle simulations, we reveal that the source of dispersed electrons is the precipitating electrons injected into the crustal fields via open field lines. These electrons display as dispersionless bursts near the source location, and as dispersed bursts as they drift away from the source. Thus, the dispersed electrons represent a later stage in the evolution of dispersionless electrons. This evolutionary process closely mirrors that observed within Earth's dipole field, affirming that the crustal fields function similarly to a mini-magnetosphere. [ABSTRACT FROM AUTHOR]

Although photochemistry-enabled escape of oxygen is a dominant atmospheric loss process at Mars today, ion outflow plays an essential role in the long-term evolution of Mars' atmosphere. Apart from heavy planetary ions such as O +, O2 +, and CO2 +, the loss of planetary protons is also important because it could be related to water loss. To study planetary proton loss due to solar wind interaction, we improve the 4-species (O +, O2 +, CO2 +, and H +) single-fluid magnetohydrodynamic (MHD) model of Mars, to a 5-species (separating planetary protons and solar wind protons) MHD model so that the two types of protons can be tracked separately. The global distributions of solar wind protons and planetary ions at low altitudes are investigated. The calculated planetary proton escape rates are larger than heavy ion loss rates and solar wind proton inflows for both solar maximum and minimum conditions. Planetary proton escape rates are 1-2 orders less than neutral hydrogen loss, suggesting that planetary protons could contribute to no >10% of the hydrogen loss under current conditions. By comparing normal cases with cases for which H-O charge exchange reactions or electron impact ionizations are switched off, we find that H-O charge exchange mainly affects densities at low altitudes, while impact ionizations exert great influence on escape rates at high altitudes. The overall results suggest the specific treatment of proton origins in models of Mars atmosphere escape provides better insight into the contributing processes, and should be included in future studies focusing on water's fate. [ABSTRACT FROM AUTHOR]

We present the first measurements of Mars discrete aurora in the extreme ultraviolet (<110 nm) and the first synoptic aurora images in the far ultraviolet (110–180 nm). Auroral emission is detected in >75% of nightside images, with patterns shifting visibly over 15–20 min. Aurora is observed most frequently in regions of open magnetic topology (where crustal magnetic fields are very weak and/or vertical), with the brightest aurora where crustal fields are strongest. We present the first disk‐averaged spectrum of discrete aurora, with several O, C, and CO features as expected for electron impact primarily on CO2. We categorize discrete auroral morphology into three types: crustal field aurora, non‐crustal field patchy aurora, and a new type we call "sinuous" aurora, an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' magnetotail. Plain Language Summary: In this study, we present near‐global images of localized aurora on Mars and the first measurements of these aurora at very short ultraviolet wavelengths (<110 nm). They are caused by energetic electrons from the solar wind smashing into Mars' upper atmosphere. We find auroras in >75% of images, with their patterns shifting visibly over 15–20 min. They are observed most frequently in regions where magnetic fields are very weak or both strong and vertical, with the brightest aurora where magnetic fields are strongest. We present the first disk‐averaged spectrum of these auroras, showing features expected for electrons striking CO2 (the most abundant gas in Mars' atmosphere). We categorize discrete auroral patterns into three types: those near strong vertical crustal magnetic field, patchy aurora near very weak crustal fields, and a new type we call "sinuous," an elongated serpentine structure that stretches thousands of kilometers into the nightside from near midnight in the northern hemisphere. These observations point to a highly dynamic environment in Mars' nightside space environment. Key Points: We present the first disk measurements of Mars discrete aurora in the EUV end FUV, with the oxygen feature at 130.4 nm being the brightestAuroras are detected in ∼75% of nightside images and are more likely where crustal fields are either very weak or both strong and verticalAn elongated, sinuous discrete aurora is discovered, extending far into the nightside. It may be related to the magnetotail current sheet [ABSTRACT FROM AUTHOR]

The Martian magnetotail exhibits a highly twisted configuration, shifting in response to changes in polarity of the interplanetary magnetic field's (IMF) dawn‐dusk (BY) component. Here, we analyze ∼6000 MAVEN orbits to quantify the degree of magnetotail twisting (θTwist) and assess variations as a function of (a) strong planetary crustal field location, (b) Mars season, and (c) downtail distance. The results demonstrate that θTwist is larger for a duskward (+BY) IMF orientation a majority of the time. This preference is likely due to the local orientation of crustal magnetic fields across the surface of Mars, where a +BY IMF orientation presents ideal conditions for magnetic reconnection to occur. Additionally, we observe an increase in θTwist with downtail distance, similar to Earth's magnetotail. These findings suggest that coupling between the IMF and moderate‐to‐weak crustal field regions may play a major role in determining the magnetospheric structure at Mars. Plain Language Summary: MAVEN magnetic field data are analyzed to understand factors that may influence the magnetotail structure at Mars. The Martian magnetotail lobes are observed to be twisted and the degree of this twist can vary. In this work, we calculate the degree of tail twist and monitor how it changes. To understand how the twist changes, we examine these variations as a function of Mars crustal field location, Mars season, and downtail distance away from Mars. Key Points: Mars' magnetotail can be twisted up to 60 deg away from its expected location based on interplanetary magnetic field (IMF) draping, much greater than Earth's tail twistMAVEN observations show that Mars' tail exhibits larger twisting for +BY IMF orientation, compared to −BY IMFMars crustal magnetic fields may play a significant role in shaping the twisted structure of the Martian magnetotail [ABSTRACT FROM AUTHOR]

We investigated the formation mechanism of a molecular ion plume and its contribution to ion escape based on Mars Atmosphere and Volatile EvolutioN (MAVEN) observations from November 2014 to October 2019 and numerical models. Here, we report a CO2+‐rich plume event and a statistical study of the molecular ion plume. MAVEN observed a CO2+‐rich plume event, in which the maximum CO2+ escape flux is approximately 4.2 × 106 cm−2s−1, on 28 August 2015 under strong solar wind dynamic pressure conditions. A numerical simulation using strong solar wind dynamic pressure conditions from the event suggested that the molecular ion plume is formed by deep penetration of the solar wind‐induced electric field, which is caused by strong solar wind dynamic pressure. A statistical study showed that CO2+ plume events tend to be observed under high solar wind dynamic pressure and strong electric field conditions. This tendency is consistent with the formation mechanism of the molecular ion plume suggested by the event study. The O2+ plume does not show the same tendency. This is because O2+ ions are abundant in the high‐altitude ionosphere, and O2+ plumes can be formed even under weak solar wind conditions. The subsolar crustal magnetic fields tend to prevent the formation of the molecular ion plume by shielding the ionosphere from the solar wind. The escape rate ratio O+:O2+:CO2+ $\left({\mathrm{O}}^{+}:{\mathrm{O}}_{2}^{+}:{\text{CO}}_{2}^{+}\right)$ is approximately 45:53:3 during the whole statistical survey period, suggesting that a molecular ion plume from the ionosphere is a non negligible ion escape channel from Mars. Plain Language Summary: It has been considered that Mars had a thick atmosphere to sustain liquid water on the surface approximately 4 billion years ago, while the Martian atmosphere is thin at the present time. Ion escape to space is one of the candidates that contributes to atmospheric loss. Both atomic oxygen ions (O+) and molecular ions, such as O2+ and CO2+, can contribute to ion escape from Mars. However, the escape mechanism and contribution of molecular ions are far from understood. Based on observations by the Mars Atmosphere and Volatile EvolutioN mission, we investigate energetic ion escape channels from Mars, which are accelerated by electric fields. The results show that the strong pressure of the solar wind facilitates energetic molecular ion escape, while crustal magnetization on the Martian surface prevents it. The escape rate ratio O+:O2+:CO2+ $\left({\mathrm{O}}^{+}:{\mathrm{O}}_{2}^{+}:{\text{CO}}_{2}^{+}\right)$ is approximately 45:53:3, suggesting the importance of energetic molecular ion escape from Mars. Key Points: When observed, polar plume flux of O2+ ${\mathrm{O}}_{2}^{+}$ (CO2+ ${\text{CO}}_{2}^{+}$) is 4 (2) times higher than O+, but they are 4 (23) times less frequently observed than O+The CO2+ ${\text{CO}}_{2}^{+}$ plume becomes detectable only under high solar wind dynamic pressure conditions, while the O2+ ${\mathrm{O}}_{2}^{+}$ plume is detected more frequentlySubsolar crustal magnetic fields tend to prevent the formation of molecular ion plumes, especially under low dynamic pressure conditions [ABSTRACT FROM AUTHOR]

Discrete aurorae have been observed at magnetized planets such as Earth and Jupiter, triggered by accelerated electrons. Similar aurorae have also been observed at Mars with only localized strong crustal magnetisms. However, our understanding of this phenomenon at Mars is still limited. In particular, direct and quantitative comparisons of the auroral and its source electron events are lacking as these two types of observations are usually made at different times and/or locations. In this study, we establish empirical criteria to select electron events ("auroral electrons") that could trigger detectable auroral emissions with Mars Atmosphere and Volatile EvolutioN measurements, thereby enabling a direct statistical comparison. We find auroral electrons share similar statistical characteristics to those previously reported for discrete auroral events. This study bridges the gap between electron observations and auroral detections and enables collaborations across different Mars missions, as well as comparative planetary studies of discrete aurora. Plain Language Summary: Aurorae have been observed in the polar regions at magnetized planets such as Earth and Jupiter, triggered by accelerated electrons. Similarly, localized auroral emissions have also been reported at Mars with no global dipole field but localized strong crustal magnetisms. However, our understanding of aurorae at Mars is still very limited in terms of their characteristics and the source electrons. In particular, direct and quantitative comparisons of the auroral and electron events are lacking as these two types of observations are usually made at different times and/or locations because of the limitation of single‐spacecraft observations. This study establishes empirical criteria to select auroral electrons and statistically compare these two types of observations. This study bridges the gap between the electron observations and auroral detections and enables collaborations across different Mars missions, as well as studies of auroral phenomena at different planets. Key Points: We establish empirical criteria to select electron events that could trigger detectable auroral observationsWe find similar statistical behaviors in selected electron events as discrete auroral events and explore the cause of statistical trendsThis study bridges the gap of these two types of observations and motivates collaborations across different Mars missions [ABSTRACT FROM AUTHOR]

Through phase-space mapping 6 yr of H+, O+, and O 2 + distributions measured in situ by the Mars Atmosphere and Volatiles EvolutioN (MAVEN) orbiter, we derive semiempirical average energetic neutral atom (ENA) distributions near Mars. Differential fluxes of H-ENAs and O-ENAs are estimated by line-integrating ENA production and loss rates in the average phase-space ion distributions. By repeating the procedure for a systematic series of vantage points, we produce synthetic ENA observations for virtual circular orbits with radii twice that of Mars, revealing the angular dependence of the observer’s position. Accounting for known ENA production and loss terms, we find that H+ in the upstream solar wind yields total antisunward H-ENA fluxes of ∼3 Ă— 105 cmâˆ'2 sâˆ'1. The dayside magnetosheath produces H-ENA angular-differential fluxes of up to 3 Ă— 105 cmâˆ'2 sâˆ'1 srâˆ'1, while the magnetosheath flanks populate the planet’s nightside with H-ENA fluxes in the range of 104â€"105 cmâˆ'2 sâˆ'1 srâˆ'1. The O-ENA environment is dominated by a near-isotropic ∼105 cmâˆ'2 sâˆ'1 srâˆ'1 relatively low-energy (tens of eV) population originating in the top-side ionosphere, particularly on the dayside. Lower fluxes (103â€"104 cmâˆ'2 sâˆ'1 srâˆ'1) of energetic O-ENAs (≳100 eV) are mainly found on the nightside and above the electric polar regions of the induced magnetosphere. Asymmetries in the ENA flow are largely limited to the energetic O-ENA populations, while the H-ENA distribution is mostly symmetric around the Sunâ€'Mars line. We discuss how synthetic ENA observations can provide insight into the near-Mars space environment, including the planet’s plasma environment and exosphere. [ABSTRACT FROM AUTHOR]

We analyzed Mars Atmosphere and Volatile Evolution (MAVEN) and Venus Express (VEX) data of magnetic flux ropes observed in both Mars and Venus nightside induced magnetosphere, in order to understand their statistical characteristics and possible formation processes. We statistically identified 382 (286) events at Mars (Venus) via the minimum variance analysis and investigated the flux rope properties including their geometrical configurations of axial core field direction and spatial distributions. Interestingly, there is no significant difference with respect to the variety of the flux rope axial orientation between Mars and Venus, indicating that about at least one fourth of events do not necessitate magnetotail reconnection for their formations. However, the spatial distribution shows that flux ropes at Venus tend to be observed near the central plasma sheet, whereas those at Mars are more spread out. Moreover, the events tend to be observed more frequently in the -E hemisphere, where the solar wind electric field (ESW) is pointing toward the planet, rather than in +E hemisphere, where the ESW direction is away from the planet. The geographical distribution shows that flux ropes at Mars tend to be observed more frequently in the southern hemisphere where the strong crustal magnetic fields are primarily distributed. Therefore, such a tendency indicates that the Martian crustal magnetic fields could play significant roles on generating flux ropes in the nightside magnetosphere of Mars. [ABSTRACT FROM AUTHOR]

Japan Aerospace Exploration Agency (JAXA) plans a Phobos sample return mission (MMX: Martian Moons eXploration). In this study, we review the related works on the past climate of Mars, its evolution, and the present climate and weather to describe the scientific goals and strategies of the MMX mission regarding the evolution of the Martian surface environment. The MMX spacecraft will retrieve and return a sample of Phobos regolith back to Earth in 2029. Mars ejecta are expected to be accumulated on the surface of Phobos without being much shocked. Samples from Phobos probably contain all types of Martian rock from sedimentary to igneous covering all geological eras if ejecta from Mars could be accumulated on the Phobos surface. Therefore, the history of the surface environment of Mars can be restored by analyzing the returned samples. Remote sensing of the Martian atmosphere and monitoring ions escaping to space while the spacecraft is orbiting Mars in the equatorial orbit are also planned. The camera with multi-wavelength filters and the infrared spectrometer onboard the spacecraft can monitor rapid transport processes of water vapor, dust, ice clouds, and other species, which could not be traced by the previous satellites on the sun-synchronous polar orbit. Such time-resolved pictures of the atmospheric phenomena should be an important clue to understand both the processes of water exchange between the surface/underground reservoirs and the atmosphere and the drivers of efficient material transport to the upper atmosphere. The mass spectrometer with unprecedented mass resolution can observe ions escaping to space and monitor the atmospheric escape which has made the past Mars to evolve towards the cold and dry surface environment we know today. Together with the above two instruments, it can potentially reveal what kinds of atmospheric events can transport tracers (e.g., H2O) upward and enhance the atmospheric escape. [ABSTRACT FROM AUTHOR]

The motion of charged particles is governed by electromagnetic forces at high altitudes at Mars and thus the characterization of electrostatic potential and electric fields is important for understanding ion escape at Mars. In this study, we utilize measurements from the Mars Atmosphere and Volatile EvolutioN mission to derive electrostatic potentials above the collisional atmosphere at Mars. We find averaged potentials to be up to ∼100 V in the magnetosheath and down to ∼−70 V in the tail, with respect to the upstream. We then derive electric fields based on averaged potential maps, ranging ∼0.01−0.1 $\sim 0.01-0.1$ V/km. These data‐derived electric fields are in good agreement with ambipolar electric fields from a multi‐fluid magnetohydrodynamic (MHD) model. MHD results also reveal that these large electric fields mainly originate from the electron pressure gradient in the magnetosheath and in the transition region from the hot solar wind flow to the cold ionospheric flow. This work provides the first data‐based characterization of global ambipolar electric fields at Mars (outside of the main ionosphere). Plain Language Summary: The motion of charged particles is governed by electric and magnetic force at high altitudes at Mars and thus the characterization of electric fields is important for understanding ion escape, a form of atmospheric escape, at Mars. In this study, we utilize measurements from the Mars Atmosphere and Volatile EvolutioN mission to derive electric fields at high altitudes, which occur at density and/or temperature gradients in a collisionless plasma to maintain charge neutrality with highly mobile electrons and much slower moving ions. These data‐derived electric fields are in good agreement with electric fields from a magnetohydrodynamic model. Model results also reveal the plasma source of these electric fields. Our characterization of these electric fields provides a better understanding of the interaction between Mars and the Sun and, to a large extent, Mars' atmospheric escape. Key Points: This work provides the first data‐based characterization of global ambipolar electric fields at Mars (outside of the main ionosphere)We find averaged ambipolar potentials ranging from ∼−70 to +100 V and ambipolar electric fields of ∼0.01−0.1 $\sim 0.01-0.1$ V/kmMHD results show a good agreement with data‐derived electric fields and also suggest the plasma source of these ambipolar electric fields [ABSTRACT FROM AUTHOR]

The Emirates Mars Mission (EMM) – Hope Probe – was developed to understand Mars atmospheric circulation, dynamics, and processes through characterization of the Mars atmosphere layers and its interconnections enabled by a unique high-altitude (19,970 km periapse and 42,650 km apoapse) low inclination orbit that will offer an unprecedented local and seasonal time coverage over most of the planet. EMM has three scientific objectives to (A) characterize the state of the Martian lower atmosphere on global scales and its geographic, diurnal and seasonal variability, (B) correlate rates of thermal and photochemical atmospheric escape with conditions in the collisional Martian atmosphere, and (C) characterize the spatial structure and variability of key constituents in the Martian exosphere. The EMM data products include a variety of spectral and imaging data from three scientific instruments measuring Mars at visible, ultraviolet, and infrared wavelengths and contemporaneously and globally sampled on both diurnal and seasonal timescale. Here, we describe our strategies for addressing each objective with these data in addition to the complementary science data, tools, and physical models that will facilitate our understanding. The results will also fill a unique role by providing diagnostics of the physical processes driving atmospheric structure and dynamics, the connections between the lower and upper atmospheres, and the influences of these on atmospheric escape. [ABSTRACT FROM AUTHOR]

We report seasonal and dust‐related variations in neutral and ion species (CO2, O, and N2, and CO2+, O2+, O+, and N+, respectively) in the dayside Martian upper atmosphere between altitudes of ∼150 and ∼250 km observed by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft. The sinusoidal seasonal variations in CO2+ and O2+ densities are clearly identified, while that of O+ is less discernible. These observed variations in ion densities are well reproduced by a photochemical equilibrium model for CO2+ and O+ densities when we combine them with solar cycle variations. Furthermore, we find a decrease in O, O+, and O2+ densities in the whole altitude range at Ls = 342–346 in MY 33 during a regional dust event. The decrease in O density would lead to decreases in O+ and O2+ densities in the ionosphere through ion‐neutral reactions. Observed variations in ion and neutral species associated with the season and a regional dust storm are also confirmed in pressure coordinates. Observations show that the CO2+/O+ ratio at a given pressure level in the ionosphere varies by a factor of ∼3, which can modify the composition of ion outflow from the Martian atmosphere. Plain Language Summary: Early Mars had a dense CO2 atmosphere that could sustain a significant amount of liquid water, but present Mars only has a 7–9 mbar CO2 atmosphere. Many CO2 gases are considered to have been lost to space. One of the mechanisms for atmospheric escape on Mars is the acceleration of the ion species in the ionosphere by the interaction with the solar wind. Since ionospheric ions can be the source of escaping ions, variability in the number density in the ionosphere might affect the escape rates. The Neutral and Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile Evolution spacecraft can provide a long‐term data set of number densities and composition in the ionosphere and thermosphere. We investigated the compositional change in number densities in the ionosphere and thermosphere because they are coupled through ion‐neutral reactions. Seasonal variations in neutral and ion densities are observed, and we find that the number densities of O, O+, and O2+ decrease when a regional dust storm occurs in the lower atmosphere. Key Points: Seasonal variations in CO2+ and O2+ in the ionosphere are significant, while those in O+ are less discernibleThe observed variations in CO2+ and O+ are well reproduced by a photochemical equilibrium modelDecreases in O, O+, and O2+ densities in the thermosphere and ionosphere are found during a regional dust storm [ABSTRACT FROM AUTHOR]

Crustal magnetic fields influence a range of plasma processes at Mars, guiding the flow of energy from the solar wind into the planet's atmosphere at some locations while shielding the atmosphere at others. In this study we investigate how the topology of crustal fields varies with changes in the direction of the incoming interplanetary magnetic field (IMF). Using plasma measurements from Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Global Surveyor (MGS), we identify magnetic topology throughout the Martian ionosphere and perform a statistical analysis of crustal magnetic field topology during different IMF configurations. We find that the topology of crustal field cusp regions is dependent on IMF direction and that cusps transition between open and closed topology regularly as they rotate through the nightside of Mars. Finally, we determine that cusps often become topologically closed due to reconnection with open magnetic fields in the Martian magnetotail, creating large closed loops that connect the dayside and nightside of Mars. Key Points: Interplanetary magnetic field direction determines which crustal magnetic field cusps are open or closed to the solar windCrustal fields frequently transition between closed and open topology as they rotate through the nightside of MarsAs cusp fields reconnect with open field lines in the magnetotail, they form large closed field lines linking the dayside and nightside of Mars [ABSTRACT FROM AUTHOR]

From February to March 2019, the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft repeatedly observed aurora near periapsis over Mars' southern strong crustal fields. During these orbits, the Solar Wind Electron Analyzer observed accelerated electrons at similar locations to where the auroras were observed, resembling the inverted‐V structure observed near Earth's auroral region. In this study, we present a case study of such an acceleration event, where we estimate a field‐aligned electrostatic potential drop of ∼440 V. We determine the field‐aligned current from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing reasonably well with the estimated net electron current carried by acceleration electrons with a maximum of 2.5 μA/m2. Similar to Earth, the potential drop develops when the ambient plasma cannot sustain the imposed field‐aligned current. We also estimate the potential layer to be located above 750‐ to 850‐km altitude and the associated electric field to be ∼0.6 V/m. Key Points: We estimate a field‐aligned potential drop of 440 V during an inverted‐V electron acceleration event concurring with auroral observationsThe current density estimated from the observed magnetic perturbation reaches 1.1 μA/m2, agreeing with estimated net electron currentSimilar to Earth, this potential drop develops when the ambient plasma is unable to support the necessary field‐aligned current [ABSTRACT FROM AUTHOR]

We present a study of changes in Martian magnetic topology induced by upstream solar wind ram pressure variations. Using electron energy spectra and pitch angle distributions measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we classify the topology of magnetic field lines in the Martian space environment across a range of solar wind conditions. We find that during periods of high solar wind dynamic pressure, draped fields are pushed to lower altitudes on the dayside of the planet, compressing closed fields. At the same time, open topology becomes more prevalent on the nightside due to the broadening of crustal cusp regions. The result is a decrease in closed topology at all locations around Mars, suggesting that the Martian ionosphere becomes significantly more exposed to solar wind energy input during high solar wind pressure. This could likely contribute to elevated levels of ion escape during these periods. Plain Language Summary: Planetary atmospheres are constantly bombarded by energy and radiation from the Sun, and over time this energy input can strip away an atmosphere to space. This process has been particularly devastating at Mars, where the loss of most of its original atmosphere has left the planet cold and desolate. This loss may have been in part due to the planet's lack of a global magnetic field. While the Earth possesses a large magnetic field that diverts much of the solar energy input, Mars does not. However, Mars does possess smaller pockets of magnetic field that are rooted in the planet's crust and extend outward through the atmosphere, a unique feature in the solar system. These magnetic fields are able to shield Mars's atmosphere on a small scale but are susceptible to changes in the strength of solar activity. Here we show that during these periods of increased solar output, the magnetic pockets across Mars are compressed to a smaller size, leaving the planet's atmosphere more exposed and more likely to escape to space. Key Points: High solar wind pressure decreases the presence of closed topology across MarsOn the dayside, draped fields extend to lower altitudes during high solar wind pressureOn the nightside, crustal cusps with open topology expand during high solar wind pressure [ABSTRACT FROM AUTHOR]

The prolonged low solar extreme ultraviolet (EUV) flux conditions provided by the deep solar minimum between solar cycles 24 and 25 can be expected to have amplified the relative influence of any solar wind driven effects on the Martian corona. As such, this solar minimum has provided an opportunity to study solar wind effects on processes that produce the energetic neutral particles that populate the oxygen corona, particularly atmospheric sputtering by precipitating energetic ions, and dissociative recombination of O 2 +. To probe the corona, we use a novel, fully empirical divergence-based technique for estimating coronal oxygen densities. This method solves the continuity equation for ions to obtain local ionization rates that are, in turn, converted to neutral densities by accounting for ionization via electron impact, charge exchange, and photoionization. We apply this technique to combined measurements of ion flux, electron flux, solar EUV flux, and magnetic fields from the Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter at Mars. We separately combine MAVEN measurements during relatively low and high upstream solar wind flux, F sw , conditions and derive the relevant fluxes for each range of conditions. We find that fluxes of planetward energetic O + upstream of Mars increase by roughly × 3 during high F sw conditions. We derive radial density profiles in the range 2.0–2.7 Mars radii and find that coronal hot oxygen densities scale with radial distance, R , as R − 2 from 300 cm − 3 to 100 cm − 3 in this range. We perform χ 2 parameter-space searches for exobase densities and effective temperatures to evaluate any potential differences in the hot O populations that populate the corona. Neither the radial profiles, nor the χ 2 search, reveal any significant effects of increased F sw on the Martian oxygen corona. The best estimates for exobase properties center around (6 – 7) × 1 0 3 atoms/cm 3 and 0.3 eV/k B for both cases, with large correlated uncertainties. Based on the uncertainties inherent to the method and the measurements, we conclude that solar wind driven effects must be limited to roughly a factor of 2 and that further investigations to empirically constrain the corona over a wider range of altitudes are needed to investigate solar wind driven effects. [ABSTRACT FROM AUTHOR]

Abstract: We develop a parametric fit to the results of a detailed magnetohydrodynamic (MHD) study of the response of ion escape rates (O+, and ) to strongly varied solar forcing factors, as a way to efficiently extend the MHD results to different conditions. We then use this to develop a second, evolutionary model of solar forced ion escape. We treat the escape fluxes of ion species at Mars as proportional to the product of power laws of four factors – that of the EUV flux R euv , the solar wind particle density R ρ , its velocity (squared) , and the interplanetary magnetic field pressure , where forcing factors are expressed in units of the current epoch-averaged values. Our parametric model is: , where ϕ(i) is the escape flux of ion i. We base our study on the results of just six provided MHD model runs employing large forcing factor variations, and thus construct a successful, first-order parametric model of the MHD program. We perform a five-dimensional least squares fit of this power law model to the MHD results to derive the flux normalizations and the indices of the solar forcing factors. For O+, we obtain the values, 1.73×1024 s−1, 0.782, 0.251, 0.382, and 0.214, for ϕ 0, α, β, γ, and δ, respectively. For , the corresponding values are 1.68×1024 s−1, −0.393, 0.798, 0.967, and 0.533. For , they are 8.66×1022 s−1, −0.427, 1.083, 1.214, and 0.690. The fit reproduces the MHD results to an average error of about 5%, suggesting that the power laws are broadly representative of the MHD model results. Our analysis of the MHD model shows that by itself an increase in R EUV enhances O+ loss, but suppresses the escape of and , whereas increases in solar wind (i.e., in , and , with R euv constant) favors the escape of heavier ions more than light ions. The ratios of escaping ions detectable at Mars today can be predicted by this parametric fit as a function of the solar forcing factors. We also use the parametric model to compute escape rates over martian history. This second parametric model expresses ion escape functions of one variable (per ion), ϕ(i)= ϕ 0(i)(t/t 0)−ξ(i). The ξ(i) are linear combinations of the epoch-averaged ion escape sensitivities, which are seen to increase with ion mass. We integrate the and oxygen ion escape rates over time, and find that in the last 3.85Gyr, Mars would have lost about mbars of , and of water (from O+ and ) from ion escape. [Copyright &y& Elsevier]

Abstract: The influence of solar EUV and solar wind conditions on ion escape at Mars is investigated using ion data from the Aspera-3 instrument on Mars Express, combined with solar wind proxy data obtained from the Mars Global Surveyor (MGS) spacecraft. A solar EUV flux proxy based on data from the Earth position, scaled and shifted in time for Mars, is used to study relatively long time scale changes related to solar EUV variability. Data from May 2004 until November 2005 has been used. A clear dependence on the strength of the subsolar magnetic field as inferred from MGS measurements is seen in the ion data. The region of significant heavy ion flows is compressed and the heavy ion flux density is higher for high subsolar magnetic field strength. Because of the difference in outflow area, the difference in estimated total outflow is somewhat less than the difference in average flux density. We confirm previous findings that escaping planetary ions are mainly seen in the hemisphere into which the solar wind electric field is pointed. The effect is more pronounced for the high subsolar magnetic field case.The average ion motion has a consistent bias towards the direction of the solar wind electric field, but the main motion is in the antisunward direction. The antisunward flow velocity increases with tailward distance, reaching above at 2 to 3 martian radii downtail from Mars for ions. Different ion species reach approximately the same bulk flow energy. We did not find any clear correlation between the solar EUV flux and the ion escape distribution or rate, probably because the variation of the solar EUV flux over our study interval was too small. The results indicate that the solar wind and its magnetic field directly interacts with the ionosphere of Mars, removing more ions for high subsolar magnetic field strength. The interaction region and the tail heavy ion flow region are not perfectly shielded from the solar wind electric field, which accelerates particles over relatively large tail distances. [Copyright &y& Elsevier]

Abstract: Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) is a subsurface and topside ionosphere radar sounder aboard the European Space Agency spacecraft Mars Express, in orbit at Mars since 25 December 2003, and in operation since 17 June 2005. The ionospheric sounding mode of MARSIS is capable of detecting the reflection of the sounding wave from the martian surface. This ability has been used in previous work to show that the surface reflection is absorbed and disappears during periods when high fluxes of energetic particles are incident on the ionosphere of Mars. These absorption events are believed to be the result of increased collisional damping of the sounding wave, caused by increased electron density below the spacecraft, in turn caused by impact ionization from the impinging particles. In this work we identify two absorption events that were isolated during periods when the surface reflection is consistently visible and when Mars is nearly at opposition. The visibility of the surface reflection is viewed in conjunction with particle and photon measurements taken at both Mars and Earth. Both absorption events are found to coincide with Earth passing through solar wind speed and ion flux signatures indicative of a corotating interaction region (CIR). The two events are separated by an interval of approximately 27 days, corresponding to one solar rotation. The first of the two events coincides with abruptly enhanced particle fluxes seen in situ at Mars. Simultaneous with the particle enhancement there are an abrupt decrease in the intensity of electron oscillations, typically seen by the Mars Express particle instrument ASPERA-3 between the magnetic pileup boundary and the martian bow shock, and a sharp drop in the solar wind pressure, seen in the proxy quantity based on MGS magnetometer observations. The decrease in oscillation intensity is therefore the probable effect of a relaxation of the martian bow shock. The second absorption event does not show a particle enhancement and complete ASPERA-3 data during that time are unavailable. Other absorption events are the apparent result of solar X-ray and XUV enhancements. We conclude that surface reflection absorption events are sometimes caused by enhanced ionospheric ionization from high energy particles accelerated by the shocks associated with a CIR. A full statistical analysis of CIRs in relation to observed absorption events in conjunction with a quantitative analysis of the deposition of ionization during space weather events is needed for a complete understanding of this phenomenon. If such analyses can be carried out, radar sensing of the martian ionosphere might be useful as a space weather probe. [Copyright &y& Elsevier]

The deep Solar Cycle (SC) 23 minimum and the modestly active SC 24 maximum have produced generally weaker solar events and heliospheric conditions. Surprisingly, on September 10, 2017 late in the declining phase of SC 24, some of the strongest solar activity erupted from active region (AR) 12673, including an X8.2-class solar flare and a fast coronal mass ejection (CME). Associated with this activity are the solar energetic particles (SEPs), which may be accelerated locally at the flare site or by the moving shock front propagating ahead of the CME. Although AR 12673 was not centrally facing Mars (it was located about 67 degrees east in heliolongitude from the Sun-to-Mars line), the solar events impacted the local space weather environment. To name a few, some of the effects 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. In this presentation we will give an overview of the observations at Mars obtained from various Mars missions, including MAVEN, MEX, and MSL. Numerical results from the Wang-Sheeley-Arge (WSA)-Enlil-cone modeling system together with observations at Earth/L1 and STEREO-A will also be presented to provide global heliospheric context of the event period. [ABSTRACT FROM AUTHOR]