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Mars' dayside ionosphere is maintained primarily by ionization from solar ultraviolet photons and subsequent chemical reactions, with small contributions from other mechanisms such as impact ionization and charge exchange. In December 2023, the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission observed the impact of an interplanetary coronal mass ejection (ICME) on Mars' ionosphere, including strongly enhanced fluxes of suprathermal electrons. We show that this enhancement in suprathermal electron fluxes increased ion production from electron impact, so that dayside electron impact ionization rates exceeded photoionization rates during the ICME. This change in ion production mechanisms led to unusually high densities of the minor ions C+ ${\mathrm{C}}^{+}$ and O++ ${\mathrm{O}}^{++}$. Space weather events are known to increase ion escape rates, so changes in ion composition during space weather events have important implications for atmospheric evolution. We show that scaling nominal loss rates to account for space weather may underestimate carbon loss from Mars' atmosphere. Plain Language Summary: Dayside planetary ionospheres are primarily produced through interactions between atmospheric neutral gases and sunlight. Impact by energetic particles, especially electrons, typically only contributes a small amount of plasma. We observed unusually high densities of the minor ions C+ ${\mathrm{C}}^{+}$ and O++ ${\mathrm{O}}^{++}$ in Mars' ionosphere during a space weather event in December 2023, when a large bubble of magnetized plasma launched from the Sun impacted the planet. This plasma bubble compressed the Mars magnetosheath, pushing suprathermal electrons to lower altitudes, where they impacted the atmosphere and significantly increased plasma production through impact ionization. Changes in the production mechanisms of the ionosphere during space weather events lead to changes in its density and composition. This is important because space weather events are known to increase the amount of atmospheric gas escaping from a planet, which can have important implications for how the atmosphere evolves over millions of years. Key Points: Unexpected increases in C+ and rarely detected O++ densities were observed during the December 2023 space weather event at MarsThese ions were produced by electron impact ionization from the intense electron fluxes associated with the space weather eventElectron impact ionization exceeded photoionization during the event, which temporarily altered the composition of ions escaping Mars [ABSTRACT FROM AUTHOR]

Boundaries between space plasmas occur in numerous contexts and scales, from astrophysical jets to planetary magnetospheres. Mass and momentum transport across boundaries poses a fundamental problem in magnetospheric physics. Kelvin-Helmholtz instability (KHI) is a promising mechanism to facilitate transport. Although previous studies have suggested KHI occurrence in various space plasmas, theory predicts that compressibility prevents KHI excitation at boundaries with large density gradients because of previously considered boundary structures where density varies with velocity. Based on the observations of a large density gradient boundary by MAVEN at Mars, where we can observe an extreme case, in this study, we show that it is the entropy, instead of the previously considered density, that varies with the velocity in the real velocity-sheared boundary. The entropy-based boundary structure places the velocity shear in a lower-density region than the traditional density-based structure and weakens the compressibility effect. This new boundary structure thus enables KHI excitation even at large density gradient boundaries, such as at the ionopause of unmagnetized planets and the plasmapause of magnetized planets. The result suggests the ubiquitous occurrence of KHI in the plasma universe and emphasizes its important role in planetary cold plasma escape from unmagnetized planets. [ABSTRACT FROM AUTHOR]

The typical subsolar stand‐off distance of Mars' bow shock is of the order of a solar wind ion convective gyroradius, making it highly non‐planar to incident ions. Using spacecraft observations and a test particle model, we illustrate the impact of the bow shock curvature on transient structures which form near the upstream edge of moving foreshocks caused by slow rotations in the interplanetary magnetic field (IMF). The structures exhibit noticeable decrease in the solar wind plasma density and the IMF strength within their core, are accompanied by a compressional shock layer, and are consistent with foreshock bubbles (FBs). Ion populations responsible for these structures include backstreaming ions that only appear within the moving foreshock and reflected ions with hybrid trajectories that straddle between the quasi‐perpendicular and quasi‐parallel bow shocks during slow IMF rotations. Both ion populations accumulate near the upstream edge of the moving foreshock which facilitates FB formation. Plain Language Summary: Planets in the solar system are continuously impacted by the solar wind, a plasma flow originating at the Sun and propagating through the interplanetary medium at high speeds. The solar wind also carries a magnetic field which at times contains twists or discontinuities. The discontinuities are associated with large scale electric currents that can have planar shapes. A planetary obstacle significantly modulate the solar wind plasma and the interaction of solar wind discontinuities with the modulated plasma upstream of the planet leads to formation of transient structures. Due to their relatively large size, these structures can significantly impact and destabilize plasma boundaries at lower altitudes closer to the surface. The results of this paper improve our understanding of solar wind interactions and formation of transient structures upstream of Mars. Key Points: Foreshock bubbles can form upstream of MarsSlow field rotations can cause foreshock bubbles while reflected ions from the quasi‐perpendicular bow shock contribute to their formationUnique ion kinetic scale processes exist around foreshock structures at Mars due to the different interaction size scale [ABSTRACT FROM AUTHOR]

Though ion escape to space is an important mechanism for atmospheric loss at Mars, the processes that accelerate ions to escape velocity have not been fully identified and quantified. The lowest altitude where suprathermal planetary ions appear is an important source region for ion escape, where electromagnetic forces and waves begin to energize ions to escape velocity. We have conducted a statistical study of O2+ ${\mathrm{O}}_{2}^{+}$ distribution functions measured by Mars Atmosphere and Volatile EvolutioN SupraThermal And Thermal Ion Composition in order to identify the region where suprathermal tails appear. At all solar zenith angles, suprathermal ions appear just above the exobase region, where the mean free path between collisions exceeds the neutral gas scale height. O2+ ${\mathrm{O}}_{2}^{+}$ temperature profiles are also presented. We also investigate the effects of crustal magnetism, finding that crustal fields protect planetary plasma on the dayside and enhance energization on the nightside. Plain Language Summary: Most ions in Mars' atmosphere lack enough energy to escape Mars' gravity, but ion escape to space is known to be important for atmospheric loss in the current epoch. The processes that accelerate ions to escape energy are not yet understood. We have conducted a statistical study of ion distributions in order to locate where energized ions appear. We find that ion energization begins just above the collisional atmosphere at all local times. Crustal magnetism appears to inhibit energization on the dayside and enhance energization on the nightside. Key Points: Ion temperature profiles calculated from Mars Atmosphere and Volatile EvolutioN SupraThermal And Thermal Ion Composition data are presented as a function of solar zenith angle (SZA) and altitude in the Mars ionosphereSuprathermal components appear in ion distribution functions starting just above the exobase region at all SZAsCrustal magnetic fields appear to reduce low‐altitude energization of planetary plasma on the dayside and enhance it on the nightside [ABSTRACT FROM AUTHOR]

Measurement of the dense cold thermal plasma in planetary ionospheres via orbiting spacecraft is challenging because ion energies are small (0–4 eV), densities can vary by four orders of magnitude, composition varies with altitude, spacecraft charging varies in time and must be measured very accurately, and instrumental effects (e.g., detector dead‐time and background) can be significant. The SupraThermal And Thermal Ion Composition instrument team has recently released a new set of data products that contain density moments of the primary ion species at Mars, including those derived at periapsis, subject to the full suite of calibration factors required. This article discusses the challenges associated with deriving these densities and provides examples of the key caveats that users of the data should be aware of. A preliminary statistical study of this new data set focuses on the structure and variability of Mars' ionosphere, demonstrating that solar zenith angle effects, the crustal magnetic fields, and electron precipitation on the nightside, drive the strongest structural features, consistent with photochemical theory and previous studies. Dayside ionospheric density profiles are highly repeatable below altitudes of 200 km, marking the region where photochemistry and collisions dominate. In the upper dayside ionosphere (altitudes >300–400 km) changes in the solar wind dynamic pressure on timescales of Mars Atmosphere and Volatile EvolutioN's orbit (hr) drive the largest (factors of 1–3) variability in ionospheric density. In contrast variability in ionospheric density peaks between 150 and 250 km altitude on the nightside (factors of 1–2), consistent with electron precipitation driving ionization in this region. Plain Language Summary: Making accurate measurements of the ions present in planetary atmospheres via orbiting spacecraft is difficult due to the large changes in conditions encountered throughout spacecraft orbits. This paper describes the techniques implemented to address the full array of difficulties associated with measuring planetary ions at Mars using measurements made by the SupraThermal And Thermal Ion Composition instrument. These techniques are used to calculate ion densities in Mars' atmosphere and we perform a preliminary investigation of this new data set, revealing several important characteristics of how ions behave in Mars' atmosphere. In particular, we find that on the dayside of the planet, there is a transition region between the upper and lower atmosphere where planetary ions behave differently. Above this transition region planetary ions are influenced by forces that arise in the space environment about Mars, while below this transition region, planetary ions are well shielded from these effects. Our results provide insight into the processes that shape Mars' atmosphere and tell us how energy is transported through the Mars system. Key Points: The calibration challenges and caveats of ion densities derived from SupraThermal And Thermal Ion Composition observations are discussedSolar zenith angle effects and the crustal magnetic fields drive the strongest structural features in ionospheric densityA variety of processes drive ionospheric variability throughout different regions of the Martian ionosphere [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]

The primary objective of the magnetometer and electron reflectometer (MAG/ER) experiment on Lunar Prospector (LP) is to investigate the nature and origin of the moon's magnetic fields. Measurements by early […]

In situ measurements of ionospheric and thermospheric temperatures are experimentally challenging because orbiting spacecraft typically travel supersonically with respect to the cold gas and plasma. We present O2+ ${\mathrm{O}}_{2}^{+}$ temperatures in Mars' ionosphere derived from data measured by the SupraThermal And Thermal Ion Composition instrument onboard the Mars Atmosphere and Volatile EvolutioN spacecraft. We focus on data obtained during nine special orbit maneuvers known as Deep Dips, during which MAVEN lowered its periapsis altitude from the nominal 150 to 120 km for 1 week in order to sample the ionospheric main peak and approach the homopause. We use two independent techniques to calculate ion temperatures from the measured energy and angular widths of the supersonic ram ion beam. After correcting for background and instrument response, we are able to measure ion temperatures as low as 100 K with associated uncertainties as low as 10%. It is theoretically expected that ion temperatures will converge to the neutral temperature at altitudes below the exobase region (∼180–200 km) due to strong collisional coupling; however, no evidence of the expected thermalization is observed. We have eliminated several possible explanations for the observed temperature difference between ions and neutrals, including Coulomb collisions with electrons, Joule heating, and heating caused by interactions with the spacecraft. The source of the energy maintaining the high ion temperatures remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere. Plain Language Summary: A small fraction of a planet's atmosphere, the ionosphere, exists as a plasma made of positive ions produced when sunlight removes an electron from a neutral particle. We measured the temperatures of ions in Mars' lower ionosphere for the first time since 1976 using an instrument called SupraThermal And Thermal Ion Composition on the Mars Atmosphere and Volatile EvolutioN spacecraft. In the part of the ionosphere with the most ions, ions were expected to have the same temperature as the neutral atmosphere because ions and neutrals collide with each other many times per second. However, we found that during the day, the ions were more than 100 K hotter than the neutrals. We eliminated many possible sources of energy for the ions, including sunlight, collisions with electrons, chemical reactions, and electric fields in Mars' atmosphere. The source of the energy that keeps the ions hot remains unclear, suggesting that a fundamental piece of physics is missing from existing models of the Martian ionosphere. Key Points: We present ionospheric O2+ temperatures down to 100 K derived from Mars Atmosphere and Volatile EvolutioN SupraThermal And Thermal Ion Composition measurements from 120 to 300 km altitudeIonospheric O2+ is warmer than the neutral gas more than 7 scale heights below the exobase, which requires an active source of ion heatingThe source of the ion heating remains unclear, indicating a gap in our understanding of thermalization in planetary ionospheres [ABSTRACT FROM AUTHOR]

We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of low frequency steepened fast magnetosonic waves in the Martian magnetosphere and ionosphere. Solar wind pressure pulses generated in the upstream foreshock region impact the magnetopause and generate the magnetosonic waves within the magnetosphere, in a process analogous to the production of magnetic Pc pulsations in the terrestrial magnetosphere. The draped nature of the IMF about Mars, combined with the near‐perpendicular propagation of these waves across the magnetic field, act to channel these waves into the nightside ionosphere, where they are observed in their non‐linear steepened form. Coincident‐in‐time ion observations show that the light (H+) and heavy (O+, O2+, CO2+) planetary ion distribution functions possess significant suprathermal energetic tails, arising from wave‐particle interactions with the steepened waves. The short gyro period and small gyro radius of the protons, relative to the steepened waves, results in proton heating via adiabatic compression. In contrast, the long gyro period of the heavy ions relative to the wave frequency leads to nonadiabatic heating via wave‐trapping processes. The light and heavy ion species are heated above escape energy by these waves, even down close to the exobase. A limited statistical study of 101 neighboring orbits found that similar wave events occurred on 28% of orbits analyzed, suggesting that such wave‐heating events may be important drivers of the Mars nightside ionospheric dynamics and energy budget. Our discussion includes placing our results in the context of solar wind energy transfer to the ionospheres of unmagnetized and magnetized bodies in general. Key Points: Linear and steepened magnetosonic waves are observed in the near Mars environmentDraped nature of IMF about Martian obstacle "channels" magnetosonic waves into nightside ionosphereAdiabatic and nonadiabatic wave‐particle interaction processes heat light and heavy planetary ions [ABSTRACT FROM AUTHOR]

Betatron cooling, a plasma process losing particle energy in the perpendicular direction but reserving particle energy in the field‐aligned direction, is a consequence of magnetic depression under the conservation of magnetic moment. Such process has been widely studied in the Earth's magnetosphere but has never been reported in other planetary environment. Here, by utilizing the Mars Atmosphere and Volatile EvolutioN (MAVEN) measurements, we report two events of betatron cooling in the Martian magnetotail. In one of the events, betatron cooling occurs in the suprathermal and energetic ranges of electrons, whereas in the other event, it occurs in the thermal‐energy range. We quantitatively reproduce these two processes by using an analytical model. Gratifyingly, the cooling factor derived from the analytical model agrees well with the observation of magnetic depression. These results, for the first time demonstrating the betatron‐cooling effect beyond the Earth, are useful to understand the electron dynamics in the planetary magnetosphere. Plain Language Summary: Under conservation of the magnetic moment, electrons can lose energy in the perpendicular direction but reserve energy in the field‐aligned direction when magnetic field strength becomes weaker. Such process, called betatron cooling, has not been reported in the extraterrestrial planetary environment. Using the Mars Atmosphere and Volatile EvolutioN (MAVEN) data and an analytical model, for the first time, we quantitatively demonstrate and reproduce this cooling process of electrons in the Martian magnetotail, and the cooling factor derived from the analytical model agrees well with the observation. Our study promotes the understanding of the electron dynamics in the planetary magnetosphere. Key Points: We provide the first evidence of electron betatron cooling in Martian magnetotailWe quantitatively reproduce this cooling process by using an analytical modelThe cooling factor derived from the model agrees very well with the observation [ABSTRACT FROM AUTHOR]

This observational study demonstrates that the magnitude and location of energization of electrons in the Martian magnetosheath is more complex than previous studies suggest. Electrons in Mars's magnetosheath originate in the solar wind and are accelerated by an electric field when they cross the bow shock. Assuming that this acceleration is localized solely to the shock, the field-aligned electron distributions in the sheath are expected to be highly asymmetric. However, such an asymmetry is not observed in this study. Based on the analysis here, it is suggested that an additional parallel acceleration takes place downstream of the Martian bow shock. This additional acceleration suppresses the expected asymmetry of the electron distribution. Consequently, along a flux tube in the magnetosheath that is tied on both ends to the bow shock the difference in energization between parallel and anti-parallel electrons is less than about 20 eV. Where this energization difference is expected to be maximal, we find the energization difference to be at most ...25% of the predicted value. [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]

Plasma instabilities have been studied for many decades in the terrestrial ionosphere via remote and in‐situ measurement techniques. The study of these kilometer‐scale instabilities has provided crucial insight into coupling between the neutral atmosphere and ionosphere. A special high‐time cadence observation mode developed for the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission has demonstrated for the first time that kilometer‐scale density fluctuations exist at Mars. Density and associated magnetic fluctuations are observed to occur both in and out of phase with each other, suggesting that multiple instability mechanisms are active, similar to Earth. The unique solar wind interaction with Mars and its localized crustal magnetic fields appears to play an important role in the formation of these irregularity events, which form preferentially during meridionally pointing magnetic field within the ionosphere. This is in contrast to Earth where the strong dipole magnetic field dominates in the terrestrial ionosphere. Key Points: Kilometer‐scale ionospheric density irregularities are fully resolved for the first time at MarsA range of irregularity characteristics suggests that multiple formation mechanisms are active at Mars, as at EarthThe unique interaction between Mars and the solar wind appears to play an important role in irregularity formation [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]

We describe a new method to analyze the properties of plasma waves and apply it to observations made upstream from Mars by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission. The slow measurement cadence of most charged particle instrumentation has limited the application of analysis techniques based on correlations between particle and magnetic field measurements. We show that we can extend the frequency range of applicability for these techniques, for a subset of waves that remain nearly coherent over multiple wave periods, by subsampling velocity distribution function measurements and binning them by the wave phase. This technique enables the computation of correlations and transport ratios for plasma waves previously inaccessible to this technique at Mars. By computing the cross helicity, we find that most identified waves propagate upstream in the plasma frame. This supports the conclusions of previous studies but enables a clearer determination of the intrinsic wave mode and characteristics. The intrinsic properties of observed waves with frequencies close to the proton cyclotron frequency have little spatial variability but do have large temporal variations, likely due to seasonal changes in the hydrogen exosphere. In contrast, the predominant characteristics of waves at higher frequencies have less temporal variability but more spatial variability. We find several indications of the presence of multiple wave modes in the lower frequency wave observations, with unusual wave properties observed for propagation parallel to the magnetic field and for background magnetic fields nearly perpendicular to the solar wind flow. [ABSTRACT FROM AUTHOR]

We present Mars Atmosphere and Volatile EvolutioN (MAVEN) observations of periodic (∼25 s) large‐scale (hundreds of km) magnetosonic waves propagating into the Martian dayside upper ionosphere. These waves adiabatically modulate the superthermal electron distribution function, and the induced electron temperature anisotropies drive the generation of observed electromagnetic whistler waves. The localized (in altitude) minimum in the ratio fpe/ fce provides conditions favorable for the local enhancement of efficient wave‐particle interactions, so that the induced whistlers act back on the superthermal electron population to isotropize the plasma through pitch angle scattering. These wave‐particle interactions break the adiabaticity of the large‐scale magnetosonic wave compressions, leading to local heating of the superthermal electrons during compressive wave "troughs." Further evidence of this heating is observed as the subsequent phase shift between the observed perpendicular‐to‐parallel superthermal electron temperatures and compressive wave fronts. This heating mechanism may be important at other unmagnetized bodies. Key Points: Large‐scale magnetosonic waves heat ionospheric electrons via the generation of whistler waves and subsequent wave‐particle interactionsThe observed heating mechanism demonstrates energy transfer from the solar wind to the Mars upper ionosphereSuch a heating mechanism may be important at other unmagnetized bodies such as Venus and comets [ABSTRACT FROM AUTHOR]

Electron velocity distributions in Mars's magnetosheath show a systematic erosion of the energy spectrum with distance downstream from the bow shock. Previous attempts to model this erosion invoked assumptions to promote electron ionization impact collisions with Mars's neutral hydrogen exosphere. We show that the near collision‐free magnetosheath requires a kinetic description; the population of electrons at any location is a convolution of electrons arriving from more distant regions that ultimately map directly to the solar wind. We construct a simple model that captures all the essential physics. The model demonstrates how the erosion of the electron distributions is the result of the trapping, escape, and replacement of electrons that traverse the global bow shock; some are temporarily confined to the expanding cavity formed by the cross‐shock electrostatic potential. The model also has implications for the ability of solar wind electrons to reach altitudes below the pileup boundary. Plain Language Summary: All the planets are embedded in a supersonic flow of ionized gas originating from the Sun's hot atmosphere. Thus, upstream of a planet we see shock waves that slow, heat, and divert the flow around the planet. At planets such as Mars, which lack an internal magnetic field, that shock and the sheath of diverted flow are quite close to the top of the atmosphere. Thus, it has been tempting to attribute the evolution of the gas within the sheath by invoking collisions between, for example, the shocked solar wind electrons and neutral constituents from the planet. However, such collisions are actually quite infrequent. We have developed a simple collision‐free model that shows how the apparent reduction in energy of the shocked solar wind electrons is the result of electron escape back into the exterior flow and their effective replacement by less energized electrons. This approach also highlights the ability of exterior electrons to penetrate more deeply into the atmosphere. Key Points: This study develops an idealized collisionless model for electrons in Mars's magnetosheathThe model reproduces the erosion of higher energy electron phase space within the magnetosheath [ABSTRACT FROM AUTHOR]

We investigate Martian ultralow frequency (ULF) electromagnetic waves generated by local plasma instabilities below the Martian bow shock. Recent Mars Atmosphere and Volatile EvolutioN (MAVEN) observations have shown that ULF waves generated upstream of the Martian bow shock can propagate down to the upper ionosphere, possibly facilitating heavy ion escape from Mars by heating the ionospheric plasma. In contrast to the upstream waves oscillating near the upstream proton cyclotron frequency, we identify narrow band ULF magnetic field fluctuations with frequencies near the local proton cyclotron frequency (fcp(local)) from MAVEN data. In addition to expected proton cyclotron waves locally generated in the magnetosheath, we newly identify compressional narrow band emissions near fcp(local) (and its harmonics for some cases) in the dayside upper ionosphere and in the nightside magnetotail. The dayside waves are preferentially observed for high solar extreme ultraviolet (EUV) conditions and are often associated with ring/shell‐like, hot protons of magnetosheath origin in the presence of cold, dense ionospheric protons. The nightside waves exhibit distinct preference for high‐solar‐EUV, strong‐solar‐wind conditions, under which both warm and cold protons are enhanced. The observed properties of these compressional waves are generally consistent with a proton Bernstein mode instability driven by a positive perpendicular slope in proton velocity distribution functions. The excited waves can cause perpendicular heating of thermal protons, thereby transferring energy from precipitating hot protons to cold ionospheric protons. Key Points: MAVEN observes narrow band ULF waves near the local proton cyclotron frequency below the Martian bow shockCompressional narrow band emissions are newly identified in the dayside upper ionosphere and in the nightside magnetotailThe wave‐particle interaction facilitates energy transfer from hot protons of magnetosheath origin to cold ionospheric protons [ABSTRACT FROM AUTHOR]

A combination of statistical studies and 18 case studies have been used to investigate the structure of the induced Martian magnetosphere. The different plasma and magnetic pressure forces on the dayside of the induced magnetosphere of Mars have been studied using 3.5 years of Mars Atmosphere and Volatile Evolution (MAVEN) and Mars Express (MEX) observations. We present estimates of typical values for the dominant pressure terms, that is, the thermal pressures of the ionosphere and the magnetosheath, the magnetic pressure of the magnetic pile‐up region, and the solar wind dynamic pressure. For 18 typical orbits the altitudes and relative distances of the pressure balance boundaries, the photoelectron boundary, the ion composition boundary, and the induced magnetosphere boundary are estimated. The magnetic pile‐up boundary is discussed but not further studied since earlier characterizations of the magnetic pile‐up boundary do not agree with our results. This study focuses on the transition region between the ionosphere and the magnetosheath on the dayside of Mars. We show that earlier definitions of the photoelectron boundary, ion composition boundary, and induced magnetosphere boundary do not characterize the transition region well, mainly because each boundary is based on measurements from only one or two instruments. In order to characterize the transition region correctly, changes in magnetic field strength and fluctuations, dominant ion species, electron and ion densities and energy distributions need to be considered. This article confirms a complex interaction between Mars and the solar wind and can explain why previous studies have had difficulties to describe the force balance. Key Points: This article presents an overview of the Martian dayside magnetospheric structure based on the dominant pressure termsTypical altitudes of the pressure balance boundaries, the PEB, ICB, and IMB are providedWe show that earlier defined boundaries are not a sufficient characterization of the ionosphere/magnetosheath transition region [ABSTRACT FROM AUTHOR]

Suprathermal electron depletions are structures of the nightside ionosphere of Mars resulting from an equilibrium between electron loss and creation processes. Photoionization of oxygen and carbon dioxide by UV and EUV photons is the main ionization process of the Martian atmosphere. The observation of suprathermal electron depletions is strongly unexpected in the portion of the Martian environment where photoionization can occur. This region is delimited by the UltraViolet (UV) terminator, behind which no UV ionizing photons are detected. In this study suprathermal electron depletions are used to determine the position of the UV terminator thanks to MAVEN observations. The MAVEN spacecraft is now in its fourth year of data recording and has already covered more than one Martian year, a large range of latitude, local time, and solar zenith angle in the nightside down to 110‐km altitude. This coverage enables us to determine the approximate position of the UV terminator over one Martian year. We then investigate the variation of its position on the dawnside and duskside and depending on seasons. Our results are compared with models of the Martian atmosphere and in situ data of the atmospheric composition which all highlight an asymmetry between the duskside and the dawnside at equinox. However, models show an inversion in the position of the dusk and the dawn UV terminator at perihelion and aphelion, which cannot yet be confirmed or disproved by the data. Key Points: The approximate position of the UltraViolet terminator can be determined thanks to suprathermal electron depletionsThe UV terminator is at a mean altitude of 123 km above the optical terminator over one Martian yearThe UV terminator is higher on the duskside than on the dawnside at equinox and models predict an inversion at aphelion and perihelion [ABSTRACT FROM AUTHOR]

We report, for the first time, strong evidences that a fast Fermi mechanism is taking place at the Mars bow shock. The MAVEN spacecraft observations from the Solar Wind Electron Analyzer instrument show electron flux spikes with energies up to ∼1.5 keV. These spikes are associated with sunward propagating electrons and appear when the interplanetary field line threading the spacecraft is connected near the Martian bow shock tangency point. The observed loss cone distribution is a salient feature of these backstreaming electrons as the phase space density peaks on a ring centered along the magnetic field direction. Moreover, the data show no evidence of any effect due to a hypothetical cross‐shock electric potential on the observed angular distributions. Although similar distributions are seen at the terrestrial bow shock, the quantitative analysis of the measurements strongly indicates that the electrons are produced at the shock foot and escape upstream before exploring the entire shock structure. Key Points: Flux spikes associated with sunward propagating energetic electrons observed upstream of the Martian bow shockFast Fermi process operating at the foot of the Martian shock structure rather than the rampSimilarities and strong contrasts with the terrestrial foreshock [ABSTRACT FROM AUTHOR]

Current sheets (CSs) play a crucial role in the storage and conversion of magnetic energy in planetary magnetotails. Using high‐resolution magnetic field data from MAVEN spacecraft, we report the existence of super thin current sheets (STCSs) in the Martian magnetotail. The typical half‐thickness of the STCSs is ~5 km, and it is much less than the gyroradius of thermal protons (ρp). The STCSs are embedded into a thicker sheet with L ≥ ρp forming a multiscale current configuration. The formation of STCS does not depend on ion composition, but it is controlled by the small value of the normal component of the magnetic field at the neutral plane (BN). A number of the observed multiscale CSs are located in the parametric map close to the tearing‐unstable domain, and thus, the inner STCS can provide an additional free energy to excite ion tearing mode in the Martian magnetotail. Plain Language Summary: We investigate the structure of cross‐tail current sheets (CSs) in the Martian magnetotail. The CSs play a crucial role in the conversion of magnetic energy into kinetic and thermal energy of plasma particles via magnetic reconnection. Signatures of magnetic reconnection were reported in the Martian magnetotail, but the mechanisms leading to reconnection onset are still unknown. Previous observations in the Earth magnetotail showed that the stretching of the CS magnetic configuration leads to formation of thin CS embedded into a thicker one. Such multiscale configuration can be a source of free energy for excitation of ion tearing mode. The development of this mode can generate X‐type and/or O‐type magnetic configuration and trigger reconnection. By using MAVEN spacecraft observations, we reported the formation of multiscale CS structure in the Martian magnetotail with the inner super thin current sheet (STCS) having a half‐thickness much less than proton gyroradius. We showed that the STCS can be in the metastable state. Thus, the CS thinning and the formation of STCS can lead to a new metastable equilibrium, in which the CS undergoes topological changes enabling the process of fast magnetic reconnection. Key Points: Super thin and intense current layers with a half‐thickness approximately few kilometers embedded into a thicker CS are observed in the Martian magnetotailSuper thin inner current layers are formed in the CS with small BN at the neutral plane (BN/B0 < 0.3) regardless of ion compositionA number of such multiscale CSs are located in the parameter space close to the tearing‐unstable domain [ABSTRACT FROM AUTHOR]

Open and draped magnetic field topologies are important at Mars because they can provide ionospheric particles a path to escape to space. Four years of Mars Atmosphere and Volatile EvolutioN data are analyzed in this study, demonstrating that the altitude at which the ionospheric density drops below 102 cm−3 is essentially coincident with the altitude down to which open and draped magnetic field lines are observed in the ionosphere. During times of enhanced solar wind dynamic pressure, a greater fraction of the magnetic topology was observed as open or draped (as opposed to closed) above densities of 102 cm−3. The altitudes at which the ionospheric density fell below 102 cm−3, and the magnetic field topology transitioned from closed to open or draped, also decreased during higher dynamic pressure conditions. Times of enhanced solar wind dynamic pressure thus appear to drive greater penetration of draped magnetic field into the ionosphere, enhancing the rate of reconnection between draped and crustal magnetic fields and producing more open field. Such observations may have implications for the long‐term evolution of the Martian ionosphere; the historic solar wind is thought to have been denser and faster than present‐day conditions, and "quiet time" conditions may have been equivalent to extreme dynamic pressure events today. Depending on past atmospheric conditions at Mars, draped topology may have routinely penetrated deep into the ionosphere, and quiet time rates of ionospheric escape to space may thus have been much greater for early Mars than today. Key Points: Four years of MAVEN data are analyzed to investigate the penetration of draped magnetic field into the Martian upper ionosphereThe upper extent of the Martian ionosphere coincides with the transition from closed to open and draped magnetic field topologiesOpen and draped magnetic field topologies are observed at lower altitudes during higher solar wind dynamic pressure conditions [ABSTRACT FROM AUTHOR]

Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data, revealing the presence of substantial ion outflow on the dayside of Mars. Extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the morphology of the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. Above 300 km, various ionospheric species present a roughly constant density scale height around 100 km on the dayside and 180 km on the nightside. An evaluation of the ion force balance, appropriate for regions with near‐horizontal magnetic field lines, suggests the presence of supersonic ion outflow predominantly driven by the ambient magnetic pressure, with characteristic dayside and nightside flow velocities of 4 and 20 km/s, respectively, both referred to an altitude of 500 km. The corresponding total ion outflow rates are estimated to be 5 × 1025 s−1 on the dayside and 1 × 1025 s−1 on the nightside. The data also indicate a prominent variation with magnetic field orientation in that the ion distribution over regions with near‐vertical field lines tends to be more extended on the dayside but more concentrated on the nightside, as compared to regions with near‐horizontal field lines. These observations should have important implications on the pattern of ion dynamics in the vicinity of Mars. Plain Language Summary: Prior to the Mars Atmosphere and Volatile Evolution mission, the only information on the composition of the Martian ionosphere came from the Viking Retarding Potential Analyzer data acquired on the dayside of Mars. Recently, extensive measurements made by the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer allow us to examine the Martian ionosphere not only in unprecedented detail but also on both the dayside and the nightside of the planet. By analyzing these data, we find that on each side, many of the detected ion species share a common density structure at altitudes above 300 km. Meanwhile, such a structure is clearly influenced by the ambient magnetic fields, which are well known to be inhomogeneous on Mars and cluster over the Southern Hemisphere. Near strong magnetic fields, the Martian ionosphere tends to be more extended on the dayside but more concentrated on the nightside. These findings reveal the presence of supersonic ion outflow on Mars. Such an ion outflow makes a significant contribution to plasma escape, which influences the long‐term evolution of the planet. Key Points: The topside ionosphere of Mars is characterized by a common ion density scale height of 100 km on the dayside and 190 km on the nightsideStrong bulk ion outflow is present on Mars, being supersonic at sufficiently high altitudes and driven by the ambient magnetic pressureThe ambient magnetic field has an obvious impact on the ion distribution, with distinct patterns on the dayside and the nightside of Mars [ABSTRACT FROM AUTHOR]

We report 8510-MHz (3.5-cm) radar observations of the Earth-crossing asteroid (ECA) 6489 Golevka (1991 JX) obtained between June 3 and June 15, 1995, at Goldstone, the Very Large Array and the Evpatoria (Ukraine) and Kashima (Japan) radio antennas. One-dimensional Doppler spectra are used to estimate the object's convex hull, refine the ephemeris, and yield four possible pole directions. Three-dimensional modeling using two-dimensional delay-Doppler images and published lightcurves unambiguously defines the pole and reveals an extraordinarily angular shape with flat sides, sharp edges and corners, and peculiar concavities. The equivalent diameter of the object is 530 [+ or -] 30 m, with moments of inertia about the (long, intermediate, short) axes proportional to (1.00, 1.38, 1.39) [+ or -] 0.1. The asteroid's pole direction is [Lambda] = 202 [+ or -] 5 [degrees], [Beta] = -45 [+ or -] 5 [degrees], and its sidereal period is P = 6.0289 [+ or -] 0.0001 h. The asteroid's circular polarization ratio, SC/OC = 0.23 [+ or -] 0.02, is lower than the average for radar-detected near-Earth asteroids and reveals only a modest degree of near-surface roughness at scales near the 3.5-cm wavelength. However, the approximately Lambertian radar scattering law implies considerable surface roughness at larger scales. The asteroid's radar scattering law is modeled as [Rho] [cos.sup.n] [Theta], with [Rho] = 0.25 [+ or -] 0.12 and n = 1.7 [+ or -] 0.7 giving an equivalent spherical albedo of 0.18 [+ or -] 0.09. This value is in the middle of the distribution of albedos of S-class asteroid's previously imaged by radar. The Hapke parameters describing the object's optical scattering properties are w = 0.173 [+ or -] 0.006, h = 0.024 [+ or -] 0.012, [B.sub.0] = 1.03 [+ or -] 0.45, g = -0.34 [+ or -] 0.02, and [bar][Theta] = 20 [+ or -] 5 [degrees]. Both the optical and the radar scattering properties are consistent with those of a typical S-class asteroid. Goldstone-VLA plane-of-sky images do not resolve the asteroid but do provide astrometry with uncertainties less than 0.1 arcsec. Integration of an orbit based on all available radar and optical astrometry shows that Golevka has an insignificant probability of collision with any planet during at least the next nine centuries. We investigate Golevka's dynamical environment, assuming uniform density. Some areas of the surface are characterized by large enough slopes that we expect that they are exposed, solid, monolithic rock. [C] 2000 Academic Press Key Words: asteroids; radar.

A complex boundary layer with a variety of charged particle and electromagnetic field signatures, including a transition between plasma predominantly of solar wind origin and plasma of planetary origin, lies between the Martian bow shock and the ionosphere. In this paper, we develop and utilize algorithms to autonomously identify and characterize this ion composition boundary (ICB), using data from the Mars Atmosphere and Volatile EvolutioN mission. We find an asymmetric ICB with a larger average thickness, lower altitude, and lower velocity shear in the hemisphere where the solar wind motional electric field points outward, as a result of the asymmetry of the mass loading process. The ICB thickness scales with the magnetosheath proton gyroradius at the top of the boundary layer but does not clearly vary with external drivers. The ICB location varies with solar wind ram pressure and crustal magnetic field strength, but does not clearly respond to solar wind Mach number or extreme ultraviolet irradiance. The ICB represents a distinct boundary for ion density and flow speed, but the magnetic field strength and direction typically do not vary significantly across the ICB. The plasma density and flow speed at the ICB vary seasonally, likely in response to variations in the neutral exosphere and/or atmosphere. However, the ICB on average remains at or below the altitude where pressure balance is achieved between the piled up magnetic field and the solar wind ram pressure, regardless of season or crustal magnetic field strength. Plain Language Summary: Planets without global intrinsic magnetic fields like that of the Earth interact in a distinctly different manner with the solar wind, a hot ionized gas (known as a plasma) that flows out through our solar system from the Sun. The interaction region includes a complex boundary layer where plasma dominated by the light ions of the solar wind transitions to plasma mainly composed of heavy ions derived from the planetary atmosphere. This boundary layer forms an interface between solar and planetary matter, and processes that occur in this unique region can transfer energy from the fast solar wind particles to the initially slower atmospheric particles, leading to the escape of atmospheric species. We investigate this compositional transition at Mars, utilizing data from the Mars Atmosphere and Volatile EvolutioN mission, to determine its location, structure, and response to external and internal influences. Key Points: The ion composition boundary (ICB) where the majority species changes from protons to heavy ions has asymmetries in location and structureThe ICB altitude varies with solar wind ram pressure but not other drivers, while the plasma properties at the ICB vary with seasonThe ICB lies at or below the altitude where the normal magnetic pressure balances the solar wind ram pressure, regardless of conditions [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]

Abstract: Cold ion outflow is one of the candidate processes to cause significant atmospheric escape from Mars. We here report on the cold dense heavy ion outflow event observed in Martian‐induced magnetotail by Mars Atmosphere and Volatile EvolutioN (MAVEN) on 4 December 2014. In the outflow event, the cold dense heavy ion outflow was observed only in south dusk downward convection electric field (E) lobe. During the event, the strong crustal magnetic field was located on dayside of Mars. It suggests that combination of minimagnetosphere and downward E facilitates the cold dense heavy ion outflow. In order to estimate the CO2+ density, we adopted a fitting method of log‐normal distribution to the O2+ time of flight distributions to eliminate O2+ contamination in the CO2+ time of flight data. Contribution of O2+, O+, and CO2+ to the total heavy ions (~134 cm−3) is about 71%, 26%, and 3%, respectively. Average heavy ion flux was ~5.4 × 107 cm−2s−1, assuming the escape velocity at 2,000‐km altitude. [ABSTRACT FROM AUTHOR]

Abstract: The identification of magnetic reconnection on the dayside of Mars has been elusive owing to the lack of comprehensive plasma and field measurements. Here we present direct measurements of dayside in situ reconnection signatures by the comprehensive particles and fields package on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft over strong crustal magnetic fields in the southern hemisphere of Mars. During a crossing of a bifurcated current sheet consisting of northward and southward magnetic fields, MAVEN recorded (i) ionospheric photoelectrons trapped on closed magnetic field lines, (ii) Hall magnetic fields and a nonzero normal field with polarity consistent with a crossing northward of the X line, and (iii) northward Alfvénic ion jets. Dayside magnetic reconnection on crustal magnetic fields could control the global configuration and topology of the Martian magnetosphere and alter the ion escape pattern from the dayside ionosphere. [ABSTRACT FROM AUTHOR]

Backstreaming electrons emanating from the bow shock of Mars reported from the Mars Atmosphere and Volatile EvolutioN/Solar Wind Electron Analyzer observations show a flux fall off with the distance from the shock. This feature is not observed at the terrestrial foreshock. The flux decay is observed only for electron energy E ≥ 29 eV. A reported recent study indicates that Mars foreshock electrons are produced at the shock in a mirror reflection of a portion of the solar wind electrons. In this context, and given that the electrons are sufficiently energetic to not be affected by the interplanetary magnetic field fluctuations, the observed flux decrease appears problematic. We investigate the possibility that the flux fall off with distance results from the impact of backstreaming electrons with Mars exospheric neutral hydrogen. We demonstrate that the flux fall off is consistent with the electron-atomic hydrogen impact cross section for a large range of energy. A better agreement is obtained for energy where the impact cross section is the highest. One important consequence is that foreshock electrons can play an important role in the production of pickup ions at Mars far exosphere. [ABSTRACT FROM AUTHOR]

Abstract: We present Mars Atmosphere and Volatile EvolutioN observations of large‐amplitude magnetosonic waves propagating through the magnetosheath into the Martian ionosphere near the subsolar point on the dayside of the planet. The observed waves grow in amplitude as predicted for a wave propagating into a denser, charged medium, with wave amplitudes reaching 25 nT, equivalent to ∼40% of the background field strength. These waves drive significant density and temperature variations (∼20% to 100% in amplitude) in the suprathermal electrons and light ion species (H+) that correlate with compressional fronts of the magnetosonic waves. Density and temperature variations are also observed for the ionospheric electrons, and heavy ion species (O+ and O 2 +); however, these variations are not in phase with the magnetic field variations. Whistler waves are observed at compressional wave fronts and are thought to be produced by unstable, anistropic suprathermal electrons. The magnetosonic waves drive significant ion and electron heating down to just above the exobase region. Ion heating rates are estimated to be between 0.03 and 0.2 eVs−1 per ion, and heavier ions could thus gain escape energy if located in this heating region for ∼10–70 s. The measured ionospheric density profile indicates severe ionospheric erosion above the exobase region, and this is likely caused by substantial ion outflow that is driven by the observed heating. The effectiveness of these magnetosonic waves to energize the plasma close to the exobase could have important implications for the long‐term climate evolution for unmagnetized bodies that are exposed to the solar wind. [ABSTRACT FROM AUTHOR]

Abstract: We perform a survey of 1‐Hz waves at Mars utilizing Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observations for a Martian year. We find that the 1‐Hz wave occurrence rate shows an apparent variation caused by masking of the waves by background turbulence during the times when the background turbulence levels are high. To correct for this turbulence masking, we select waves that occur in time intervals where the background turbulence levels are low. We find that the extreme ultraviolet flux does not affect the wave occurrence rate significantly, suggesting that the newly born pickup ions originating in the Mars's exosphere contribute minimally to the 1‐Hz wave generation. We find that the wave occurrence rates are higher for low Mach numbers and low beta values than for high Mach numbers and high beta values. Further, we find that a high percentage of 1‐Hz waves satisfy the group‐standing condition, which suggests that a high percentage of the waves seen as monochromatic waves in the spacecraft frame can be broadband waves in the solar wind frame that have group velocities nearly equal and opposite to the solar wind velocity. We infer that the wave occurrence rate trends with the Mach number and proton beta are a consequence of how the Mach numbers and beta values influence the wave generation and damping or how those parameters affect the group‐standing condition. Finally, we find that the 1‐Hz waves are equally likely to be found in both the quasi‐parallel and the quasi‐perpendicular foreshock regions. [ABSTRACT FROM AUTHOR]

Abstract: The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft arrived at Mars with the goal of determining the rates and mechanisms of atmospheric escape. Thermal hydrogen and hot oxygen escape are the two most important escape processes currently at work. Direct measurement of the escaping neutral hydrogen and oxygen atoms is impossible with current technology due to the low density and energy of escaping neutrals. However, when ionized and picked up by the solar wind, these escaping atoms can be detected by three particle detectors onboard MAVEN. By back‐tracing the trajectories of measured pickup ions, constraints can be placed on the density of neutrals at altitudes not accessible by other measurement methods. In this work, pickup H+ and O+ data from the Solar Energetic Particle (SEP), Solar Wind Ion Analyzer (SWIA), and SupraThermal and Thermal Ion Composition (STATIC) instruments are used to assess the variability of neutral H and O exospheres at Mars. From an analysis of 2.5 Earth years of MAVEN data, we show that a strong H escape seasonal dependence is observed by SWIA and STATIC with inferred H escape rates as low as 3 × 1025 s−1 near aphelion and as high as 4 × 1026 s−1 near perihelion. Hot O escape rates derived from SEP, SWIA, and STATIC data imply a much less variable hot O exosphere with escape rates fluctuating by a factor of 2 around a mean value of 9 × 1025 s−1. Both escape rates are in general agreement with the most recent theoretical, modeled, and observationally inferred rates. [ABSTRACT FROM AUTHOR]