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The effects of Martian dust storms are not only limited to lower atmospheric regime, but the increased dust storm activity could also affect the vertical structure of the constituents in the thermosphere. To this end, this paper investigates the changes in the vertical mixing of neutral and ionic species densities in the thermosphere before and during a regional (2016) and a global (2018) dust storm event; using Neutral Gas and Ion Mass Spectrometer (NGIMS)/ Mars Atmosphere and Volatile Evolution (MAVEN) observations. Care has been taken to keep a restricted solar zenith angle variation (25º) to avoid the effects of changes in solar illumination on the distribution of thermospheric species densities. Contrasting differences in the vertical distribution of neutral (CO2, CO, O, N2, Ar, He) and ionic (CO2+, O+, O2+, N2+, CO+, Ar+) atmospheric species before and during the regional and global dust storm events are noticed. [ABSTRACT FROM AUTHOR]

It is commonly believed that because of the direct solar wind interaction with the Martian atmosphere/ionosphere, the planet could have lost a significant part of its atmosphere. Closed field lines of the crustal magnetic field can weaken a transport of the ionospheric ions to the tail. Reconnection of the interplanetary magnetic field lines draping around Mars and the crustal magnetic field can also lead to a presense of sunward fluxes of planetary ions that might affect the total ion loss. The LatHyS (LATMOS Hybrid Simulation) three‐dimensional multispecies hybrid model is used here to characterize sunward fluxes of O+ ions and the magnetic field topology at Mars. It is shown that although reconnection between the interplanetary magnetic field (IMF) and the crustal magnetic fields strongly modifies the field topology, then sunward ion fluxes are rather small and do not significantly change the total ion loss. Plain Language Summary: Although Mars has no a global intrinsic magnetic field and solar wind interacts directly with the planetary atmosphere/ionosphere, the existence of strong but localized crustal magnetic field modifies the field topology around Mars. As a result, the Martian magnetosphere contains elements of the intrinsic and the induced magnetospheres. Reconnection between the interplanetary magnetic field and the crustal magnetic field can generate the plasma flows toward the planet and decrease the ionospheric losses, which is very important for the evolution of the Mars atmosphere/ionosphere. We have performed the numerical simulations of these potential effects and shown that the sunward ion fluxes are significantly less than the losses induced by the solar wind impact on the Martian ionosphere. Key Points: Hybrid simulations show a drastic change of the field topology at altitudes less than ∼1,000 km due to crustal field sourcesAlthough the magnetic field topology is modified, the sunward fluxes do not essentially affect the total ion lossSunward fluxes of oxygen ions in the tail vary between ∼5% and ∼20% compared to the anti‐sunward fluxes [ABSTRACT FROM AUTHOR]

The educational technology industry includes numerous learning providers and platforms offering cohort-based courses. In this paper, we examine, analyze, and critique one such platform called Maven. We focus our analysis on Maven because this specific platform describes itself as building 'the university of the future' and has recently received significant attention and funding, making it a compelling case study to better understand the potential roles and risks associated with education platforms operating outside of and alongside more traditional higher education institutions today and into the future. We highlight specific concerns about cohort-based platforms like Maven, including lack of transparency, risk of surveillance, lack of adequate financial support for learners, and over-reliance on social media networks as signifiers of educator/instructor qualification. Suggested benefits include adaptability, suitability to changing skills needs, and responsiveness to changing environmental scenarios. [ABSTRACT FROM AUTHOR]

The Martian bow shock has been extensively studied through magnetic field and plasma instrument observations from various Mars space missions. However, prior investigations primarily involve statistical analyses based on single spacecraft crossings, leaving the asymmetry of the Martian bow shock unstudied through simultaneous two‐spacecraft observations. In this study, utilizing simultaneous observations from Tianwen‐1 and MAVEN, we examine the instantaneous asymmetry of the Martian bow shock. We present the asymmetry of the Martian bow shock in the Mars‐Solar‐Electric and Mars‐Solar‐Orbital reference frames, possibly influenced by the solar wind motional electric field and Martian crustal magnetic field, respectively. Moreover, we suggest that the bow shock exhibits increased asymmetry under stronger solar wind motional electric field conditions. This study highlights how a two‐point observation approach offers valuable insights into the dynamic behavior of the Martian induced magnetosphere. Plain Language Summary: Scientists have been studying the Martian bow shock, which slows down the solar wind from supersonic to subsonic, using instruments on different Mars missions. But most studies only use data from one spacecraft, so we don't know much about how the bow shock varies in different places at the same time. In this study, we looked at data from two spacecraft, Tianwen‐1 and MAVEN, at the same time to see if we could find any differences. We found that the bow shock is different in the northern and southern parts of Mars in different reference frames, and this might be because of the solar wind electric field and the Martian crustal field. We also noticed that the bow shock is more different when the solar wind electric field is stronger. Such two‐spacecraft observations helps us understand more about how the magnetic environment around Mars works. Key Points: We investigated the asymmetry of Martian bow shock utilizing two‐spacecraft observations at distinct locationsThe dayside Martian crustal field may induce the north‐south asymmetry of the bow shockThe solar wind electric field may cause bow shock asymmetry in the Mars‐Solar‐Electric frame, with stronger fields increasing the effect [ABSTRACT FROM AUTHOR]

Magnetic perturbations in the Martian ionosphere are often considered as representing plasma irregularities, but thorough statistical comparisons between the two independent phenomena have been rarely conducted. In this study, we investigate the statistical relationship between total magnetic field perturbations and those of O2+ density as observed by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft at altitudes below 500 km in 2014–2023. From the 1 Hz time series of magnetic field strength, perturbation intensity is estimated by subtracting a signal smoothed with a Savitzky‐Golay filter (window size ∼36 km). Statistically, the magnetic perturbations are stronger on the dayside than at night, in the local summer hemisphere than in winter, and away from crustal magnetic anomalies than nearby. Also, day‐to‐day variability of the magnetic perturbations exhibits a weak but significant correlation with that of solar wind electron density. The statistical behavior of total magnetic field perturbations is not the same as that of O2+ density perturbations: that is, the former is not a perfect proxy for the latter. The seemingly counter‐intuitive discrepancy can be explained by effects of inhomogeneous background magnetic field strength (participating in total pressure balance across plasma perturbations) and localized ionospheric currents at altitudes unreachable by MAVEN. Plain Language Summary: Although Mars lacks a core dynamo that can create internal magnetic field as strong as the Earth, the near‐Mars space at altitudes of a few hundred kilometers (roughly corresponding to the Martian ionosphere) is filled with magnetic field coming from various sources. Small‐scale perturbation of magnetic field strength in the Martian ionosphere has often been attributed to collocated plasma density changes, but this conventional wisdom is yet to be verified by extensive data sets. In this study, we statistically investigate perturbations of magnetic field strength encountered by the MAVEN satellite at altitudes below 500 km in 2014–2023. The magnetic perturbations are stronger in the daytime than at night, in the local summer than in winter, and away from crustal magnetic anomalies than right above them. Further, their day‐to‐day changes are correlated with those of solar wind electron density surrounding Mars. These features are not in perfect agreement with those of ionospheric plasma perturbations observed by MAVEN. The difference indicates that the magnetic perturbations cannot be a straightforward proxy for those of ionospheric plasma at Mars. We discuss possible sources driving this unexpected discrepancy, such as inhomogeneous strength of background magnetic field and localized ionospheric currents at altitudes unreachable by MAVEN. Key Points: Statistics of total magnetic field perturbations at Martian low altitudes (≤500 km) are reported for 2014–2023The total field perturbations are stronger on the dayside than on the nightside and in the local summer hemisphere than in the winterThey are weaker at crustal magnetic anomalies than outside, and their statistics are different from that of plasma density perturbations [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]

Traditional Martian bow shock and magnetic pileup (magnetopause) boundary models are based on the fitting of free parameters in a prescribed formula. The form of the formula, fitted data, and considered controlling parameters distinguish the individual models from each other. However, all these models have one thing in common: the shape of the boundary and the parametric dependence assumed are fixed by the prescribed formula. The fitted data set typically consists of individual identified boundary crossings. This approach can suffer from a significant bias, as the boundary crossings are more likely to be identified in regions where a spacecraft spends more time. In this study, we use an automated region classification of the data measured by the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft to solar wind, magnetosheath, or magnetosphere. This is achieved by applying the Support Vector Machine method to individual spacecraft half-orbits (from periapsis to apoapsis or vice versa). Two different models of the locations of the bow shock and magnetic pileup boundaries are then constructed based on neural networks: i) a model trained using the classified data, and ii) a model trained using individual identified boundary crossings. As compared to formal empirical modeling efforts, the neural network models do not assume any prescribed shape/distance formula. Optimal model parameterization (considering the solar wind dynamic pressure, solar ionizing flux, crustal magnetic field magnitude, Alfvén Mach number, and interplanetary magnetic field magnitude) is discussed and the model performance evaluated. [ABSTRACT FROM AUTHOR]

In this paper, we use the key parameters data set of the Neutral Gas and Ion Mass Spectrometer from the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The particle density profiles of electrons, CO 2 + / N 2 + , CO + , O 2 + , O + , NO + , O 2 and O from 90 to 500 km have been deduced by adopting the Chapman modeling methodology. The correlation of the peak density/altitude with the solar zenith angle, the changes in the profile of the Martian ionosphere during solar flares, and the effects of Martian dust storms are analyzed. The results exhibit a positive/negative correlation between the peak density/altitude of the M2 layer and the solar zenith angle. Within the MAVEN observational record available, only three C-Class flares occurred on 26 August 2016, 29 November 2020, and 26 August 2021. The analysis reveals during these solar flare events, the electron density of the M2 layer above 200 km increases obviously. The peak density of M1 increases by 33.4%, 13.2% and 7.4%, while the peak height decreases by 0.1%, 10.2% and 4.4%, respectively. The Martian dust storm causes the peak height of the M2 layer to increase by 19.5 km, and the peak density to decrease by 4.2 ×   10 9   m − 3 . Our study shows that the Martian ionosphere is similar to the Earth's, which is of great significance for understanding the planetary ionosphere. [ABSTRACT FROM AUTHOR]

Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (<20 eV) and dense (∼>1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature. Plain Language Summary: In‐situ observations above the nightside of Mars show the presence of localized dense planetary ion fluxes at altitudes exceeding 2,000 km and escaping from the planet at high energies, comparable to that of the solar wind. These fluxes are accompanied by the reversal of sunward component of magnetic field. Unlike most atmospheric escape channels, the reported phenomenon is characterized by an increase in heavy to light ions density ratio with the distance from the planet at the observed altitudes up to nearly 5,000 km, as well as an increase in overall plasma number density inside this escape channel relative to the ambient sheath environment. This behavior is consistent with acceleration process initiated by a bent magnetic flux tube. Key Points: Ions of different species gain similar energies in the Martian magnetosheath by Hall electric fields associated with magnetic curvatureA high concentration of ionospheric ions correlates with a near void of shocked solar wind protons and a magnetic field reversalNumber density at the reversal increases with distance from Mars in comparison to the surrounding sheath at least till two Martian radii [ABSTRACT FROM AUTHOR]

The abundant observations and research established a detailed category of the terrestrial ionospheric irregularities, which significantly advanced our understanding of how the Earth system's complicated physical and chemical process generates the intermediate‐scale structures of the charged particles. Motivated by a future attempt at categorizing the Martian ionospheric irregularity, this study designs a method for naive classification of the plasma density depletion, enhancement, and oscillation based on the in situ measurements of the Martian ionosphere. The technique consists of several procedures: trend estimation, detrending and candidate extraction, and parameterization. The classification is achieved through a machine‐learning‐like process using some testing artificial density profiles. A preliminary credence test shows a good performance in separating the terrestrial low‐latitude Equatorial Plasma bubble (depletion) and mid‐latitude Median‐scale Traveling Ionospheric Disturbance (oscillation). Another detection experiment of the Martian plasma depletion events (collected by Basuvaraj et al. (2022a, https://doi.org/10.1029/2022je007302)) showed a recall rate (i.e., true positive) of 38% but with a high precision of 67.8%. Therefore, we believe the proposed method could convincingly extract different Martian ionospheric irregularities and help uncover the climatological characteristics in the future. Plain Language Summary: Ionospheric irregularities refer to the inhomogeneous distribution of the charged particles in the ionosphere, which would affect radio wave transmission and thus pose threats to tremendous human‐made assets deployed in terrestrial outer space and on other planets such as Mars. Unlike the terrestrial ionospheric irregularities, whose categories and physical mechanisms are well‐understood, the Martian counterparts are still in poor understanding. Previous studies have indicated the diversity, but the category has yet to be built. The large data set provided by the Mars Atmosphere and Volatile EvolutioN (MAVEN) mission allows a more detailed investigation of the Martian ionospheric irregularities. Therefore, an automatic method to extract the ionospheric density enhancement, depletion, and oscillation would be a first step to figuring out the exact types of the Martian ionospheric irregularities, thus helping to extend our knowledge of how the finer ionospheric structures are formed backgrounded by the region crustal magnetic field other than the global dipole field. Key Points: A modified rolling‐barrel algorithm extracts the candidate irregularitiesClassification of the candidates into depletion, enhancement, and oscillation is achieved through a few parameterization processesInitial testing shows good performance on the Martian and terrestrial ionospheric irregularities [ABSTRACT FROM AUTHOR]

In this study, we conducted a comparative analysis of gravity waves (GWs) in the Martian upper thermosphere, utilizing He and Ar densities measured by the Neutral Gas and Ion Mass Spectrometer aboard the Mars Atmosphere and Volatile EvolutioN spacecraft. Our investigation revealed that GWs are a persistent phenomenon in Martian thermospheric He densities, akin to their presence in heavier species like Ar and CO2. Intriguingly, we observed that local time and seasonal variations in He GW amplitudes exhibit an almost opposite pattern compared to those in Ar. A significant breakthrough was the identification of a temperature‐dependent transition wavelength distinctly demarcating short and long wavelengths. This transition wavelength, which increases with increasing background temperatures, profoundly influences the dominance of GW amplitudes in He and Ar. In the short‐wavelength limit, GW amplitudes in He prevail, while GW amplitudes in Ar take precedence in the long‐wavelength limit. During periods of lower temperatures during midnight and the dawn terminator, transition wavelengths fall within the observed wavelength range enabling the study of dominance of GWs in He and Ar. Conversely, higher temperatures (>180 K) during daytime and the dusk terminator eliminate transition wavelengths within our observational range. Consequently, the amplitudes of GWs in He dominate over those in Ar. Furthermore, under nominal dust conditions, GWs in both He and Ar exhibit an inverse relationship with their respective background densities. During global dust storm, this inverse relationship is maintained for GWs in He, while GWs in Ar display a slight positive correlation with background Ar densities. Plain Language Summary: Gravity waves (GWs) are a key aspect of planetary upper atmospheres, significantly impacting the local momentum and energy balances. In our study, we used the Mars upper atmospheric (altitudes of 160–200 km) helium (He) and argon (Ar) density data collected by a spacecraft to examine the behavior of GWs. We found that these GWs are always present and show up in the lighter gas like He just as much as they do in the heavier gases like Ar. But, the GWs in He show almost opposite local time and seasonal variations compared to those in Ar. We discovered a transition wavelength that decides how these waves behave. The transition wavelength depends on how hot or cold the atmosphere is. When it is hot, GWs in He dominate and when it is cold, GWs in Ar take over. The transition wavelength changes as the day goes by because the temperature changes. Lastly, we learned that when there is a lot of dust in the Mars atmosphere, GWs in both He and Ar relate differently to their respective background densities. GWs in He still behave the same way as without dust, but GWs in Ar start acting in a different way. Key Points: Gravity waves in He show nearly opposite local time and seasonal variations compared to those in ArA transition wavelength exists in the amplitude‐wavelength relation, below which GWs in He dominate, and above which GWs in Ar take overThe transition wavelength increases with rising temperatures and consequently shows variations across local times [ABSTRACT FROM AUTHOR]

Using Mars Atmosphere and Volatile Evolution observations, we characterize the variability of water vapor in the Martian thermosphere during Mars Years 32–36. Near a fixed atmospheric pressure level of ∼5 × 10−7 Pa, the typical water density is 1.7 (±1.0) × 103 cm−3 and the typical water mixing ratio is 14.6 (±9.0) ppm. Thermospheric water levels are higher during the southern spring and summer seasons when Mars is near perihelion and there is significant dust loading in the lower atmosphere. However, the seasonal variation is not the same from year‐to‐year, likely due to annual differences in dust loading. Water vapor abundance is highly correlated with the amount of dust in the lower atmosphere, and increases during both regional and global dust storms. Additionally, there is a local time asymmetry with higher mixing ratios observed during morning local times, potentially caused by atmospheric circulation. Our results support previous work that found increased dust levels allow more water to be supplied directly to the thermosphere. Plain Language Summary: Water vapor in the atmosphere of Mars can be transported to high altitudes and ultimately lost to space. Hence, understanding the distribution and variability of water throughout the atmosphere is critical for determining the processes that control water loss at Mars. We present new results on the variability of water vapor in the thermosphere of Mars, a region where few water observations exist. We find there is more water during the southern spring and summer seasons when Mars is closest to the Sun and there is a significant amount of dust in the lower atmosphere. We also find that the amount of water is highly correlated with the amount of dust in the lower atmosphere. These results validate previous studies that have shown atmospheric heating from increased dust levels allows more water to be transported to high altitudes without condensing. Key Points: We characterize the variations of thermospheric water with season, local time, and atmospheric dust content in Mars Years 32–36There is an asymmetry with respect to local time, with higher mixing ratios observed during morning local times (6–9 hr)The water abundance is highly correlated with the amount of dust in the lower atmosphere [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 Heavy cold ions at Mars are gravitationally bound to the planet unless some process provides energy to them. Observations show that cold (1 cm−3) O+/O2+ ions with bulk velocities equal to energies ∼1 keV can reach deep into the nightside Martian magnetosheath. These ions are co‐located with a change of the sign of the sunward component of the magnetic field. This magnetic field topology implies the persistence of a localized planetary ions escape channel associated with draped magnetic field lines that are convecting tailward. The observed ion populations propagate approximately in the same direction as surrounding magnetosheath flow and are likely to be almost unheated ionospheric ions from low altitudes. The paper discusses planetary ion energization via Hall electric field originated from ions and electron separation associated with magnetic field curvature.

Like its terrestrial counterpart, the Martian bow shock can exhibit foot, ramp, and overshoot substructures. The shock extent is known to vary with solar wind and season, but it remains an open question whether other features of the bow shock also vary seasonally. By comparing parameters of the shock during different seasons and solar wind conditions, we investigated whether the presence of seasonal planetary ions has any effect on the shock structure. We utilized a series of algorithms to automatically collect and analyze 7056 shock crossings from MAVEN spacecraft data between November 2014 and 2019. We compared the magnetic shock jump and overshoot amplitude as functions of upstream parameters and season. We found that the magnetic shock jump agrees to first order with Rankine‐Hugoniot predictions, and that the overshoot amplitude dependence on critical ratio, beta and Alfvén Mach number agree with the results of previous studies. These trends are identical for all seasons. We also found that the shock jump and fast Mach number correlate primarily with solar zenith angle, whereas overshoot amplitude correlates primarily with shock extent. We found that the most likely cause of the latter was an unexpected strong correlation between overshoot amplitude and the solar wind flow fast Mach number. The seasonal variability of the Sun‐Mars system does not appear to be strongly reflected in the resulting shock structure. The bow shock structure is controlled mainly by the properties of the solar wind, rather than by Mars' location in its orbit around the Sun. Plain Language Summary: The size of the Martian bow shock is known to vary with solar wind and season, but it remains an open question whether other features of the bow shock also vary seasonally. By comparing the shock during different seasons and solar wind conditions, we investigated whether the presence of seasonal planetary ions has any effect on the shock structure. We wrote an algorithm to identify whenever the MAVEN spacecraft crossed the bow shock, and measure relevant quantities in the vicinity to characterize the shock's behavior. We found that the shocks' measured magnetic structures' dependencies on solar wind parameters roughly agree with theoretical predictions and results of previous studies. We found that the primary and secondary substructures have negligible seasonal variation, while the latter is most strongly correlated with an entirely different solar wind parameter than we predicted—the solar wind flow directed magnetosonic Mach number—which is independent of the direction of the shock surface. Thus, the bow shock structure is controlled mainly by the properties of the solar wind, rather than by Mars' location in its orbit around the Sun. Key Points: We found that the bow shock magnetic field structure has at most a very weak seasonal variation in the magnetic jump and the overshootThe Martian bow shock overshoot varies much more significantly with the solar wind flow Mach number than with any local shock parametersThe Martian shock responds to solar wind variations in the same way as the Earth's shock [ABSTRACT FROM AUTHOR]

Measurements of CO2, Ar and N2 densities from the Neutral Gas and Ion Mass Spectrometer on the Mars Atmosphere and Volatile Evolution Mission (MAVEN) between 150 and 200 km altitude during 2015–2022 are analyzed to reveal diurnal (DW1), semidiurnal (SW2) and terdiurnal (TW3) solar‐synchronous tides in Mars thermosphere. Multi‐year‐mean tidal perturbations on a diurnal‐ and zonal‐mean background, corrected for solar flux variations, are reported as a function of latitude (48°S–48°N), altitude and solar longitude (Ls). The DW1, SW2 and TW3 amplitudes at for example, 180 km altitude are of order 90%–120%, 15%–20%, and ≲10% for CO2 and Ar, and roughly 2/3 these values for N2, the latter presumably due to the difference in molecular weight from the other species. Through examination of vertical phase progressions, DW1 is concluded to be mainly excited in situ, but SW2 and TW3 contain significant contributions from tides propagating upward from lower altitudes. By analogy with studies for Earth's thermosphere, the DW1 amplitudes and phases are thought to reflect the combined influences of thermal expansion and vertical winds. Points of agreement and disagreement with DW1, SW2, and TW3 amplitudes and phases derived from the Mars Climate Database are noted and interpreted. Key Points: Global‐scale solar‐synchronous tides are derived from Mars Atmosphere and Volatile Evolution Mission/Neutral Gas and Ion Mass Spectrometer measurements of CO2, Ar, and N2 at Mars, 150–200 kmThe diurnal tide originates mainly in situ; semidiurnal and terdiurnal tides show evidence of upward propagation from belowPoints of agreement and disagreement with the Mars Climate Database are noted and interpreted [ABSTRACT FROM AUTHOR]

In this paper, we use the key parameters data set of the Neutral Gas and Ion Mass Spectrometer from the Mars Atmosphere and Volatile Evolution (MAVEN) mission. The particle density profiles of electrons, CO2+/N2+, CO+, O2+, O+, NO+, O2 and O from 90 to 500 km have been deduced by adopting the Chapman modeling methodology. The correlation of the peak density/altitude with the solar zenith angle, the changes in the profile of the Martian ionosphere during solar flares, and the effects of Martian dust storms are analyzed. The results exhibit a positive/negative correlation between the peak density/altitude of the M2 layer and the solar zenith angle. Within the MAVEN observational record available, only three C-Class flares occurred on 26 August 2016, 29 November 2020, and 26 August 2021. The analysis reveals during these solar flare events, the electron density of the M2 layer above 200 km increases obviously. The peak density of M1 increases by 33.4%, 13.2% and 7.4%, while the peak height decreases by 0.1%, 10.2% and 4.4%, respectively. The Martian dust storm causes the peak height of the M2 layer to increase by 19.5 km, and the peak density to decrease by 4.2 × 109 m−3. Our study shows that the Martian ionosphere is similar to the Earth’s, which is of great significance for understanding the planetary ionosphere.

Observations of the Lyman‐α emissions from Interplanetary Hydrogen (IPH) atoms are made from Mars' orbit using a high spectral resolution instrument in echelle configuration. The measurements can uniquely be used to resolve IPH from planetary H emissions and to subsequently determine the brightness, velocity, and thermal broadening of the IPH flow along the instrument line of sight. Planned observations conducted during special IPH campaigns as well as serendipitous observations made of the planetary limb and a comet sighting, both upwind and downwind of the bulk IPH flow direction, are analyzed to determine these properties and to examine the variability of IPH brightness with solar activity through the declining phase of Solar Cycle 24. The results show that the IPH brightness trends with solar irradiance, the flow is fainter downwind than upwind, the IPH brightness is variable and non‐negligible compared with planetary emissions, and that deriving thermal properties of IPH requires higher spectral resolution than is presently available. A heliospheric interface model was used to simulate and further interpret the derived IPH properties. These results can improve the theoretical understanding of solar system dynamics, between the solar wind and the local interstellar medium, by providing empirical constraints to simulations of the global heliosphere from the inner boundary region near 1.6 AU and can guide the development of future interplanetary missions. Key Points: Interplanetary Hydrogen (IPH) Lyman‐a brightness trends with solar irradiance with fainter flow downwind than upwindIPH Lyman‐a brightness is variable and non‐negligible when viewed from Mars‐orbiting spacecraftMAVEN IPH observations can be used to constrain the inner boundary conditions of heliospheric models [ABSTRACT FROM AUTHOR]

The interaction between Mars and the solar wind results in different plasma regimes separated by several boundaries, among which the separation between the sheath flow and the ionosphere is complicated. Previous studies have provided different and sometimes opposite findings regarding this region. In this study, we utilize observations from the Mars Atmospheric and Volatile EvolutioN (MAVEN) mission to revisit boundaries within this region and perhaps reconcile some differences. More specifically, we start with the photoelectron boundary (PEB), a topological boundary that separates magnetic field lines having access to the dayside ionosphere (open or closed) from those connected to the solar wind on both ends (draped). We find that large gradients in the planetary ion densiti occur across the PEB and that the dominant ion switches from heavy planetary ions to protons near the PEB, indicating that the PEB falls within the ion composition boundary (ICB). Furthermore, our results show that the PEB is not a pressure balance boundary; rather the magnetic pressure dominates both sides of the PEB. Meanwhile, we find that the PEB is located where the shocked solar wind flow stops penetrating deeper into the ionosphere. These findings suggest the PEB marks the top of the Mars dayside ionosphere and also the interface where the sheath plasma flow deflects around the obstacle going downstream. Key Points: Large gradients in planetary ion density occur across the photoelectron boundary (PEB) and the PEB falls within the ion composition boundaryThe PEB can be considered as the top of the Mars dayside ionosphereThe PEB is not a pressure balance boundary but is located where the shocked sheath flow is diverted around the ionosphere [ABSTRACT FROM AUTHOR]

According to various orientation of the interplanetary magnetic field (IMF), the planetary shock can be either quasi‐parallel or quasi‐perpendicular. Under quasi‐parallel conditions a significant number of solar wind suprathermal particles are reflected from the shock and drift along IMF, forming an extended and highly turbulent region called the foreshock where various nonlinear plasma phenomena are observed. In this research, a case study of the structures in the foreshock region at Mars as observed by Mars Atmosphere and Volatile Evolution is performed close to Martian aphelion, when the foreshock wave activity is usually low. Data from plasma analyzer STATIC and magnetometer MAG is used to analyze ion beams angular spectrum and magnetic field dynamics. It is shown that the observed structures are consistent with Short Large‐Amplitude Magnetic Structures (SLAMS), commonly detected in foreshock regions of magnetized and unmagnetized bodies throughout the Solar system. Finally, the Alfven Mach number is calculated to analyze characteristics of the observed foreshock structures. The analysis shows that the observed SLAMS‐like structures at Mars are steepened waves formed by the ion cyclotron resonance between plasma waves propagating along the IMF and the back‐streaming scattered solar wind H+ and exospheric O+ and O2+ ions, with the dominant impact of O+ ions. The steepening process is accompanied with observation of gradients in the diffuse ion density near the bowshock, like it has been previously reported near the Earth (e.g., Scholer, 1993, https://doi.org/10.1029/92JA01875; Tsubouchi & Lembege, 2004, https://doi.org/10.1029/2003JA010014). Key Points: SLAMS‐like structures are for the first time reported to be observed in the Martian foreshock close to planetary aphelionFluxes of back‐streaming O+ and O2+ ions accelerated up to ∼650 eV are observed between the structuresMagnetic field frequency spectra in the region shows maximum close to O+ gyro frequency [ABSTRACT FROM AUTHOR]

The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high‐spectral resolution designed to resolve D and H Lyman‐α emissions. The high‐spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman‐α emissions. This work presents improved characterizations of the high‐spectral resolution mode using in‐orbit observations that allow for more robust detections of the faint D Lyman‐α emission line. Additionally, the instrument was re‐calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high‐spectral resolution instrument. Comparisons to Lyman‐α observations made with the low‐resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint‐emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data‐reduction pipeline with favorable implications for the science utility of D and H Lyman‐α emissions. Plain Language Summary: The MAVEN high resolution echelle (ECH) instrument has been used to observe ultraviolet emissions from Mars. Measurements made with MAVEN/ECH were made available to the public by using a data reduction pipeline that had been developed with in‐lab and theoretical analysis to convert observations into a form that was useable by the broader scientific community. Since MAVEN's orbit insertion in 2014, the ECH detector has been better characterized with lessons learned from in‐orbit performance. These improvements produced a new data reduction pipeline that has enhanced the detection of faint D Lyman‐α emissions that are critical to interpreting water loss from the upper atmosphere of Mars. Key Points: MAVEN/ECH uses HST/STIS, the only other UV echelle instrument deployed to space, for calibration and independent verificationThe MAVEN/ECH data reduction algorithm has been upgraded to improve detection of faint D Lyman‐α emissionsThe scientific implications of this work include doubling the previous data set of D measurements available for reliable interpretation [ABSTRACT FROM AUTHOR]

In early 2019, the Mars Atmosphere and Volatile Evolution (MAVEN) mission underwent an ∼2‐month aerobraking campaign during which time the spacecraft periapsis altitude was lowered from its nominal altitude range of 140–160 km to as low as ∼123 km. Excluding spacecraft walk‐in/out maneuvers, accelerometer measurements were made along 272 orbits with coverage spanning Ls 340–3°, latitudes ∼5°–54°S, longitudes 0–360°, and local solar time ∼22–17 hr. In this study, we perform a diagnostic analysis of the full aerobraking data set by fitting 4‐harmonic waves to mass densities. We then study the variations of these waves as a function of latitude with an emphasis on those observed previously in Mars' thermosphere by MAVEN and other missions. Additionally, we utilize data collected during the same time period from the Mars Reconnaissance Orbiter's Mars Climate Sounder to study the vertical propagation of waves originating from the middle atmosphere. Key results indicate that normalized wave amplitudes decrease with latitude, and this is consistent with the latitudinal structure of a diurnal Kelvin mode. We also observe that waves imprinted from the middle atmosphere show normalized amplitude growth with increasing altitude. A complete summary of data sets, analysis methodology, and scientific results is given. The purpose of this study is to add to the body of knowledge surrounding Martian atmospheric wave features and to provide further constraints for future numerical modeling and subsequent tidal mode identification. Plain Language Summary: Due to solar heating, thermal winds flow along the Martian surface where they impact topographic features enabling them to launch waves that propagate upward, often high into the thermosphere (above 100 km). This propagation causes global‐scale wave (tidal) motions in local mass densities, and the best way to study how these evolve is through direct observation via spacecraft orbiting the region(s) of interest. The Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft observes these tidal fluctuations in mass densities using its onboard accelerometer. Here, we study the data collected during two unique sampling periods that is, the Deep Dip 2 and 2019 Aerobraking campaigns—where the spacecraft was specifically operated below 150 km. In agreement with previous studies of this kind, our results show that (a) prominent wave components in this lower thermosphere region tend to be consistently organized by latitude, longitude, and altitude zones, and (b) the spatial coherence in wave structures is repeatable across Martian seasons. Key results indicate that normalized wave amplitudes decrease with latitude, and this is consistent with the latitudinal structure of diurnal eastward traveling, vertically propagating tidal components. We also observe that waves imprinted from the middle atmosphere show normalized amplitude growth with increasing altitude. Key Points: Analysis of Mars Atmosphere and Volatile Evolution accelerometer data collected during aerobraking indicates the presence of longitudinally varying waves in the thermosphereWaves fitted to mass densities show normalized amplitudes decrease with increasing latitudeWaves imprinted from the middle atmosphere show normalized amplitudes grow with increasing altitude [ABSTRACT FROM AUTHOR]

Abstract The Mars Atmosphere and Volatile Evolution (MAVEN) mission instrument suite includes an ultraviolet echelle spectrograph with high‐spectral resolution designed to resolve D and H Lyman‐α emissions. The high‐spectral resolution mode was previously characterized in the lab and in the cruise phase to Mars and had been calibrated using observations and models of interplanetary hydrogen Lyman‐α emissions. This work presents improved characterizations of the high‐spectral resolution mode using in‐orbit observations that allow for more robust detections of the faint D Lyman‐α emission line. Additionally, the instrument was re‐calibrated using simultaneous and comparable observations made with the Hubble Space Telescope high‐spectral resolution instrument. Comparisons to Lyman‐α observations made with the low‐resolution UV channel on the spectrometer, that had been calibrated with stars, showed consistency in the brightness values for measurements obtained at similar observational conditions. The combined upgrades to the faint‐emission fitting and new calibration techniques of the MAVEN echelle channel have resulted in an improved data‐reduction pipeline with favorable implications for the science utility of D and H Lyman‐α emissions.

The responses of Earth's and Mars' thermospheric densities to quasi‐periodic (∼27‐day) solar rotation variations in flux were measured contemporaneously by the Mars Atmosphere and Volatile EvolutioN, Geostationary Operational Environmental Satellites, and Swarm‐C satellites. While large solar rotation variability is found in both planetary thermospheres, correlation analyses performed on over 6 years of observations reveal that, independently of extreme ultraviolet flux level, Earth's daytime density response is about 10%–50% larger than Mars' at a similar density level. Important altitude dependencies in the density sensitivity to the solar rotation in solar flux are found in the Martian thermosphere, while the terrestrial thermosphere is shown to exhibit only small (±5%) day/night and latitude variations in the response. Detailed analyses focused on correlative periods in 2015–2016 and 2020 indicate important solar cycle effects in the sensitivities of both planetary thermospheres, with increased slopes under low solar flux conditions. These results provide important new insights into processes relevant to the interpretation of the sources of short‐term density variability in Mars' and Earth's thermospheres associated with solar drivers and point to the need for targeted modeling efforts along with dedicated data analyses to help resolve current unknowns in thermal balance processes. Plain Language Summary: The quasi‐periodic change in solar extreme ultraviolet (EUV) radiation from active regions on the Sun rotating with a period of about 27 days is one of the largest sources of short‐term variability in the upper atmospheres of both Mars and Earth. This work investigates the response of total mass densities in the upper atmospheres of Mars (∼160–260 km altitude) and Earth (∼450–480 km altitude) to the solar rotation variation in EUV flux, as a tracer of short‐term solar‐driven impacts on neutral densities. We use simultaneous density and solar flux observations from the Mars Atmosphere and Volatile EvolutioN, Swarm‐C, and Geostationary Operational Environmental Satellites from 2014 through 2021. Earth's daytime densities in the middle thermosphere are found to be about 10%–50% more responsive than Mars' for an equivalent altitude. Important dependencies on height are found at Mars, while small day/night and latitude dependencies are found at Earth. Correlation analyses also suggest strong solar cycle effects on the sensitivities, with significantly higher sensitivities during solar minimum. The results of this study can help to better constrain comparative planetary thermosphere simulations and resolve unknowns in thermal balance processes but also suggest the need for additional data‐ and modeling‐driven studies. Key Points: Solar rotation effects on Earth's and Mars' thermospheric densities are quantified in over 6 years of simultaneous satellite observationsEarth's middle thermospheric daytime density is 30%–50% (10%–30%) more responsive than Mars' during solar maximum (minimum) conditionsDensity sensitivities to solar rotation variability in extreme ultraviolet flux show clear dependencies on altitude and solar cycle at both planets [ABSTRACT FROM AUTHOR]

Several important processes of the Mars upper atmosphere are regulated by exospheric temperature (Texo) variations, including atmospheric escape rates. From the MAVEN mission, significant and largely periodic variability of Mars dayside Texo is now revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected throughout solar cycle 24. Two complementary methods are utilized to extract temperatures from NGIMS data sets. These NGIMS dayside mean exospheric temperatures are shown to vary by ∼80 K (∼180–260 K) over solar cycle 24. This corresponds to a ΔTexo/ΔEUV sensitivity of ∼38 K m2 mW−1, where Lyman‐α is the solar index. Previous Mars Global Surveyor derived Texo values for solar cycle 23 yielded a sensitivity of ∼45 in the same units. This close correspondence suggests that the underlying dayside energy balances are similar yet slightly different over these two solar cycles. Corresponding Mars Global Ionosphere‐Thermosphere Model simulations show that molecular thermal conduction largely balances EUV heating for the Mars dayside thermosphere, while CO2 15‐µm cooling is secondary in importance, along with global winds. It is valuable to compare this Mars sensitivity to that computed for the dayside thermosphere of Venus. Pioneer Venus dayside data sets imply a sensitivity of ∼17.5 K m2 mW−1 units for solar cycle 21, a factor of ∼2.2 smaller than computed for Mars. This is because Venus CO2 cooling strongly balances EUV heating near its peak, thereby serving as a more efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars. Plain Language Summary: Modern Earth global warming of its lower atmosphere is complemented by enhanced CO2 15‐μm cooling in its upper atmosphere. This CO2 cooling process also has a role in the CO2 dominated upper atmospheres of Venus and Mars. From the MAVEN mission, significant and largely periodic variability of Mars dayside exospheric temperatures is revealed by Neutral Gas and Ion Mass Spectrometer (NGIMS) data sets collected during the declining phase of the solar cycle (2014–2019). These NGIMS dayside mean exospheric temperatures vary by ∼80 K (180–260 K) over this weak (low sunspot number) solar cycle 24. Corresponding Global Climate Model (GCM) simulations of the Mars upper atmosphere reproduce this 80 K variability, revealing Extreme Ultraviolet heating is largely conducted downward to lower thermosphere altitudes providing net cooling, while CO2 15‐μm cooling is secondary in importance. By contrast, Pioneer Venus Orbiter dayside data sets (1978–1992) also imply a similar ∼80 K variability at Venus, now for the very strong (∼3‐times larger sunspot number) solar cycle 21. GCM simulations for Venus determine that CO2 cooling strongly balances EUV heating near its peak, thereby serving as an efficient thermostat regulating dayside temperatures. CO2 cooling is much weaker for Mars, yielding larger solar cycle variations in exospheric temperatures. Key Points: MAVEN dayside temperatures are shown to vary by 80 K (∼180–260 K) over solar cycle 24, reduced from the Mars Global Surveyor (MGS) value of 120 KMars temperature versus EUV sensitivity is similar for MAVEN and MGS exospheric measurements at 38 and 45 K m2 mW−1Mars thermal conduction balances EUV heating (160–180 km) for solar maximum conditions, while CO2 cooling is secondary [ABSTRACT FROM AUTHOR]

Gravity waves are a key mechanism that facilitates coupling between the lower and upper atmosphere of Mars. In order to better understand the mean, large‐scale impacts of gravity waves on the thermosphere, a modern whole atmosphere, nonlinear, non‐orographic gravity wave parameterization scheme has been incorporated into a three‐dimensional ground‐to‐exosphere Mars general circulation model, the Mars Global Ionosphere‐Thermosphere Model (M‐GITM). M‐GITM simulations utilizing the gravity wave parameterization indicate that significant gravity wave momentum is deposited in the thermosphere, especially within the altitude range of 90–170 km. This impacts the winds in the thermosphere; in particular, M‐GITM simulations show a decrease in speed of the wind maximum in the summer hemisphere by over a factor of two. Gravity wave effects also impact the temperatures above 120 km in the model, producing a cooler simulated thermosphere at most latitudes. M‐GITM results were also compared to upper atmospheric temperature and wind data sets from the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. Some aspects of wind data‐model comparisons improved once the gravity wave scheme was added to M‐GITM; furthermore, a cooler temperature profile produced by these new M‐GITM simulations for the MAVEN Deep Dip 2 observational campaign resulted in a closer data‐model comparison, particularly above 180 km. Overall, these modeling results show that gravity waves play an important role for the energy and momentum budget of the Martian thermosphere. Plain Language Summary: Atmospheric gravity waves are an important physical process in the upper atmosphere of Mars. To better understand the average effects of gravity waves on the temperatures and winds above 100 km, a modern numerical scheme designed to represent the relevant physics has been added to a 3‐D general circulation model, M‐GITM (Mars Global Ionosphere‐Thermosphere Model), which extends from the surface to about 250 km. Results from these M‐GITM simulations show that in the upper atmosphere, the wind maximum in the summer hemisphere decreases in speed by over a factor of two once the effects of gravity waves are added to the model. Additionally, above 120 km, the model now produces a cooler upper atmosphere, on average. The new M‐GITM results were also compared to select upper atmospheric temperature and wind data sets from the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft. Data‐model comparisons in upper atmospheric wind speeds for a January 2017 observational campaign improve with the addition of gravity wave effects, as do data‐model comparisons for upper atmospheric temperatures for a MAVEN Deep Dip campaign. Overall, these model results show that gravity waves can have significant impacts on the winds and temperature structure in the Martian upper atmosphere. Key Points: Large‐scale impacts of gravity waves in the thermosphere are examined using a modern gravity wave scheme in a Mars general circulation modelSignificant gravity wave momentum deposition is found in model simulations from 90 to 170 km altitudeGravity waves which propagate to the upper atmosphere of Mars can have an appreciable impact on thermospheric winds and temperatures [ABSTRACT FROM AUTHOR]

The Martian ionosphere, modulated by the solar wind from the topside and remnant crustal magnetic fields close to the surface, possesses unique structures different from Earth and Venus. Integrated observations by the plasma and magnetic field instruments onboard the Mars Atmosphere and Volatile Evolution spacecraft show evidence of ionospheric plasma depletions, independent of seasonal variations at Mars. During such depletions, the density of all ionospheric ion species is reduced by more than an order of magnitude, and, at the same time, the electron temperature increases abruptly. An automated algorithm for the identification of such plasma depletions is developed. Altogether, as many as 1,125 events are identified in 8,618 orbits available from October 2014 to May 2021. A statistical investigation of these events reveals that they are more prominent on the nightside, where they seem to occur primarily in the southern hemisphere, possibly in regions with strong crustal magnetic fields. While the dayside events occur mainly at altitudes above about 250 km, nightside event altitudes are typically lower. Considering the relation between spacecraft velocity and observed event duration, we suggest that the depletions are bubble‐like structures, more elongated horizontally than vertically. A possible mechanism of their formation is discussed. Plain Language Summary: Ions and electrons embedded in an atmosphere of a planet constitute the ionospheric plasma. At Mars, the ionospheric plasma has a well‐defined altitude structure similar to Earth. However, we find that the normally smooth plasma density profiles exhibit at times significant sudden plasma depletions. We use nearly 7 years of data obtained by the Mars Atmosphere and Volatile Evolution spacecraft to systematically analyze the occurrence and properties of such events. We show that, during the events, the density of all monitored ion species decreases at least by a factor of 10 and, at the same time, the temperature increases. The events are more frequent and occur at lower altitudes on the nightside than on the dayside. We show that the event geometry corresponds to bubble‐like structures rather than to layers and/or columns of decreased density. We suggest that the events may be formed due to the plasma drift away from certain regions, resulting in significant not yet understood large‐scale ionospheric anomalies. Key Points: Density depletions larger than an order of magnitude are observed for all major ionospheric ion species and electronsThe depletions are more prominent on the nightside, mainly in the southern hemisphere of MarsWhile most dayside depletions occur at altitudes above 250 km, their altitudes on the nightside are typically lower [ABSTRACT FROM AUTHOR]

A set of scientific instruments capable of high temporal resolution measurements onboard the Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft made it possible to study the structure and properties of the dayside Martian magnetosphere. The Mars plasma envelope on the dayside of the planet was discovered based on observations on the first orbiters of Mars (Vaisberg et al., 1976; Gringauz et al., 1976). The high temporal and mass resolution of MAVEN instruments made it possible to study the structure and processes in the plasma of the dayside magnetosphere, the plasma layer that exists between the solar wind heated by the shock wave and the ionosphere (Vaisberg et al., 2017). It is shown that, under different external conditions, two different types of plasma layers are formed between the ionosphere and magnetosheath on the dayside of the planet: (1) mixture of heated ionospheric ions and picked-up exospheric ions and (2) a layer of ionospheric ions accelerated by the solar wind motional electric field called "plume" (Dong et al., 2015). The first type of the Martian daytime magnetosphere is formed as a result of the interaction of oxygen exospheric ions captured and accelerated by the solar wind with the upper part of the Martian ionosphere (Vaisberg and Shuvalov, 2021). The second type of the Martian dayside magnetosphere is formed by the acceleration of an ion beam from the outer ionosphere, which then passes into a more energetic plume beam in the magnetosheath. This article discusses typical plasma populations, their properties and conditions that lead to the formation of the daytime magnetosphere from the material of the upper ionosphere. [ABSTRACT FROM AUTHOR]

Discrete aurorae have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae (termed "auroral electrons"). By utilizing empirical criteria to select auroral electrons established by Xu et al. (2022, https://doi.org/10.1029/2022GL097757), we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and fluxes of auroral electrons. We find the occurrence rate increases with upstream dynamic pressure and weakly depends on the interplanetary magnetic field strength. Meanwhile, the integrated auroral electron flux somewhat decreases with increasing upstream drivers. Auroral electron precipitation also occurs more frequently and is more intense over regions of strong crustal fields compared to weak crustal fields. Aside from emissions, auroral electrons are expected to cause significant impact ionization and enhance the plasma density locally. In this study, we also quantify the nightside ionospheric impact of auroral electron precipitation, specifically the thermal ion (O+, O2+ ${O}_{2}^{+}$, and CO2+ $C{O}_{2}^{+}$) density enhancement, with MAVEN observations. Our results show that the ion density is increased by up to an order of magnitude at low altitudes. The crustal effects on ion density profiles for nominal electron and auroral electron precipitation are also discussed. Plain Language Summary: Localized auroral emissions have been observed at Mars by multiple spacecraft, including Mars Express, Mars Atmosphere and Volatile EvolutioN (MAVEN), and most recently the United Arab Emirates Hope spacecraft. Meanwhile, there have been studies on the source particles responsible for producing these detectable aurorae, termed "auroral electrons." By utilizing previously established criteria to select auroral electrons, we conduct statistical analyses of the impact of upstream drivers on the occurrence rate and also the intensity of auroral electrons. In the meantime, auroral emission is not the sole effect of electrons impacting the collisional atmosphere, but also an enhanced local ionization. Using both MAVEN observations, we show that the ion density is increased by up to an order of magnitude at low altitudes comparing auroral electron and nonauroral electron events. Key Points: The occurrence rates of auroral electrons increase but their integrated energy fluxes somewhat decrease with increasing upstream driversWe quantify the planetary ion density enhancement resulting from auroral electron precipitation to be up to an order of magnitudeAuroral electron precipitation occurs more frequently and is more intense over strong crustal fields than weak crustal fields [ABSTRACT FROM AUTHOR]

Solar wind protons can interact directly with the hydrogen corona of Mars through charge exchange, resulting in energetic neutral atoms (ENAs) able to penetrate deep into the upper atmosphere of Mars. ENAs can undergo multiple charge changing interactions, leading to an observable beam of penetrating protons in the upper atmosphere. We seek to characterize the behavior of these protons in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that backscattered penetrating proton flux is enhanced in regions where the magnetic field strength is greater than 200 nT. We also find a strong correlation at CO2 column densities less than 5.5 × 1014 cm−2 between magnetic field strength and the observed backscattered and downward flux. We do not see significant changes in penetrating proton flux with magnetic field strengths on the order of 10 nT. Plain Language Summary: Positively charged particles in the solar wind can directly interact with hydrogen in the Martian environment. These positively charged particles can charge exchange with hydrogen atoms, becoming fast, neutral atoms that are able to navigate their way deep into the upper atmosphere of Mars. These fast neutrals can undergo further charge‐changing reactions and become positively charged once more. This paper seeks to understand how these positive particles, or penetrating protons, behave in the presence of magnetic fields using data collected by the Mars Atmosphere and Volatile EvolutioN spacecraft. We find that strong magnetic fields enhance the number of backscattered penetrating protons we observe. At low atmospheric column densities, strong magnetic fields significantly deflect these protons back to higher altitudes. At high atmospheric column densities, magnetic fields do not appear to affect these penetrating protons to the same degree as collisions with other neutral molecules. We also find that small magnetic field strengths do not affect the penetrating proton flux we observe. Key Points: Backscattered penetrating proton flux is enhanced in magnetic field regions with strengths greater than 200 nTPenetrating protons are influenced by strong magnetic fields at column densities less than 5.5 × 1014 cm−2Penetrating proton fluxes are not greatly affected by magnetic field strengths on the order of 10 nT [ABSTRACT FROM AUTHOR]

The densities of three ion species in the Martian upper atmosphere were compared during the MY33 and MY34 Martian regional and global dust storms (RDS 2016 and GDS 2018, respectively) using data from the neutral gas and ion mass spectrometer of the Mars atmosphere and volatile evolution mission. The trends of the ion species and their relative abundances in altitudes compared to some neutral species were examined from 10 September–4 October 2016 and 27 May–18 June 2018, at altitudes of 160–240 km. Both RDS 2016 and GDS 2018 caused variations in the ion species abundance of the upper atmosphere at their onsets in 18–21 September 2016 and 5–8 June 2018 respectively. The densities of O2+, CO2+, and O+ increased during RDS 2016. Meanwhile, O2+ and O+ densities decreased and CO2+ density increased during GDS 2018. Ion species' relative abundances indicate that during RDS 2016, the increase in O2+ density may be caused by the increase of CO2+ or O+ densities rather than the increase of O or CO2 densities. Meanwhile, the decrease in O2+ density during GDS 2018 may be caused by the decrease of O or O+ densities rather than the decrease in CO2+ or CO2 densities. [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]

Using data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS), we investigate the horizontal variations in composition of the Martian upper atmosphere. We focus on ratios of Ar, N2, O, He, and H2 to CO2 and their variation with local time, latitude, season, and temperature. These are the first direct measurements of H2 in the upper atmosphere, a species important for the delivery of H to this region because H and H2 escape to space. The NGIMS data nominally cover an altitude range of ∼150–300 km, from near the mesopause to above the exobase. The ratios to CO2 of He and H2 are larger toward the poles and on the nightside of the planet, where the atmosphere is colder and downwelling leads to the enrichment of these lighter species. For example, the H2 to CO2 ratio, at a CO2 density of 1 × 108 cm−3, is observed to increase by a factor of 2.8 from 52.5°S to 72.5°S and by a factor of 5.5 from 3 p.m. to 3 a.m. around the equator. We observe seasonal variation of the Ar to CO2 ratio due to deposition and sublimation of CO2 at the polar ice caps. Finally, we find good agreement between the Ar and N2 to CO2 ratios obtained by the Viking Lander Upper Atmospheric Mass Spectrometers and those obtained by NGIMS, and significant disagreement between the N2 to CO2 ratios obtained by the Imaging Ultraviolet Spectrograph on MAVEN and those obtained by NGIMS. Plain Language Summary: Using data from the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) Neutral Gas and Ion Mass Spectrometer (NGIMS), which analyzes components of a gas by their mass, we investigate the composition of the outermost portion of the Martian atmosphere. This region of the atmosphere is important because a part of it escapes to space. We explore the way in which the proportions of Ar, N2, O, He, and H2 vary relative to the main atmospheric gas, CO2, over a Martian day and with temperature, latitude, and season. These are the first direct measurements of H2 in the upper atmosphere of Mars. We find that the proportions of the lightest species, H2 and He, vary substantially throughout the Martian day and with latitude. This variation is caused by transport of these species to cooler regions: the atmosphere on the nightside of the planet and above the poles. We observe seasonal variation in the proportion of Ar, which is due to the conversion of CO2 gas to ice at the polar caps during winter. Finally, we find good agreement between measurements obtained by NGIMS and the NASA Viking missions, but significant disagreement between NGIMS and the MAVEN Imaging Ultraviolet Spectrograph. Key Points: The composition of the upper atmosphere of Mars is investigated using mass spectrometer measurements from Mars Atmosphere and Volatile EvolutioN Neutral Gas and Ion Mass SpectrometerThe variation of the ratios of Ar, N2, O, He, and H2 to CO2 with local time, latitude, and season is measuredAll species show a diurnal variation in density ratio with maxima in the early morning and magnitude inversely correlated with mass [ABSTRACT FROM AUTHOR]

Using the Mars Atmosphere and Volatile EvolutioN measurements, we observed a wave train of vortices in different developing phases of the Kelvin‐Helmholtz (K‐H) instability at the ionopause above the Martian northern hemisphere during 09:35:00–09:55:00 Universal Time on 24 March 2017. Based on the sawtooth‐like oscillations in the magnetic fields associated with the quasi‐periodic fluctuations in the ion densities, velocities, and total pressure, we identified three fully and two partially developed vortices. The wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min. The magnetic topologies associated with the vortices are observed to change from draped in the magnetosheath to open in the transition region and finally to closed approaching the ionosphere. Significant ion loss caused by the K‐H instability is observed. The total oxygen loss rate is estimated to be ∼1025 s−1, which is comparable to the global ion loss as previously estimated. Plain Language Summary: Shear‐driven Kelvin‐Helmholtz (K‐H) instability can form along the boundary of the solar wind interaction with non‐magnetic planets (such as Venus and Mars). Previous studies reported partially and fully developed K‐H vortices at the Martian induced magnetosheath. In this study, we directly observed both types of the K‐H vortices during the developing process of the K‐H instability at the Martian ionopause. The magnetic topologies associated with the K‐H vortices change from draped in the magnetosheath to open in the transition region and to closed approaching the ionosphere. The K‐H instability can be an effective mechanism that leads to ion escape at Mars. A large number of ions within the K‐H vortices are observed to escape from Mars. The loss rate of oxygen caused by the K‐H instability is estimated to be significantly high. Our results confirm that the K‐H instability is important for the Martian ion loss. Key Points: We report a wave train of Kelvin‐Helmholtz (K‐H) vortices during different developing phases of the K‐H instability at the ionopause of MarsThe wavelength of the K‐H waves is ∼2 Martian radii and the period is ∼2 min; the magnetic topologies within the K‐H vortices changedThe estimated loss rate of oxygen caused by the K‐H instability is on the order of 1025 s−1 [ABSTRACT FROM AUTHOR]

With the aid of a multi-instrument data set gathered by the Mars Atmosphere and Volatile Evolution (MAVEN) during ten selected periods, we make detailed calculations of the CO 2 + Ultraviolet Doublet (UVD) emission brightness profiles which are then compared to the Imaging Ultraviolet Spectrometer limb observations. Our calculations confirm that the photoionization of atmospheric CO 2 is the predominant process driving CO 2 + UVD emission at high altitudes, whereas the photoelectron impact ionization of CO 2 becomes more important at low altitudes. The data–model comparisons show good agreement near and above the emission peak at around 120 km with an intensity of 27–45 kR. A special case is found for period 3 coincident with a regional dust storm during which the peak altitude rose by 20 km. Of particular interest is the significant discrepancy below the peak, which is likely associated with the uncertainties in either atmospheric density or incident solar irradiance. A detailed investigation suggests that the latter uncertainty is more likely responsible for such a discrepancy, in that the solar irradiance shortward of a wavelength threshold below 30 nm should be adjusted to achieve reasonable data–model agreement over the entire altitude range. This result highlights the necessity to improve the accuracy of any solar irradiance model used for planetary aeronomical studies. [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]

Photoelectrons are produced on the dayside of Mars via solar Extreme Ultraviolet (EUV) and X‐ray ionization and are also frequently observed on the nightside as a result of field‐aligned transport. Based on the Solar Wind Electron Analyzer (SWEA) measurements made on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft, we show that the shape of the photoelectron energy distribution exhibits interesting cross‐terminator variations, in that the He II peak becomes less pronounced and the 10 − 50 eV smooth background becomes hardened, both more evident at lower altitudes. These observations could be understood as the outcome of "atmospheric absorption" via photoelectron collisions with ambient neutrals during day‐to‐night transport. Such a scenario could successfully interpret the diversity of the MAVEN SWEA observations such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar EUV and X‐ray radiation, all using the 10–50 eV spectral hardness as a diagnostic. In particular, the magnetic control of the cross‐terminator spectral reshaping is proposed as the outcome of varying day‐to‐night magnetic connectivity which naturally implies varying field‐aligned atmospheric column for "absorption." Crude estimates suggest that an "absorbing" atmospheric column of ≈1.5 × 1015 cm−2 is required for a significant photoelectron spectral reshaping beyond the EUV terminator. Therefore the observations reported here may shed light on the extension of the cross‐terminator magnetic field lines, as complementary to the day‐to‐night magnetic connectivity inferred from the occurrence of nightside photoelectrons. Plain Language Summary: Photoelectrons are an omnipresent population of the Martian ionosphere, which are produced exclusively on the dayside via photoionization and are also frequently observed on the nightside as a result of cross‐terminator transport. They are of broad interest because they make significant contributions to local ionization, heating, and airglow emission in the Martian upper atmosphere. In addition, the occurrence of nightside photoelectrons has been commonly used as a diagnostic of the day‐to‐night magnetic connectivity near Mars. Photoelectrons may also undergo significant spectral reshaping beyond the EUV terminator, an issue that has been paid little attention to but is potentially important for shedding light on the extension of the magnetic connectivity. This study is devoted to such a topic using SWEA measurements made on board the MAVEN spacecraft. We propose that collisions with atmospheric neutrals continuously modify the photoelectron energy distribution from a highly non‐thermal, structured one toward a thermal, featureless one. This scenario successfully interprets many aspects of the observed cross‐terminator variation, such as the dawn‐dusk and north‐south asymmetries, the impacts of crustal magnetization, upstream solar wind conditions, and solar activity. Key Points: The shape of the photoelectron energy distribution varies systematically across the Martian terminatorThe variation is the direct outcome of atmospheric absorption, which is more pronounced at low altitudesBoth crustal magnetization and upstream solar wind conditions affect the variation by modifying the field‐aligned atmospheric column [ABSTRACT FROM AUTHOR]