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The study of atmospheric emissions from orbit to probe the middle and upper atmosphere of Mars, which started with the Mariner missions, is living a golden era thanks to the European Space Agency (ESA) Mars Express mission and other subsequent missions built upon its success, including the ESA ExoMars Trace Gas Orbiter (TGO) mission. Here we summarize the most relevant information obtained by the analysis of atmospheric emissions data from Mars Express and TGO, about the temperature and density structure, the atmospheric dynamics, the chemistry and the atmospheric escape to space. Mars Express also opened a new field of research on Mars with the discovery of aurorae on the planet. We present here the most outstanding results collected by Mars Express about aurorae. Finally, we also discuss how later measurements by other missions have complemented Mars Express and TGO results, and the potential future developments relevant to this field of research. [ABSTRACT FROM AUTHOR]

The Martian ionosphere originates from the ionization of the planetary neutral atmosphere by solar radiation. This conductive layer is embedded within the thermosphere and exosphere of Mars where it forms a highly variable interaction region with the solar wind. The Martian ionosphere has been continuously observed by the three plasma instruments MaRS, MARSIS and ASPERA-3 on Mars Express for the last 20 years (>10 Martian years). Those long-term observations laid a solid foundation for what we know today about the Martian ionosphere, and provided numerous opportunities for collaboration and coordinated observations with other missions. This review describes the most significant achievements of Mars Express for the ionosphere, such as the dynamics and structures of both day and nightside, its variability and couplings with the lower atmosphere, as well as the improvement of atmospheric and ionosphere modelling. Mars Express has also provided a better characterization of the role of several external and internal drivers in controlling the ionosphere, such as the Martian crustal magnetic fields, solar activity, seasons, dust lifting from the surface, and even the direct interaction of the Martian ionosphere with the coma of an Oort-cloud comet (C/2013 A1, Siding Spring). [ABSTRACT FROM AUTHOR]

Measurements of CO2, Ar, N2, and O densities between 150 and 200 km from the Mars Atmosphere and Volatile Evolution Neutral Gas and Ion Mass Spectrometer during February 2015 to February 2022 are analyzed to provide a comprehensive analysis of their longitudinal wavenumber k = 1, 2, and 3 components. Variations in density amplitudes (Ak) with solar flux are marginally detectable during this period. The Ak binned and averaged in latitude, local solar time and Ls are referenced to diurnal- and zonal-mean backgrounds in accord with how tides and stationary planetary waves (SPWs) are defined in theory and modeling. The resulting global Ak distributions are the interference patterns formed by superposition of diurnal tides, SPWs and/or semidiurnal tides; consequently, a simple dependence on species mass consistent with thermal expansion (diffusive equilibrium) that might exist for some individual wave components is obscured. Additionally, vertical winds likely contribute to deviations from diffusive equilibrium. Complementary analyses of the Mars Climate Database indicate that the major contributors to the Ak are DE2, SE1, DE1, and SPW1, 2, 3; support the absence of significant variability due to solar flux; and indicate a more well-defined sensitivity to species mass. The Ak and their phases (longitudes of maxima) for the whole data set are available as part of Supporting Information S1. [ABSTRACT FROM AUTHOR]

This is the second part of Stolzenbach et al. (2023, https://doi.org/10.1029/2022JE007276), named hereafter Paper I, extends the period to the end of MY 34 and the first half of MY 35. This encompasses the end phase of the MY 34 Global Dust Storm (GDS), the MY 34 C‐Storm, the Aphelion Cloud Belt (ACB) season of MY 35, and an unusual early dust event of MY 35 from LS 30° to LS 55°. The end of MY 34 overall aerosol size distribution shows the same parameters for dust and water ice to what was seen during the MY 34 GDS. Interestingly, the layered water ice vertical structure of MY 34 GDS disappears. The MY 34 C‐Storm maintains condition like the MY 34 GDS. A high latitude layer of bigger water ice particles, close to 1 μm, is seen from 50 to 60 km. This layered structure is linked to an enhanced meridional transport characteristic of high intensity dust event which put the MY 34 C‐Storm as particularly intense compared to non‐GDS years C‐Storms as previously suggested by Holmes et al. (2021, https://doi.org/10.1016/j.epsl.2021.117109). Surprisingly, MY 35 began with an unusually large dust event (Kass et al., 2020, https://ui.adsabs.harvard.edu/abs/2020AGUFMP039...01K) found in the Northern hemisphere during LS 35° to LS 50°. During this dust event, the altitude of aerosol first detection is roughly equal to 20 km. This is close to the values encountered during the MY 34 GDS, its decay phase and the C‐Storm of the same year. Nonetheless, no vertical layered structure was observed. Plain Language Summary: Mars has a peculiar tendency, in one in every three Martian year, the entire planet is covered by dust suspended in the atmosphere. These events are referred as Global Dust Storms (GDS). The dust is lifted by fierce winds from the ground up to 80 km high. The mixture of water ice crystals and dust particle in the air of Mars change how the incoming Sun light warms the atmosphere. It is then of a crucial importance to properly characterize the nature and size of the aerosols, especially during a GDS, to better understand the dynamics of the Martian atmosphere. The spacecraft (ESA/Roscosmos) ExoMars Trace Gas Orbiter has been studying the Martian atmosphere since April 2018 and observed a GDS but also other and less intense dust storms. These data help us distinguish the aerosols nature (dust and water ice) and sizes during these distinct types of dust events. Our study confirms that, globally, the particles of dust and water ice are quite small, close to 1 μm or even less. Their distribution varies a lot, meaning that one may find a lot of different particle sizes or lot of similar sized particles. Key Points: MY 34 C‐Storm aerosol effective size and vertical structure is similar to MY 34 Global Dust Storm (GDS)MY 35 early dust event aerosol shows similar effective size and vertical structure as MY 34 GDSMY 34 GDS leads to more intense C‐Storm and an unusual early storm at the start of MY 35 [ABSTRACT FROM AUTHOR]

Since the beginning of the Trace Gas Orbiter (TGO) science operations in April 2018, its instrument "Nadir and Occultation for MArs Discovery" (NOMAD) supplies detailed observations of the IR spectrums of the Martian atmosphere. We developed a procedure that allows us to evaluate the composition and distribution's parameters of the atmospheric Martian aerosols. We use a retrieval program (RCP) in conjunction with a radiative forward model (KOPRA) to evaluate the vertical profile of aerosol extinction from NOMAD measurements. We then apply a model/data fitting strategy of the aerosol extinction. In this first article, we describe the method used to evaluate the parameters representing the Martian aerosol composition and size distribution. MY 34 GDS showed a peak intensity from LS 190° to 210°. During this period, the aerosol content rises multiple scale height, reaching altitudes up to 100 km. The lowermost altitude of aerosol's detection during NOMAD observation rises up to 30 km. Dust aerosols reff were observed to be close to 1 μm and its νeff lower than 0.2. Water ice aerosols reff were observed to be submicron with a νeff lower than 0.2. The vertical aerosol structure can be divided in two parts. The lower layers are represented by higher reff than the upper layers. The change between the lower and upper layers is very steep, taking only few kilometers. The decaying phase of the GDS, LS 210°–260°, shows a decrease in altitude of the aerosol content but no meaningful difference in the observed aerosol's size distribution parameters. Plain Language Summary: Mars' atmosphere is filled with dust and water ice particles carried by the winds. These aerosols affect the way sunlight is distributed in the atmosphere and on the surface, and this directly affects temperature. In addition, approximately every three Martian years, Mars experiences what is known as a "global dust storm." This type of dust storm covers the entire red planet in dust. It affects the temperature and water vapor content of the Martian atmosphere. Determining and assessing aerosol properties, number, size and mass during and after a global dust storm is of crucial importance to understanding its underlying mechanisms. Here, we develop an analysis scheme to study the size, nature, number and distribution of Martian aerosols. Our study confirms that, overall, dust and water ice particles are quite small, close to 1 μm or even smaller, and that a global dust storm affects the intensity of other storms that follow. Key Points: Retrieval of Martian aerosols key properties from NOMAD‐SO data during the MY 34 GDSMesospheric dust and water ice reff are mainly ∼1 μm and ≤0.5 μm respectively during the MY 34 GDS and its decay phaseDuring the MY 34 GDS, the effective variance shows a slight N/S asymmetry and values mainly ≤0.2 [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, the non‐orographic gravity waves (GW) parameterization of the Mars Planetary Climate Model (PCM) previously implemented by Gilli et al. (2020, https://doi.org/10.1029/2018JE005873) is revisited and extended to the exobase (∼250 km). The simulations performed with the new scheme correct some known biases in the modeled thermal tide amplitudes and polar warming, improving the agreement with Mars Climate Sounder (MCS) observed thermal structures and tides below ∼100 km. Additionally, we find that the simulated densities above 150 km are compatible with NGIMS (Neutral Gas and Ion Mass Spectrometer) measured abundances. Large drag depositions ranging up to >∼950 m s−1 sol−1 are induced at altitude of 90–170 km due to the wave saturation (breaking) and depletion, leading to winds damped to magnitudes of ∼150–225 and ∼80 m s−1 in the zonal and meridional directions, respectively. Resulting temperature variations are ∼±10–30 K or 5%–10% at most latitudes except in the polar regions (where they can reach ∼±30–60 K). The results indicate that non‐orographic GW play a significant role in the dynamics of the middle‐upper atmosphere of Mars via the induced transfer of momentum and energy from the lower atmosphere. Plain Language Summary: Atmospheric gravity (buoyancy) waves are oscillations that result from flows over topography (orographic gravity waves (GW)) or perturbations caused by convective forcing, front systems, and jet streams (non‐orographic GW). In this paper, we model non‐orographic GW as a Gaussian wave packet, which is the sum of several monochromatic waves of random wavenumbers and frequencies. The wave packets are launched vertically from low altitudes (∼6 km), where the mean Mars Planetary Boundary Layer is located. We reproduce several features of the thermal structure and tides observed by the MCS by implementing our scheme to the Mars PCM. The scheme has a strong effect on the model's wind fields above an altitude of 35–40 km, transporting momentum from the source layers to higher layers, and even up to the thermosphere (altitude >∼100 km). The temperature and density variations generated by the scheme are comparable to the observations made by the Neutral Gas and Ion Mass Spectrometer. Our results demonstrate that non‐orographic GW play a crucial role in the momentum‐energy transport that affects the dynamics of the middle and upper atmosphere of Mars. Key Points: A stochastic parameterization of non‐orographic gravity waves is implemented up to the exobase (∼0–250 km)Simulation results are in very satisfying agreement with Mars Climate Sounder observed thermal structures and tidesThe simulated upper atmospheric densities are compatible with Neutral Gas and Ion Mass Spectrometer abundances [ABSTRACT FROM AUTHOR]

The recently discovered super-Earth LP 890-9 c is an intriguing target for atmospheric studies as it transits a nearby, low-activity late-type M-dwarf star at the inner edge of the Habitable Zone. Its position at the runaway greenhouse limit makes it a natural laboratory to study the climate evolution of hot rocky planets. We present the first 3D-Global Climate Model exo-Venus model for a modern Venus-like atmosphere (92 bar surface pressure, realistic composition, and H2SO4 radiatively-active clouds), applied to the tidally-locked LP 890-9 c to inform observations by JWST and future instruments. If LP 890-9 c has developed into a modern exo-Venus, then the modelled temperatures suggest that H2SO4 clouds are possible even in the substellar region. Like on modern Venus, clouds on LP 890-9 c would create a flat spectrum. The strongest CO2 bands in transmission predicted by our model for LP 890-9 c are about 10 ppm, challenging detection, given JWST estimated noise floor. Estimated phase curve amplitudes are 0.9 and 2.4 ppm for continuum and CO2 bands, respectively. While pointing out the challenge to characterise modern exo-Venus analogues, these results provide new insights for JWST proposals and highlight the influence of clouds in the spectrum of hot rocky exoplanet spectra. [ABSTRACT FROM AUTHOR]

We present CO density profiles up to about 100 km in the Martian atmosphere obtained for the first time from retrievals of solar occultation measurements by the Nadir and Occultation for Mars Discovery (NOMAD) onboard ExoMars Trace Gas Orbiter (TGO). CO is an important trace gas on Mars, as it is controlled by CO2 photolysis, chemical reaction with the OH radicals, and the global dynamics. However, the measurements of CO vertical profiles have been elusive until the arrival of TGO. We show how the NOMAD CO variations describe very well the Mars general circulation. We observe a depletion of CO in the upper troposphere and mesosphere during the peak period, LS = 190°–200°, more pronounced over the northern latitudes, confirming a similar result recently reported by Atmospheric Chemistry Suite onboard TGO. However, in the lower troposphere around 20 km, and at least at high latitudes of the S. hemisphere, NOMAD CO mixing ratios increase over 1,500 ppmv during the GDS (Global Dust Storm) onset. This might be related to the downwelling branch of the Hadley circulation. A subsequent increase in tropospheric CO is observed during the decay phase of the GDS around LS = 210°–250° when the dust loading is still high. This could be associated with a reduction in the amount of OH radicals in the lower atmosphere due to lack of solar insolation. Once the GDS is over, CO steadily decreases globally during the southern summer season. A couple of distinct CO patterns associated with the Summer solstice and equinox circulation are reported and discussed. Plain Language Summary: CO is an extremely interesting trace species in the Martian atmosphere. It has been used for both dynamical and photochemical studies of the atmosphere. But its vertical distribution has not been systematically measured until the arrival of the Exomars Trace Gas Orbiter (TGO). We use observations of the NOMAD (Nadir and Occultation for Mars Discovery) spectrometer onboard TGO to retrieve full profiles of mixing ratios of CO up to 100 km with a good vertical resolution. The retrievals cover two Martian seasons during which a global dust storm event occurred. We have found the behavior of CO during this event to be governed by local chemistry as well as by the long range transport. During the dust storm, CO mixing ratios are depleted all over the globe while over the southern high latitudes, we discover an increase in CO due to transport from low latitudes during the end of the southern winter. The dynamical effect of global transport is found in the vertical distribution of CO during the southern summer. Another important result, where the local chemistry might be at play is the increase of CO in the low altitudes over low and midlatitudes during the decay phase of the GDS. Key Points: Global map of CO profiles from Nadir and Occultation for Mars Discovery solar occultation observations during the first year of Trace Gas Orbiter operations is presented for the first timeDuring the onset of the 2018 global dust storm (GDS), the CO volume mixing ratios (VMRs) are found to be depleted by 28% at 50 km compared to the average CO VMR valuesHigh CO abundance at tropospheric altitudes is observed over NH during decay of the GDS when the atmospheric dust loading is high [ABSTRACT FROM AUTHOR]

We present vertical profiles of temperature and density from solar occultation (SO) observations by the "Nadir and Occultation for Mars Discovery" (NOMAD) spectrometer on board the Trace Gas Orbiter during its first operational year, which covered the second half of Mars Year 34. We used calibrated transmittance spectra in 380 scans, and apply an in‐house pre‐processing to clean data systematics. Temperature and CO2 profiles up to about 90 km, with consistent hydrostatic adjustment, are obtained, after adapting an Earth‐tested retrieval scheme to Mars conditions. Both pre‐processing and retrieval are discussed to illustrate their performance and robustness. Our results reveal the large impact of the MY34 Global Dust Storm (GDS), which warmed the atmosphere at all altitudes. The large GDS aerosols opacity limited the sounding of tropospheric layers. The retrieved temperatures agree well with global climate models (GCM) at tropospheric altitudes, but NOMAD mesospheric temperatures are wavier and globally colder by 10 K in the perihelion season, particularly during the GDS and its decay phase. We observe a warm layer around 80 km during the Southern Spring, especially in the Northern Hemisphere morning terminator, associated to large thermal tides, significantly stronger than in the GCM. Cold mesospheric pockets, close to CO2 condensation temperatures, are more frequently observed than in the GCM. NOMAD CO2 densities show oscillations upon a seasonal trend that track well the latitudinal variations expected. Results uncertainties and suggestions to improve future data re‐analysis are briefly discussed. Plain Language Summary: The detailed variation of temperature and density with altitude is of paramount importance to characterize the atmospheric state and to constrain the chemistry and dynamics as a whole. The Nadir and Occultation for Mars Discovery (NOMAD) instrument on board the Trace Gas Orbiter (TGO) has among its key targets the characterization of the thermal state with unprecedented vertical resolution. This is the target of this work, where we analyzed transmittance spectra obtained from the NOMAD solar occultation channel, with a state‐of‐the‐art retrieval scheme, adapted from Earth to Mars conditions and geometry. We applied it to the first year of TGO observations, which covered the last two Mars seasons of Mars Year 34. The results permit to study the temperature structure up to 90 km and its seasonal and latitudinal variations, revealing the impact of the MY34 Global Dust Storm, a warm layer at mesospheric altitudes not present in climate models, more frequent cold pockets than in current global climate models, and generally, colder temperature at those altitudes, all of which can be of importance for the validation of these climate models. Key Points: Temperature and density profiles up to 90 km are retrieved from Nadir and Occultation for Mars Discovery (NOMAD) first year of solar occultations, covering two seasons of Mars Year 34NOMAD temperatures agree well with climate model predictions below 50 km but are wavier and globally colder by about 10 K at high altitudesWe report large thermal tides producing warm layers at 80 km in the morning terminator. Also strong warming by the 2018 global dust storm [ABSTRACT FROM AUTHOR]

During Mars dust storms, atmospheric heating and expansion moves the ionospheric peak upward. Typically, peak altitude increases by no more than 10 km, and this increase occurs simultaneously with the expansion of the dust storm. However, Felici et al. (2020), https://doi.org/10.1029/2019JA027083, using the Mars Atmosphere Volatile EvolutioN (MAVEN) Radio Occultation Science Experiment (ROSE), reported an unusually large increase of ∼20 km at southern latitudes in early October 2016 during a modest dust storm. Here, we investigate why the ionospheric peak altitude increased so much in these observations. We extend the time series of ionospheric peak altitude values beyond the limited extent of the ROSE observations by applying a one‐dimensional photochemical model, in which neutral atmospheric conditions are based on in situ MAVEN Neutral Gas Ion Mass Spectrometer observations at similar latitudes and solar zenith angles to those observed by ROSE. We find that the ionospheric peak altitude was highest throughout October 2016 yet both the local and global atmospheric dust loading were greatest 1 month earlier. We hypothesize that (a) a portion of the unusually large 20 km enhancement in peak altitude and (b) the unusual delay between the greatest dust loading and the highest peak altitude were both associated with the occurrence of perihelion, which maximizes solar heating of the atmosphere, in late October 2016. Plain Language Summary: Dust storms always cause the top of the Martian ionosphere, a weakly ionized region in the upper atmosphere, to move to higher heights, typically by ∼10 km, due to the heating and expansion of the atmosphere caused by the storm. However, during a moderate regional dust storm in 2016, the ionospheric peak height elevated by ∼20 km. This study examines the reasons for this unusual increase. Simulations using a photochemical model showed that the ionospheric peak remained elevated for the whole month of October. However, the dust storm peaked in September, roughly a month before maximum ionospheric height. This is quite unusual as the ionospheric peak usually occurs at the highest heights during the peak of storms as maximum atmospheric heating and expansion occur during this period. Thus, this time delay suggests that the dust storm event does not solely cause the atypical enhancement in the ionospheric peak altitude. We suggest that the occurrence of Martian perihelion, Mars's closest point to the Sun in its orbit, and associated enhancement in solar heating in October 2016 might have contributed significantly to the thermal expansion of the atmosphere and hence toward the observed enhancement in the ionospheric peak height. Key Points: The ionospheric peak altitudes in the southern latitudes enhanced by ∼20 km during the regional dust storm of 2016Dust loading maximized during mid‐September but ionospheric peak heights reached their maximum only in October, suggesting a time delayThe occurrence of perihelion in October 2016 might have caused the time delay and also could have contributed to the unusual enhancement [ABSTRACT FROM AUTHOR]

We report the first detailed study of the diurnal thermal structure of upper mesosphere/lower thermosphere (∼80 to 160 km) of Mars from stellar occultation observations by the Imaging Ultraviolet Spectrograph (IUVS) aboard the NASA Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft. Due to stray light contamination, analyses of this data set to date have been confined to the nighttime events. This study makes use of a revised algorithm for removal of stray light from occultation spectra to retrieve the dayside events as well. The dayside is observed to be warmer than the nightside, with the maximum day/night difference of ∼30 K in the lower thermosphere, ∼20 K around the mesopause, with little diurnal variations at lower altitudes. This is consistent with the radiative time constant which is of the order of 1 Mars day in the 3×10−4 $3\times {10}^{-4}$ to 3×10−3 $3\times {10}^{-3}$ Pa region. The data also shows that the regions at pressure less than 2×10−3 $2\times {10}^{-3}$ Pa are under strong solar control with no prominent migrating tidal signatures. In contrast, on Earth, the radiative time constant near the mesopause is ∼10 Earth days and the temperature variations due to tides are quite large. The Mars Climate Database shows a diurnal trend opposite to the data in the mesosphere, with the dayside mesopause predicted to be cooler than the nightside by ∼10 K along with signatures of a vertically propagating tide. The IUVS data set provides an unprecedented constraint on the structure of the Martian mesosphere. Plain Language Summary: The dayside Mars Atmosphere and Volatile EvolutioN/Imaging Ultraviolet Spectrograph stellar occultation observations are often contaminated by stray light and are not processed by the current data reduction pipeline. We have developed an improved algorithm to retrieve these dayside events as well, therefore expanding the usable data set. There have been very few observations of Mars in 80–160 km altitude range, a region that is influenced by the wave perturbations from lower atmosphere and solar forcing from above, with no data set providing a complete local time coverage to study the day/night differences. Therefore, we have enabled the first detailed study of the diurnal thermal structure of Mars in this region by salvaging the dayside events. We find the dayside atmosphere to be always warmer than the nightside at these altitudes. Though it is expected but has not been established for the Martian mesosphere. In contrast, the Mars Climate Database shows an opposite diurnal trend at the mesopause, along with inconsistent tidal signatures with the data. The mesopause is controlled by the energy balance between radiative processes. These data‐model inconsistencies thus imply that our current understanding of the dynamics and structure of the Martian mesosphere is inadequate and this data set can provide important constraints on the models. Key Points: First detailed study of the diurnal thermal structure of Mars for 10−5−10−2 ${10}^{-5}-{10}^{-2}$ Pa pressure levels from 3003 stellar occultationsDayside is observed to be warmer than the nightside at these pressure levels with no detectable migrating tidal signaturesGCM predictions are inconsistent with the data and show dayside mesopause cooler than the nightside, with a vertically propagating tide [ABSTRACT FROM AUTHOR]

The nature and origins of longitudinal density structures in Mars' thermosphere between 100 and 120 km altitude, and -30° and -75° latitude around winter solstice (Ls = 70--122°), as measured by the accelerometer on the ExoMars Trace Gas Orbiter (TGO) Mission, are investigated. The methodology consists of deducing the responsible waves by comparing the observed longitude versus local time phase progressions of the structures with those of probable candidate waves in the Mars Climate Database (MCD) climatology. The observed wave-1, wave-2, and wave-3 structures are interpreted as originating mainly from SPW1; (SPW2, S0, DE1); and (SPW3, DE2), respectively, where SPWm denotes a stationary planetary wave with zonal wavenumber s = m, S0 is the zonally symmetric semidiurnal tide, and DE1(DE2) are the eastward-propagating diurnal tides with s = -1(-2). All of these waves exist in the MCD with amplitudes as large as those observed, although not wholly in the narrow height-latitude regime accessed by TGO. This disparity between the MCD and TGO amplitude structures is hypothesized to result from the extreme intrusion of the polar-region eastward winter jet into the thermosphere in the MCD. The winter polar jet blocks poleward expansion of DE1 and DE2 beyond -30° latitude, and also impacts the structures of SPW2, S0 and SPW3 that arise from in situ nonlinear interaction between DE1 and DE2, respectively, with the diurnal migrating tide (DW1). [ABSTRACT FROM AUTHOR]

The nature and origins of longitudinal density structures in Mars’ thermosphere between 100 and 120 km altitude, and −30° and −75° latitude around winter solstice (Ls = 70–122°), as measured by the accelerometer on the ExoMars Trace Gas Orbiter (TGO) Mission, are investigated. The methodology consists of deducing the responsible waves by comparing the observed longitude versus local time phase progressions of the structures with those of probable candidate waves in the Mars Climate Database (MCD) climatology. The observed wave‐1, wave‐2, and wave‐3 structures are interpreted as originating mainly from SPW1; (SPW2, S0, DE1); and (SPW3, DE2), respectively, where SPWm denotes a stationary planetary wave with zonal wavenumber s = m, S0 is the zonally symmetric semidiurnal tide, and DE1(DE2) are the eastward‐propagating diurnal tides with s = −1(−2). All of these waves exist in the MCD with amplitudes as large as those observed, although not wholly in the narrow height‐latitude regime accessed by TGO. This disparity between the MCD and TGO amplitude structures is hypothesized to result from the extreme intrusion of the polar‐region eastward winter jet into the thermosphere in the MCD. The winter polar jet blocks poleward expansion of DE1 and DE2 beyond −30° latitude, and also impacts the structures of SPW2, S0 and SPW3 that arise from in situ nonlinear interaction between DE1 and DE2, respectively, with the diurnal migrating tide (DW1).Key Points: Longitudinal density structures measured by Mars TGO accelerometer are interpreted with insights provided by Mars Climate Database (MCD)Stationary planetary waves, some excited by in situ wave‐wave interactions, play a prominent role in explaining the density structuresThe MCD‐TGO comparison suggests that the intensity and vertical‐latitudinal extent of the winter polar jet may be overestimated in the MCD [ABSTRACT FROM AUTHOR]

Elevated stratopauses are typically associated with prolonged disturbed conditions in the Northern Hemisphere polar winter. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and the Microwave Limb Sounder (MLS) observed a short‐lived and highly zonally asymmetric stratopause at mesospheric altitudes in November 2009, the earliest in the season reported so far. The Arctic climatological winter stratopause vanished, and MIPAS and MLS measured temperatures of 260 K at 82 km and 250 K at 75 km, respectively, in a region smaller than in typical midwinter elevated stratopause events. Planetary wave activity was initially high. Zonal mean zonal winds and the poleward temperature gradient northward of 70°N stayed reversed during 7 days, but the mesosphere did not cool. Wave activity dropped until the eastward stratospheric winds resumed and a strong vortex restored in the mesosphere. The stratopause emerged at high altitudes, staying there for 2–5 days. It was accompanied by enhanced downward transport. It took the stratopause 9 days to move down to its typical winter altitudes. Key Points: MIPAS and MLS observed a highly zonally asymmetric Arctic stratopause at high altitudes in November 2009 accompanied by a strong descentThe event has similarities with typical elevated stratopause events, but it is of smaller scale in terms of duration and extentThis is the first time a high‐altitude stratopause observed from space this early in the winter season is reported [ABSTRACT FROM AUTHOR]

Elevated stratopauses are typically associated with prolonged disturbed conditions in the Northern Hemisphere polar winter. The Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) and the Microwave Limb Sounder (MLS) observed a short‐lived and highly zonally asymmetric stratopause at mesospheric altitudes in November 2009, the earliest in the season reported so far. The Arctic climatological winter stratopause vanished, and MIPAS and MLS measured temperatures of 260 K at 82 km and 250 K at 75 km, respectively, in a region smaller than in typical midwinter elevated stratopause events. Planetary wave activity was initially high. Zonal mean zonal winds and the poleward temperature gradient northward of 70°N stayed reversed during 7 days, but the mesosphere did not cool. Wave activity dropped until the eastward stratospheric winds resumed and a strong vortex restored in the mesosphere. The stratopause emerged at high altitudes, staying there for 2–5 days. It was accompanied by enhanced downward transport. It took the stratopause 9 days to move down to its typical winter altitudes. Key Points: MIPAS and MLS observed a highly zonally asymmetric Arctic stratopause at high altitudes in November 2009 accompanied by a strong descentThe event has similarities with typical elevated stratopause events, but it is of smaller scale in terms of duration and extentThis is the first time a high‐altitude stratopause observed from space this early in the winter season is reported [ABSTRACT FROM AUTHOR]

Using a new observational technique, the NGIMS (Neutral Gas and Ion Mass Spectrometer) instrument on the MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft has the unique capability to measure horizontal thermospheric winds. Measured along the orbit track from periapsis (∼150 km) to ∼200 km, these are the first in situ observations of thermospheric winds at Mars. Significantly, this also means that simulated winds from a global circulation model can be compared to in situ observations from this part of the Martian atmosphere for the first time. In this study, observations from five NGIMS neutral wind campaigns have been compared to simulations from the Mars Global Ionosphere‐Thermosphere Model (M‐GITM), a ground to exosphere 3‐D general circulation model. By comparing NGIMS neutral wind observations to model simulations, the processes driving the winds and their variations in the upper atmosphere are examined. These comparisons show that for certain observational periods, the M‐GITM simulated winds can generally replicate the magnitude and/or direction of the NGIMS wind observations, while in others, significant differences occur. In general, wind observations from NGIMS campaigns with large orbit‐to‐orbit variability are not well replicated by M‐GITM, while campaigns with higher observed wind speeds are better captured by the model. Additionally, using these data‐model comparisons, the relative role of normal solar forcing and corresponding differential heating in driving thermospheric winds at Mars is found to be variable, likely acting as the primary driver under some conditions and secondary to other physical processes under others. Plain Language Summary: The MAVEN (Mars Atmosphere and Volatile Evolution) spacecraft, which is currently in orbit around Mars, has been taking monthly measurements of the speed and direction of the winds in the upper atmosphere of Mars between about 150 and 200 km above the surface. The observed wind speeds and directions change with time and location, and sometimes fluctuate quickly. These measurements are compared to simulations from a computer model of the Mars atmosphere called M‐GITM (Mars Global Ionosphere‐Thermosphere Model). This is the first comparison between direct measurements of winds in the upper atmosphere of Mars and simulated winds and is important because it can help to inform us what physical processes are acting on the observed winds. Some wind measurements have similar wind speeds or directions to those predicted by the M‐GITM model, but sometimes, there are large differences between the simulated and measured winds. The disagreements between wind observations and model simulations suggest that processes other than normal solar forcing may become relatively more important during these observations and alter the expected circulation pattern. Understanding the processes that drive the winds in the upper atmosphere of Mars provides key context for understanding how the atmosphere behaves as a whole system. Key Points: New thermospheric wind observations at Mars are compared to a global circulation model for the first timeWinds during observational campaigns with high average wind speeds and little variability were best replicated in model simulationsData‐model disagreements suggest processes other than normal solar forcing may become important and alter the expected circulation pattern [ABSTRACT FROM AUTHOR]

Abstract: We analyze 10 years of Mars Express total electron content (TEC) data from the Mars Advanced Radar for Subsurface and Ionospheric Sounding (MARSIS) instrument. We describe the spatial, seasonal, and solar cycle behavior of the Martian TEC. Due to orbit evolution, data come mainly from the evening, dusk terminator and postdusk nightside. The annual TEC profile shows a peak at Ls = 25–75° which is not related to the solar irradiance variation but instead coincides with an increase in the thermospheric density, possibly linked with variations in the surface pressure produced by atmospheric cycles such as the CO2 or water cycles. With the help of numerical modeling, we explore the contribution of the ion species to the TEC and the coupling between the thermosphere and ionosphere. These are the first observations which show that the TEC is a useful parameter, routinely measured by Mars Express, of the dynamics of the lower‐upper atmospheric coupling and can be used as tracer for the behavior of the thermosphere. [ABSTRACT FROM AUTHOR]

The Martian mesosphere and thermosphere, the region above about 60 km, is not the primary target of the ExoMars 2016 mission but its Trace Gas Orbiter (TGO) can explore it and address many interesting issues, either in-situ during the aerobraking period or remotely during the regular mission. In the aerobraking phase TGO peeks into thermospheric densities and temperatures, in a broad range of latitudes and during a long continuous period. TGO carries two instruments designed for the detection of trace species, NOMAD and ACS, which will use the solar occultation technique. Their regular sounding at the terminator up to very high altitudes in many different molecular bands will represent the first time that an extensive and precise dataset of densities and hopefully temperatures are obtained at those altitudes and local times on Mars. But there are additional capabilities in TGO for studying the upper atmosphere of Mars, and we review them briefly. Our simulations suggest that airglow emissions from the UV to the IR might be observed outside the terminator. If eventually confirmed from orbit, they would supply new information about atmospheric dynamics and variability. However, their optimal exploitation requires a special spacecraft pointing, currently not considered in the regular operations but feasible in our opinion. We discuss the synergy between the TGO instruments, specially the wide spectral range achieved by combining them. We also encourage coordinated operations with other Mars-observing missions capable of supplying simultaneous measurements of its upper atmosphere. [ABSTRACT FROM AUTHOR]

Global distributions of the CO2 vmr (volume mixing ratio) in the mesosphere and lower thermosphere (from 70 up to ~140 km) have been derived from high-resolution limb emission daytime MIPAS (Michelson Interferometer for Passive Atmospheric Sounding) spectra in the 4.3 µm region. This is the first time that the CO2 vmr has been retrieved in the 120-140 km range. The data set spans from January 2005 to March 2012. The retrieval of CO2 has been performed jointly with the elevation pointing of the line of sight (LOS) by using a non-local thermodynamic equilibrium (non-LTE) retrieval scheme. The non-LTE model incorporates the new vibrational-vibrational and vibrational-translational collisional rates recently derived from the MIPAS spectra by Jurado-Navarro et al. (2015). It also takes advantage of simultaneous MIPAS measurements of other atmospheric parameters (retrieved in previous steps), such as the kinetic temperature (derived up to ~100 km from the CO2 15 µm region of MIPAS spectra and from 100 up to 170 km from the NO 5.3 µm emission of the same MIPAS spectra) and the O3 measurements (up to ~100 km). The latter is very important for calculations of the non-LTE populations because it strongly constrains the O(³P) and O(¹D) concentrations below ~100 km. The estimated precision of the retrieved CO2 vmr profiles varies with altitude ranging from ~1% below 90 km to 5% around 120 km and larger than 10% above 130 km. There are some latitudinal and seasonal variations of the precision, which are mainly driven by the solar illumination conditions. The retrieved CO2 profiles have a vertical resolution of about 5-7 km below 120 km and between 10 and 20 km at 120-140 km. We have shown that the inclusion of the LOS as joint fit parameter improves the retrieval of CO2, allowing for a clear discrimination between the information on CO2 concentration and the LOS and also leading to significantly smaller systematic errors. The retrieved CO2 has an improved accuracy because of the new rate coefficients recently derived from MIPAS and the simultaneous MIPAS measurements of other key atmospheric parameters (retrieved in previous steps) needed for non-LTE modelling like kinetic temperature and O3 concentration. The major systematic error source is the uncertainty of the pressure/temperature profiles, inducing errors at midlatitude conditions of up to 15% above 100 km (20% for polar summer) and of ~5% around 80 km. The errors due to uncertainties in the O(¹D) and O(³P) profiles are within 3-4% in the 100-120 km region, and those due to uncertainties in the gain calibration and in the near-infrared solar flux are within ~2% at all altitudes. The retrieved CO2 shows the major features expected and predicted by general circulation models. In particular, its abrupt decline above 80-90 km and the seasonal change of the latitudinal distribution, with higher CO2 abundances in polar summer from 70 up to ~95 km and lower CO2 vmr in the polar winter. Above ~95 km, CO2 is more abundant in the polar winter than at the midlatitudes and polar summer regions, caused by the reversal of the mean circulation in that altitude region. Also, the solstice seasonal distribution, with a significant pole-to-pole CO2 gradient, lasts about 2.5 months in each hemisphere, while the seasonal transition occurs quickly. [ABSTRACT FROM AUTHOR]

MIPAS global Sun-synchronous observations are almost fixed in local time. Subtraction of the descending and ascending node measurements at each longitude only includes the longitudinal oscillations with odd daily frequencies nodd from the Sun's perspective at 10:00. Contributions from the background atmosphere, daily-invariant zonal oscillations and tidal modes with even-parity daily frequencies vanish. We have determined longitudinal oscillations in MIPAS temperature with nodd and wavenumber k = 0-4 from the stratosphere to 150 km from April 2007 to March 2012. To our knowledge, this is the first time zonal oscillations in temperature have been derived pole to pole in this altitude range from a single instrument. The major findings are the detection of (1) migrating tides at northern and southern high latitudes; (2) significant k = 1 activity at extratropical and high latitudes, particularly in the Southern Hemisphere; (3) k = 3 and k = 4 eastward-propagating waves that penetrate the lower thermosphere with a significantly larger vertical wavelength than in the mesosphere; and (4) a migrating tide quasi-biennial oscillation in the stratosphere, mesosphere and lower thermosphere. MIPAS global measurements of longitudinal oscillations are useful for testing tide modeling in the mesosphere and lower thermosphere region and as a lower boundary for models extending higher up in the atmosphere. [ABSTRACT FROM AUTHOR]

MIPAS global sun-synchronous observations are almost locked in local time. Subtraction of the descending and ascending node measurements at each longitude only contain the longitudinal oscillations with odd daily frequencies nodd from a solar perspective at 10A.M. Contributions of the background atmosphere, persistent (on a daily basis) longitudinal oscillations and tidal modes with even daily frequencies vanish. We have determined MIPAS temperature longitudinal oscillations with nodd and wavenumber k=0-4 from 20 to 150 km from April 2007 to March 2012. To our knowledge, 5 this is the first time temperature zonal oscillations are derived in this altitude range globally from a single instrument. The major findings are the detection of: 1) migrating tides at Northern and Southern high latitudes; 2) significant k = 1 activity at extra-tropical and highlatitudes, particularly in the SH; 3) k = 3 and k = 4 eastward propagating waves that penetrate in the lower thermosphere with a significantly larger vertical wavelength than in the mesosphere; 4) a quasi-biennial oscillation of the migrating tide mainly originated in the stratosphere and propagated to the MLT. MIPAS global measurements of longitudinal oscillations are useful for testing tide modeling in the MLT region and as a lower boundary of models extending higher up in the atmosphere. [ABSTRACT FROM AUTHOR]

In order to further evaluate the behavior of ionospheric variations at Mars, we investigate the Martian ionosphere-thermosphere (IT) perturbations associated with non-migrating thermal tides using over four years of Mars Atmosphere and Volatile Evolution (MAVEN) in situ measurements of the IT electron and neutral densities. The results are consistent with those of previous studies, namely strong correlation between the tidal perturbations in electron and neutral densities on the dayside at altitudes ~150–185 km, as expected from photochemical theory. In addition, there are intervals during which this correlation extends to higher altitudes, up to ~270 km, where diffusive transport of plasma plays a dominant role over photochemical processes. This is significant because at these altitudes the thermosphere and ionosphere are only weakly coupled through collisions. The identified non-migrating tidal wave variations in the neutral thermosphere are predominantly wave-1, wave-2, and wave-3. Wave-1 is often the dominant wavenumber for electron density tidal variations, particularly at high altitudes over crustal fields. The Mars Climate Database (MCD) neutral densities (below 300 km along the MAVEN orbit) shows clear tidal variations which are predominantly wave-2 and wave-3, and have similar wave amplitudes to those observed. [ABSTRACT FROM AUTHOR]