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See detailMartian visible and ultraviolet dayglow: altitude, latitudinal and seasonal variations observed with NOMAD/TGO
Gérard, Jean-Claude ULiege; Aoki, Shohei ULiege; Gkouvelis, Leonardos ULiege et al

Conference (2020, October 02)

The OI 557.7 nm green line has been measured in the Martian dayglow for the first time with the UVIS visible-ultraviolet spectrograph on board ESA’s Trace Gas Orbiter (Gérard et al., 2020). The first ... [more ▼]

The OI 557.7 nm green line has been measured in the Martian dayglow for the first time with the UVIS visible-ultraviolet spectrograph on board ESA’s Trace Gas Orbiter (Gérard et al., 2020). The first observations started in April 2019 in a special mode where the spacecraft is tilted to observe the limb with the UVIS nadir channel (Vandaele et al., 2015, Patel et al., 2017). The instrument detected the presence of bright green dayglow emission on every of those observations. The main peak altitude is located near 80 km, and its intensity varies as a result of the changing distance from sun, the local time and latitude of the observations. A second, less pronounced, emission peak is observed near 110 km. Photochemical model simulations (Gkouvelis et al., 2018) used the MCD density distribution (Forget et al., 1999) have been made to understand the sources of this airglow emission. It is able to reproduce the altitude and the brightness of the airglow layer. It indicates that the green line dayglow on Mars is essentially produced by photodissociation of CO2 molecules by solar far ultraviolet radiation (Fox & Dalgarno, 1979). A fraction of the oxygen atoms is formed in the 1S metastable state that produces the green emission. In this presentation, we describe additional dayside observations obtained since December 2019. For this purpose, the spacecraft has been used in a special mode where it is re-oriented so that the UVIS channel observed the sunlit limb (Lopez-Valverde et al., 2018). We analyse the observed limb profile variations and the changing altitude of the peak emission resulting from the variations of the pressure levels in the mesosphere (Gkouvelis et al., 2020). The measured intensities are compared with model calculations of the O(1S) density in the conditions of the observations. The ratio of ultraviolet spectral features relative to the oxygen emission also observed with UVIS will also be analysed. [less ▲]

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See detailDetection of green line emission in the dayside atmosphere of Mars from NOMAD-TGO observations
Gérard, Jean-Claude ULiege; Aoki, Shohei ULiege; Willame, Yannick et al

in Nature Astronomy (2020)

The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green colour1. It is also observed as a thin layer of glow ... [more ▼]

The oxygen emission at 557.7 nm is a ubiquitous component of the spectrum of the terrestrial polar aurora and the reason for its usual green colour1. It is also observed as a thin layer of glow surrounding the Earth near 90 km altitude in the dayside atmosphere2,3 but it has so far eluded detection in other planets. Here we report dayglow observations of the green line outside the Earth. They have been performed with the Nadir and Occultation for Mars Discovery ultraviolet and visible spectrometer instrument on board the European Space Agency’s ExoMars Trace Gas Orbiter. Using a special observation mode, scans of the dayside limb provide the altitude distribution of the intensity of the 557.7 nm line and its variability. Two intensity peaks are observed near 80 and 120 km altitude, corresponding to photodissociation of CO2 by solar Lyman α and extreme ultraviolet radiation, respectively. A weaker emission, originating from the same upper level of the oxygen atom, is observed in the near ultraviolet at 297.2 nm. These simultaneous measurements of both oxygen lines make it possible to directly derive a ratio of 16.5 between the visible and ultraviolet emissions, and thereby clarify a controversy between discordant ab initio calculations and atmospheric measurements that has persisted despite multiple efforts. This ratio is considered a standard for measurements connecting the ultraviolet and visible spectral regions. This result has consequences for the study of auroral and airglow processes and for spectral calibration. [less ▲]

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See detailIsobar Altitude Variations in the Upper Mesosphere Observed With IUVS‐MAVEN in Response to Martian Dust Storms
Gkouvelis, Leonardos ULiege; Gérard, Jean-Claude ULiege; Gonzalez-Galindo, Francisco et al

in Geophysical Research Letters (2020), 46

We report limb measurements of the oxygen dayglow emission at 297.2 nm performed during four Martian dust storms. The emission peak provides a good remote sensing tool to probe changes of the altitude of ... [more ▼]

We report limb measurements of the oxygen dayglow emission at 297.2 nm performed during four Martian dust storms. The emission peak provides a good remote sensing tool to probe changes of the altitude of the 39 mPa pressure level for the first time during dust storms. We illustrate the time variation of these changes and compare them with the infrared opacity in the lower atmosphere. We find that the 39 mPa level rises in response to the increase in dust opacity. It reaches a plateau, and additional dust load does not significantly increase its altitude. Numerical simulations with the LMD global circulation model shows a similar response, except for the event observed during MY33 regional storm when the model fails to reproduce the observed variations. Observations collected during the onset of the global dust storm in June 2018 show that the upper atmosphere rapidly responds within two Martian days to the increased amount of tropospheric dust. [less ▲]

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See detailAirglow remote sensing of the seasonal variation of the Martian upper atmosphere: MAVEN limb observations and model comparison
Gkouvelis, Leonardos ULiege; Gérard, Jean-Claude ULiege; Ritter, Birgit ULiege et al

in Icarus (2020), 341

We apply a new technique to remotely sense the seasonal altitude variation of the isobar level in this region. We present results from observations of airglow limb profiles collected during two Martian ... [more ▼]

We apply a new technique to remotely sense the seasonal altitude variation of the isobar level in this region. We present results from observations of airglow limb profiles collected during two Martian years with the Imaging UltraViolet Spectrograph (IUVS) onboard MAVEN. IUVS has collected tens of thousands of Martian airglow limb profiles since the Fall of 2014. We analyze the global dayglow dataset of the 297.2 nm emission corresponding to the O(3P-1S) forbidden transition. The vertical profile of this unique emission shows two peaks: one around 120 km and a second one near 80 km. We use both peaks to trace the altitude changes of the isobars in this atmospheric region. We map and compare them with the seasonal variations of the O(1S) with CO UV doublet peak. We find that the altitude of the level varies by up to 20 km. We show that the dayglow model overpredicts the lower peak altitude by 2–5 km from the observations while it underpredicts the upper peak altitude with the same offset. The best agreement near 80 km is obtained by decreasing the CO density by the Mars Climate Database (MCD) by about 40%. [less ▲]

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See detailConcurrent Observations Of Magnetic Reconnection From Cluster, IMAGE and SuperDARN: A Comparison Of Reconnection Rates And Energy Conversion
Matar, Jessy ULiege; Hubert, Benoît ULiege; Yao, Zhonghua ULiege et al

in Journal of Geophysical Research: Space Physics (2020), 125(4),

Auroral emissions reflect energy dissipation in the atmosphere, while the energy mostly originates from the magnetospheric dynamics of the planet. In the Earth's magnetosphere, the interaction between the ... [more ▼]

Auroral emissions reflect energy dissipation in the atmosphere, while the energy mostly originates from the magnetospheric dynamics of the planet. In the Earth's magnetosphere, the interaction between the solar wind and the terrestrial magnetic field produces magnetic reconnection ultimately causing the appearance of auroras in the polar regions. In this study, we revisit two reconnection events previously identified using ESA Cluster observations during conditions of moderate geomagnetic activity, in the magnetotail. We compare these in situ detections with simultaneous global auroral images from the NASA-IMAGE satellite and ionospheric convection measurements from SuperDARN. We show for the first time a direct correspondence between the electric voltage estimated at ionospheric altitude and the reconnection rate at the reconnection site. We compute the total auroral precipitation power and compare it to the energy released from the reconnection site. We deduce that the precipitated electron power of the aurora and the electron energy deposition rate from the reconnection site are of the same order of magnitude, which means that the thermal energy released by reconnection is primarily transferred to the ionosphere and contributes to the auroral activity. ©2020. American Geophysical Union. All Rights Reserved. [less ▲]

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See detailExamining Local Time Variations in the Gains and Losses of Open Magnetic Flux During Substorms
Mooney, Michaela K.; Forsyth, Colin; Rae, I. Jonathan et al

in Journal of Geophysical Research: Space Physics (2020), 125(4),

The open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by ... [more ▼]

The open magnetic flux content of the magnetosphere varies during substorms as a result of dayside and nightside reconnection. The open flux can be calculated from the area of the polar cap, delineated by the open-closed field line boundary (OCB). This study presents a superposed epoch analysis of the location of the OCB and the change in the magnetic flux content in individual nightside MLT sectors during substorm growth, expansion, and recovery phases. Far ultraviolet (FUV) observations from the IMAGE satellite are used to derive a proxy of the OCB location. In the hour prior to substorm onset, the total nightside flux content increases by up to 0.12 GWb on average, resulting in an equatorward expansion of the OCB. Following substorm onset, the OCB contracts toward the pole as the open magnetic flux content decreases by up to 0.14 GWb on average, but the rate of decrease of the total nightside open flux content differs by 5–66% between the three IMAGE far ultraviolet instruments. The OCB does not contract poleward uniformly in all nightside magnetic local time (MLT) sectors after substorm onset. Close to the substorm onset MLT sector, the OCB contracts immediately following substorm onset; however, the OCB in more dawnward and duskward MLT sectors continues to expand equatorward for up to 120 minutes after substorm onset. Despite the continued increase in flux in these sectors after substorm onset, the total nightside flux content decreases immediately at substorm onset, indicating that the nightside reconnection rate exceeds the dayside rate following substorm onset. ©2020. The Authors. [less ▲]

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See detailDetection of medium scale traveling ionospheric disturbances with TIMED/GUVI limb observations at mid and low latitude regions
Wautelet, Gilles ULiege; Hubert, Benoît ULiege; Gérard, Jean-Claude ULiege

Poster (2019, December 13)

Medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities at mid-latitudes but also occur in low-latitude regions. Whether they are due to the ... [more ▼]

Medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities at mid-latitudes but also occur in low-latitude regions. Whether they are due to the propagation of atmospheric gravity waves originating from the lower atmosphere or related to sporadic E layers, their harmonic signature is a common feature that allows them to be easily identified. MSTIDs have been extensively studied and characterized during the last two decades, mainly using GNSS measurements, ground-based all-sky imagers, radars or ionosondes. However, only few studies aimed to describe their vertical structure using remote sensing observations from space, which is helpful to understand their propagation and their dissipation processes. NASA’s TIMED mission was launched in December 2001 on a 74° inclination orbit at an altitude of 625 km, which allowed covering both low and high-latitude regions. The Global Ultraviolet Imager (GUVI) instrument aimed at remotely sense, among others, the ionospheric ion and electron densities. GUVI performed disk observations and limb scans in five FUV wavelength channels, making it an ideal tool to characterize the vertical structure of the ionosphere as well as to contextualize the study. The purpose of this work is to use GUVI limb scans to characterize MSTIDs preliminary detected by GNSS before December 2007, until the limb scanning mode failed. We first select a few MSTID cases during maximum background conditions of the Total Electron Content (TEC) computed by GNSS ground stations. Then, coincidental GUVI limb scans of the OI-135.6 nm emission are analyzed to characterize the vertical structure of the MSTIDs. The comparison is completed by the analysis of ionosonde profiles collected in the vicinity of the region where the MSTIDs have been previously detected by GNSS and GUVI. [less ▲]

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See detailInversion of occultation observation of a dusty atmosphere using hypergeometric functions.
Hubert, Benoît ULiege; Gérard, Jean-Claude ULiege; Gkouvelis, Leonardos ULiege et al

Poster (2019, December 12)

Occultation of solar radiation by a planetary atmosphere is a very accurate method to obtain high signal vs noise spectral measurement of the properties of the atmospheric gas, not only owing to the ... [more ▼]

Occultation of solar radiation by a planetary atmosphere is a very accurate method to obtain high signal vs noise spectral measurement of the properties of the atmospheric gas, not only owing to the overwhelmingly large photon flux from our host star, but also because the method is nearly not dependent on instrument calibration. On the other hand, the method can only be applied near the terminator. Using occultation techniques in other regions of the atmosphere can nevertheless be done using stars as a radiation source, but the instrument then has to be more sensitive to cope with the severely reduced photon flux. The method nevertheless remains independent on the absolute calibration of the instrument. Occultation observation directly provides the optical thickness (or the extinction coefficient) of the absorbing and scattering constituents when multiple scattering can be safely neglected. Under those conditions, the measurement gives the line-of-sight integrated density of the absorbing and scattering constituents, and simultaneous measurements at several wavelength are then needed to discriminate between the effects of the several species. Retrieval of the vertical density profile of the different constituents requires an inversion method, basically an inverse Abel transform when a spherical (or cylindrical) symmetry assumption can be made. Efficient inverse Abel transform methods rely on least squares fit techniques taking advantage of easy-to-compute analytical indefinite integrals constructed from the Abel transform integral operator. In the case of a dusty atmosphere, the contribution of dusts to the extinction depends on the properties of the dust grains controlling their scattering cross section, which is generally represented using the so-called alpha parameter appearing as an exponent of the wavelength in the expression of the cross section. As the properties of the dusts vary with altitude, so does the alpha parameter, which severely complicates the computation of the indefinite integrals needed for the inverse Abel transform fitting. We propose a method that allows to express those indefinite integrals using Gauss’s hypergeometric 2F1 function, which can be applied to the observation of the Earth as well as of planet Mars, as it is done by the ESA EXOMARS-NOMAD instrument. [less ▲]

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See detailPotential of TIMED/GUVI limb observations for medium-scale traveling ionospheric disturbances study at mid-latitudes
Wautelet, Gilles ULiege; Hubert, Benoît ULiege; Gérard, Jean-Claude ULiege

Conference (2019, November 18)

At mid-latitudes, medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities. During daytime, the common source of MSTIDs is the propagation of ... [more ▼]

At mid-latitudes, medium-scale traveling ionospheric disturbances (MSTIDs) are the most recurrent type of ionospheric irregularities. During daytime, the common source of MSTIDs is the propagation of atmospheric gravity waves whose origin is generally found in the lower atmosphere. In the nighttime hours, the Perkins instability induces another type of MSTIDs that is correlated with the appearance of sporadic E layers, sometimes leading to spread-F signatures in ionograms. MSTIDs climatology and characterization have been extensively described during the last two decades, mainly using GNSS measurements. However, only few studies are devoted to the description of their vertical structure and the monitoring of their propagation into the ionosphere, which is helpful to understand their dissipation processes and their physical origin. The NASA’s TIMED mission was launched in December 2001 on a 74° inclination low-Earth orbit at an altitude of 625 km, which allowed to cover both low and high-latitude regions. The Global Ultraviolet Imager (GUVI) instrument aimed at remotely sense, among others, the ionospheric ion and electron densities. GUVI performs disk observations and limb scans in five FUV wavelength channels, making it an ideal tool to characterize the vertical structure of the ionosphere as well as to contextualize the study. The purpose of this work is to use GUVI limb scans to characterize MSTIDs preliminary detected by GNSS in mid-latitudes before December 2007, after which the instrument exclusively supplied disk observations. We first select a few MSTID cases during solar maximum conditions that were observed in the Total Electron Content (TEC) by GNSS ground stations. Then, we combine our dataset with GUVI limb observation of the OI-135.6 nm emission to characterize the vertical structure of the MSTIDs. At last, concurrent observations from ionosondes located in the vicinity of the region where the GNSS and GUVI data were obtained will also provide an interesting cross-comparison dataset. [less ▲]

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See detailKinetic Monte Carlo Model for the Precipitation of High-Energy Protons and Hydrogen Atoms into the Atmosphere of Mars Taking into Account the Measured Magnetic Field
Shematovich, V.I.; Bisikalo, D.V.; Gérard, Jean-Claude ULiege et al

in Astronomy Reports (2019), 63(10), 835-845

Results of model computations of the interaction of the high-energy protons and hydrogen atoms (H/H+) precipitating into the Martian atmosphere are presented. These computations were performed using a ... [more ▼]

Results of model computations of the interaction of the high-energy protons and hydrogen atoms (H/H+) precipitating into the Martian atmosphere are presented. These computations were performed using a modification of the kinetic Monte Carlo model developed earlier for the analysis of the data from the MEX/ASPERA-3 instrument onboard the Mars Express spacecraft and the MAVEN/SWIA instrument onboard the MAVEN spacecraft. In this modification of the model, an arbitrary (three-dimensional) structure of the magnetic field of Mars is taken into account for the first time. With local measurements of all three components of the magnetic field, not only the flux of protons penetrating into the atmosphere, but also the degradation of the H/H+ flux along the spacecraft orbit and the formation of upward fluxes of protons and hydrogen atoms scattered by the atmosphere, can now be described. A comparison of simulations and measurements of proton fluxes at low altitudes are used to infer the efficiency of charge exchange between the solar wind and the extended Martian hydrogen corona. It was found that the induced magnetic field plays a very important role in the formation of the proton flux back-scattered by the atmosphere and strongly controls its magnitude. [less ▲]

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See detailThe OI-135.6 nm Nighttime Emission in ICON-FUV Images: A New Tool for the Observation of Classical Medium-Scale Traveling Ionospheric Disturbances?
Wautelet, Gilles ULiege; Hubert, Benoît ULiege; Gérard, Jean-Claude ULiege et al

in Journal of Geophysical Research. Space Physics (2019), 124

The National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON) mission will study the close relationship between the ionosphere, the atmospheric weather, and space weather using ... [more ▼]

The National Aeronautics and Space Administration Ionospheric Connection Explorer (ICON) mission will study the close relationship between the ionosphere, the atmospheric weather, and space weather using in situ and remote sensing instruments proving plasma density, temperature, ion drift velocity, and thermospheric wind velocity over the equatorial region. In particular, the far ultraviolet (FUV) instrument will image the terrestrial limb in two wavelength channels. During nighttime, only the channel characterizing the bright 135.6-nm emission of atomic oxygen will be used. The purpose of this study is to simulate FUV nightglow measurements under quiet as well as disturbed ionospheric conditions. Classical medium-scale traveling ionospheric disturbances (MSTIDs), which are understood as the ionospheric signature of atmospheric gravity waves, are one of the main sources of ionospheric variability. Here, we simulate their potential appearance in the FUV instrument data. The simulation model produces FUV images used as input to identify and characterize MSTIDs. MSTID propagation parameters can be retrieved under specific geometrical configurations between the FUV lines of sight and propagation direction of the MSTID, which differs depending on the limb or sublimb observing geometry. The largest MSTID signature is expected during equinoxes under solar maximum periods, for MSTID periods of less than 30 min. The weak brightness of the 135.6-nm multiplet under solar minimum conditions is the main limitation to the MSTID detection on the nightside. Future MSTID detection algorithms would have to cope with very low signal-to-noise ratio, in particular during solstices and under solar minimum conditions. [less ▲]

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See detailSeasonal density variations in the Martian thermosphere from observations of UV dayglow by MAVEN/IUVS
Gérard, Jean-Claude ULiege; Gkouvelis, Leonardos ULiege; Ritter, Birgit ULiege et al

Conference (2019, September)

Our simulations demonstrate that the peak altitudes of the CO2+ UVD, the Cameron bands and the OI 297.2 nm emissions are good proxies of the amount of CO2 above the emission peaks. In particular, the peak ... [more ▼]

Our simulations demonstrate that the peak altitudes of the CO2+ UVD, the Cameron bands and the OI 297.2 nm emissions are good proxies of the amount of CO2 above the emission peaks. In particular, the peak altitudes ZM of the UVD, Cameron and 297.2 nm emissions are directly controlled by the slant CO2 column density overlying the emission peaks. The characteristics of the OI 130 and 135 nm profiles depend on both the O and the CO2 distribution [3]. We take advantage of this feature, combined with the unprecedented quality and coverage of the IUVS limb observations, to map the variations of these emissions observed during the 4 years of the MAVEN mission. In particular, we present the parallel seasonal variations of ZM as a function of latitude and local time of the different emissions. An example for the Cameron and UVD emissions is shown in Fig. 3. We demonstrate that the changes of altitude ZM of different isobar levels co-vary and result from both seasonal surface-atmosphere exchanges and atmospheric dust load following global dust storms. In general, the MCD CO2 density requires only minor scaling to match the observations. [less ▲]

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See detailAnalysing the Martian Atmosphere with the O 130.4 and 135.6 nm emission observed by MAVEN/IUVS
Ritter, Birgit ULiege; Gérard, Jean-Claude ULiege; Gkouvelis, Leonardos ULiege et al

Scientific conference (2019, September)

We analyze limb observations of dayglow emissions from atomic oxygen in the Martian thermosphere and present highlights of the results

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See detailCO2 variations in the Martian lower thermosphere from IUVS-MAVEN airglow observations.
Gkouvelis, Leonardos ULiege; Gérard, Jean-Claude ULiege; Ritter, Birgit ULiege et al

Poster (2019, September)

UVS-MAVEN limb observations have been performed since 2014. We have analyzed almost four years of observations focusing on the O(1S) 297.2 nm dayglow emission line. We have developed an automatic ... [more ▼]

UVS-MAVEN limb observations have been performed since 2014. We have analyzed almost four years of observations focusing on the O(1S) 297.2 nm dayglow emission line. We have developed an automatic methodology to retrieve the CO2 column densities near 80 km, a region difficult to probe by other techniques. We present nearly two Martian years of observations of pressure variations at different latitudes and comparisons with MCD model predictions. Generally, the best agreement is reached following scaling down of the MCD values from 0.3 to 0.8 to fit the observations. This result was previously expected on the basis of model comparisons with ultraviolet occultation measurements. The data used in this study have been downloaded from NASA's Planetary Data. [less ▲]

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See detailDevelopment of VUV multilayer coatings for SMILE-UVI instrument
Loicq, Jerôme ULiege; Fleury-Frenette, Karl ULiege; Blain, Pascal ULiege et al

in International Conference on Space Optics — ICSO 2018 (2019, July 12)

he Ultraviolet Imager (UVI) instrument is a very challenging imager developed in the frame of the SMILE-ESA mission. The UV camera will consist of a single imaging system targeted at a portion of the ... [more ▼]

he Ultraviolet Imager (UVI) instrument is a very challenging imager developed in the frame of the SMILE-ESA mission. The UV camera will consist of a single imaging system targeted at a portion of the Lyman-Birge-Hopfield (LBH) N2 wavelength band. The baseline design of the imager meets the requirements to record snapshots of auroral dynamics with sufficient spatial resolution to measure cusp processes (100 km) under fully sunlit conditions from the specified apogee of the spacecraft. To achieve this goal, the UVI instrument utilizes a combination of four on-axis mirrors with an intensified FUV CMOS based camera. The mirrors will be coated with spectral selective interferometric layers to provide most of the signal filtering. The objective of these filters is to select the scientific waveband between 160 and 180 nm. The combined four mirrors have to give an out-of-band rejection ratio as high as possible to reject light from solar diffusion, dayglow and unwanted atomic lines in a range of 10-8 – 10-9. Different multilayer coatings are considered and optimized according to the π- multilayer equation for different H/L ratio and for different angles of incidence. Our theoretical evaluation shows a least a modification of the reflectance spectrum as a function of the angle of incidence, so that the optical beams hitting the different mirrors can have different optical properties depending on the optical fields and the distribution of the rays on the pupil. In this paper the effect of fields and coating homogeneity on the spectral throughput of the UVI instrument will be assessed and described. [less ▲]

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See detailMAVEN‐IUVS observations of the CO2+ UV doublet and CO Cameron bands in the Martian thermosphere: Aeronomy, seasonal and latitudinal distribution.
Gérard, Jean-Claude ULiege; Gkouvelis, Leonardos ULiege; Ritter, Birgit ULiege et al

in Journal of Geophysical Research. Space Physics (2019), 124

We analyze two Martian years of dayglow measurements of the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 298-299 nm with the Imaging UltraViolet Spectrograph on board the Maven orbiter. We ... [more ▼]

We analyze two Martian years of dayglow measurements of the CO Cameron bands and the CO2+ ultraviolet doublet (UVD) at 298-299 nm with the Imaging UltraViolet Spectrograph on board the Maven orbiter. We show that the altitude and the brightness of the two emissions peaks are strongly correlated, although data were collected over a wide range of latitudes and seasons. veraged limb profiles are presented and compared with numerical simulations based on updated calculations of the production of the CO (a3Π) and the CO2+ (B 2Σ) states. The model simulations use the solar flux directly measured on board MAVEN with the Extreme Ultraviolet Monitor (EUVM) and the neutral densities provided by the Mars Climate Database (MCD) version 5.3, adapted to the conditions of the observations. We show that the altitude and the shape of the sample limb profiles are well reproduced using the MCD neutral atmosphere. The simulated peak intensities of the CO2+ UVD and Cameron bands are in good agreement considering the uncertainties on the excitation cross sections and the calibration of the IUVS and EUVM instruments. No significant adjustment of the electron impact cross section on CO2 to produce the a3Π state is needed. Seasonal-latitudinal maps of the Cameron and UVD peak altitude observed during two Martian years show variations as large as 23 km. Model simulations of the amplitude of these changes are in fair agreement with the observations except during the southern summer dust period (Ls = 270°-320°) when the calculated rise of the dayglow layer is underestimated. [less ▲]

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See detailNo detection of methane on Mars from early ExoMars Trace Gas Orbiter observations
Korablev, O.; Vandaele, A. C.; Montmessin, F. et al

in Nature (2019), 568

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today 1 . A number of different measurements of methane show evidence of ... [more ▼]

The detection of methane on Mars has been interpreted as indicating that geochemical or biotic activities could persist on Mars today 1 . A number of different measurements of methane show evidence of transient, locally elevated methane concentrations and seasonal variations in background methane concentrations 2–5 . These measurements, however, are difficult to reconcile with our current understanding of the chemistry and physics of the Martian atmosphere 6,7 , which—given methane’s lifetime of several centuries—predicts an even, well mixed distribution of methane 1,6,8 . Here we report highly sensitive measurements of the atmosphere of Mars in an attempt to detect methane, using the ACS and NOMAD instruments onboard the ESA-Roscosmos ExoMars Trace Gas Orbiter from April to August 2018. We did not detect any methane over a range of latitudes in both hemispheres, obtaining an upper limit for methane of about 0.05 parts per billion by volume, which is 10 to 100 times lower than previously reported positive detections 2,4 . We suggest that reconciliation between the present findings and the background methane concentrations found in the Gale crater 4 would require an unknown process that can rapidly remove or sequester methane from the lower atmosphere before it spreads globally. [less ▲]

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See detailMartian dust storm impact on atmospheric H 2 O and D/H observed by ExoMars Trace Gas Orbiter
Vandaele, A. C.; Korablev, O.; Daerden, F. et al

in Nature (2019), 568

Global dust storms on Mars are rare 1,2 but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere 3 , primarily owing to ... [more ▼]

Global dust storms on Mars are rare 1,2 but can affect the Martian atmosphere for several months. They can cause changes in atmospheric dynamics and inflation of the atmosphere 3 , primarily owing to solar heating of the dust 3 . In turn, changes in atmospheric dynamics can affect the distribution of atmospheric water vapour, with potential implications for the atmospheric photochemistry and climate on Mars 4 . Recent observations of the water vapour abundance in the Martian atmosphere during dust storm conditions revealed a high-altitude increase in atmospheric water vapour that was more pronounced at high northern latitudes 5,6 , as well as a decrease in the water column at low latitudes 7,8 . Here we present concurrent, high-resolution measurements of dust, water and semiheavy water (HDO) at the onset of a global dust storm, obtained by the NOMAD and ACS instruments onboard the ExoMars Trace Gas Orbiter. We report the vertical distribution of the HDO/H 2 O ratio (D/H) from the planetary boundary layer up to an altitude of 80 kilometres. Our findings suggest that before the onset of the dust storm, HDO abundances were reduced to levels below detectability at altitudes above 40 kilometres. This decrease in HDO coincided with the presence of water-ice clouds. During the storm, an increase in the abundance of H 2 O and HDO was observed at altitudes between 40 and 80 kilometres. We propose that these increased abundances may be the result of warmer temperatures during the dust storm causing stronger atmospheric circulation and preventing ice cloud formation, which may confine water vapour to lower altitudes through gravitational fall and subsequent sublimation of ice crystals 3 . The observed changes in H 2 O and HDO abundance occurred within a few days during the development of the dust storm, suggesting a fast impact of dust storms on the Martian atmosphere. © 2019, The Author(s), under exclusive licence to Springer Nature Limited. [less ▲]

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See detailCharacteristics of Mars UV dayglow emissions from atomic oxygen at 130.4 and 135.6 nm: MAVEN/IUVS limb observations and modeling.
Ritter, Birgit ULiege; Gérard, Jean-Claude ULiege; Gkouvelis, Leonardos ULiege et al

in Journal of Geophysical Research. Space Physics (2019)

We present an overview of two Martian years oxygen dayglow limb observations of the ultraviolet (UV) emissions at 130.4 nm and 135.6 nm. The data have been collected with the IUVS instrument on board the ... [more ▼]

We present an overview of two Martian years oxygen dayglow limb observations of the ultraviolet (UV) emissions at 130.4 nm and 135.6 nm. The data have been collected with the IUVS instrument on board the MAVEN spacecraft. We use solar flux measurements of EUVM on board MAVEN to remove the solar induced variation and show the variations of the maximum limb brightness and altitude with season, SZA and latitude, which reflects the strong variability of the Martian atmosphere. The 130.4 and 135.6 nm peak brightness and altitudes are strongly correlated and behave similarly. Both emissions are modeled for selected data using Monte Carlo codes to calculate emissions arising from electron impact on O and CO2. Additional radiative transfer calculations are made to analyze the optically thick 130.4 nm emission. Model atmospheres from the Mars Climate Database serve as input. Both simulated limb profiles are in good agreement with the observations despite some deviations. We furthermore show that the observed 130.4 nm brightness is dominated by resonance scattering of the solar multiplet with a contribution (15-20%) by electron impact on O. Over 95% of the excitation at 135.6 nm arises from electron impact on O. Simulations indicate that the limb brightness is dependent on the oxygen and CO2 content, while the peak emission altitude is mainly driven by the CO2 content because of absorption processes. We deduce [O]/[CO2] mixing ratios of 3.1% and 3.0% at 130 km for datasets collected at LS=350° in Martian years 32 and 33. [less ▲]

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