MARTIAN ULTRAVIOLET AURORA: RESULTS OF MODEL SIMULATIONS

We present recent modeling results based on observations performed with the UV spectrographs on board the Mars Express and MAVEN missions.Two types of aurora are discussed: the localized and transient discrete aurora and the more stable diffuse aurora observed during periods of active solar periods.CODYMA

MARS OXYGEN GREEN LINE DAYGLOW FROM NOMAD/UVIS AND MODEL COMPARISON

The UVIS (UV and Visible Spectrometer) channel of the NOMAD (Nadir and Occultation for MArs Discovery) spectrometer onboard the ExoMars Trace Gas Orbiter performs limb observations of the dayside of the Mars atmosphere in both the visible and the ultraviolet domains since April 2019. The recently discovered visible emissions of the oxygen green line at 557.7 nm has here been investigated. The variations of the limb profile of this emission are studied over seasons. These average limb profiles are compared to photochemical model simulations with MAVEN/EUVM solar flux and the LMD GCM as inputs of the model. The global shape of the profile and the intensities are generally well reproduced. However, the peak altitude can sometimes be underestimated by the model and needs an adjustment of the CO2 density to reproduce the observations. We also compare the variations of the green line intensities over some individual UVIS limb tracking observations (observations of the atmosphere at a quasi-constant altitude) to model simulations and demonstrate a very good agreement. Finally, we show that the intensity and altitude of the lower emission peak are correlated with the solar Ly-α flux as expected from the theory of its production

Abel transform of exponential functions for planetary and cometary atmospheres with application to observation of 46P/Wirtanen and to the OI 557.7 nm emission at Mars.

Line-of-sight integration of emissions from planetary and cometary atmospheres is the Abel transform of the emission rate, under the spherical symmetry assumption. Indefinite integrals constructed from the Abel transform integral are useful for implementing remote sensing data analysis methods, such as the numerical inverse Abel transform giving the volume emission rate compatible with the observation. We obtain analytical expressions based on a suitable, non-alternating, series development to compute those indefinite integrals. We establish expressions allowing absolute accuracy control of the convergence of these series depending on the number of terms involved. We compare the analytical method with numerical computation techniques, which are found to be sufficiently accurate as well. Inverse Abel transform fitting is then tested in order to establish that the expected emission rate profiles can be retrieved from the observation of both planetary and cometary atmospheres. We show that the method is robust, especially when Tikhonov regularization is included, although it must be carefully tuned when the observation varies across many orders of magnitude. A first application is conducted over observation of comet 46P/Wirtanen, showing some variability possibly attributable to an evolution of the contamination by dust and icy grains. A second application is considered to deduce the 557.7 nm volume emission rate profile of the metastable oxygen atom in the upper atmosphere of planet Mars

A chemical survey of exoplanets with ARIEL

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet’s birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25–7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and well-defined planet sample within its 4-year mission lifetime. Transit, eclipse and phase-curve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10–100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H2O, CO2, CH4 NH3, HCN, H2S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performed – using conservative estimates of mission performance and a full model of all significant noise sources in the measurement – using a list of potential ARIEL targets that incorporates the latest available exoplanet statistics. The conclusion at the end of the Phase A study, is that ARIEL – in line with the stated mission objectives – will be able to observe about 1000 exoplanets depending on the details of the adopted survey strategy, thus confirming the feasibility of the main science objectives.Peer reviewedFinal Published versio

Observation de l’atmosphère de Vénus par le spectromètre imageur VIRTIS-M de Venus-Express : analyse des émissions nocturnes de O2 et OH

Venus, the second planet of the solar system, has a very dense CO2-dominated atmosphere. Above 50 km, its dynamics is usually decomposed into two main circulation patterns. The first one, the Retrograde Superrotating Zonal (RSZ) circulation, controls atmospheric layers below 65 km of altitude. This motion is related to the retrograde rotation of the planet. The second circulation operates above 120 km. This Subsolar-Antisolar (SS-AS) circulation generates a flux from the dayside to the nightside of Venus. It originates from the strong temperature gradients at the top of the atmospheric layer. Between 65 and 120km, the circulation is more complex and no in situ measurement has been performed to study this region of the atmosphere. However, it is possible to use minor atmospheric constituents and their spectral signatures as dynamic tracers to better understand this region. For example, oxygen atoms are produced by photodissociation of CO2 molecules which dominate the Venusian atmosphere. They are then carried by the SS-AS circulation to the planet nightside, where they recombine into O2 molecules in several metastable excited states. Their de-excitation produces measurable emissions, named nightglow which may be qualitatively investigated. This thesis focuses on the study of these emission phenomena. Data have been acquired by the Venus Express spacecraft, in a quasi-polar elliptical orbit around Venus since April 2006. More specifically, observations have been made with the VIRTIS-M instrument, a multispectral imager. As VIRTIS observes in the visible and near infrared domains, some molecular oxygen and hydroxyl transitions can be detected in the data. The main goal of this study has been to extract quantitative information from these observations and to analyze both the density of constituents (such as excited molecular oxygen, atomic oxygen and ozone) and the dynamical processes involved in this region of the Venusian atmosphere. In a first part, data acquired at 1.27 µm in nadir mode have been processed and analyzed in order to study the O2(a1Δg→X3Σg-) infrared atmospheric transition. Data processing consists in correcting the geometrical effects associated with the view angle, the cloud reflection and the thermal contribution. Data analysis following emission patches in individual data sets points out a large variability of the phenomenon, both in terms of brightness and localization. Emission peaks vary from 0.5 to 6 MegaRayleighs (MR) and may be observed over the entire southern hemisphere of the planet, which is the observable part in nadir mode. However, once the individual data are grouped together to generate a statistical map, our analysis shows that the emission at 1.27 µm is located around the antisolar point, which confirms the SS-AS circulation predominance. This map is improved in the northern hemisphere by adding vertical intensity profiles derived from limb images. These profiles are deconvolved to take into account VIRTIS-M spatial resolution and transformed by the Abel inversion to get a local profile of the volume emission rate. A vertical integration of these profiles simulates a nadir observation and completes the bidimensional statistical map of the O2(a1Δg) emission. The intensity reaches 1.6 MR at the antisolar point and the mean nightside value is 0.5 MR. This map, combined with limb profiles, allows to generate a tridimensional distribution of the emission. It shows that the emitting layer is located at ~96.5 km. These results, combined with a tridimensional distribution of the CO2 density (generated with the VTS3 model or measurements from the SPICAV spectrometer on board Venus Express) allows to generate a 3-D map of the atomic oxygen density. The mean nightside density value is 2.0x1011 cm-3 at 103.4 km. This empirical map validates the VTGCM model, as no measurements of the atomic oxygen density had ever been performed in this region of the Venus atmosphere. Other oxygen transitions have been detected in the visible domain (Migliorini et al., 2012): the Herzberg II (c1Σu-→X3Σg-) and Chamberlain (A’3Δu→a1Δg) transitions. Using CO2 and O density profiles derived from our previous study, these transitions have been modeled. Some reaction parameters, whose laboratory measurements are insufficient or inexistent, have thus been estimated. The distribution of the Herzberg I (A3Σu→X3Σg-) transition has also been simulated. Other emission limb profiles have also been extracted from the VIRTIS-M database: the OH(Δv=1) and OH(Δv=2) Meinel emission bands of the hydroxyl molecule. First, these profiles have been processed to subtract a stray signal. The simultaneous statistical study shows that IOH(Δv=1)= 0.60 MR and IOH(Δv=2)=0.23 MR at ~97 km and that their intensity are correlated. The spectral analysis with synthetic spectra demonstrates that only v’≤4 vibrational levels are populated. These emissions have been modeled taking into account excited OH production, deactivation by collisions and reaction and spontaneous emission loss. The CO2 and O density profiles derived from the oxygen study have been used. The quenching coefficients have been adjusted to consider the temperature of the emitting layer and two quenching mechanisms by CO2 have been implemented. This model showed that collisional quenching by single quantum jump (Δv=1) best reproduces the observations. Likewise, an ozone density of 5.8x106 cm-3 at 96.5 km (for the best case) is in good agreement with the recent SPICAV O3 detection. Finally, the study of simultaneous OH(Δv=1) and O2(a1Δg) limb profiles showed a very high spatial correlation of these two emissions. This result has been explained by the role of atomic oxygen as a common precursor for the formation of both molecular oxygen and hydroxyl.Vénus, deuxième planète du système solaire, possède une atmosphère très dense en CO2 dont la dynamique, au-delà de 50 km, est usuellement décomposée en deux circulations principales. La première, la Super-rotation Rétrograde Zonale (RSZ), régit les couches de l’atmosphère situées en-deçà de 65 km d’altitude. Il s’agit d’un mouvement lié à la rotation rétrograde de la planète. La deuxième circulation opère au-delà de 120 km d’altitude. Il s’agit de la circulation Subsolaire-Antisolaire (SS-AS) qui génère un flux depuis la face diurne vers la face nocturne de Vénus. Cette circulation est due aux importants gradients de température régnant au sommet de la couche atmosphérique. Entre 65 et 120 km, la circulation est plus complexe et aucune mesure in situ ne permet d’étudier cette région de l’atmosphère. Il est en revanche possible d’utiliser les constituants atmosphériques minoritaires et leurs signatures spectrales en tant que traceurs de la dynamique pour en apprendre davantage sur cette région. Ainsi, par exemple, les atomes d’oxygène issus de la photodissociation du CO2 majoritairement présent dans l’atmosphère vénusienne sont transportés vers la face nocturne de la planète par la circulation SS-AS, où ils se recombinent pour former une molécule de O2 dans différents états excités. Leur désexcitation produit alors une émission lumineuse (nightglow) qu’il est possible d’analyser quantitativement. Cette thèse porte donc sur l’étude de tels phénomènes lumineux à partir des données acquises par la sonde Venus Express en orbite elliptique quasi-polaire autour de Vénus depuis avril 2006 et, plus particulièrement, grâce aux observations du spectromètre imageur VIRTIS-M à son bord. VIRTIS observant dans les domaines du visible et du proche infrarouge, les données permettent de mettre en évidence les émissions de différentes transitions de la molécule d’oxygène mais également du radical hydroxyle. Son objectif principal est d’extraire de ces observations l’information quantitative sur la densité de constituants tels que l’oxygène moléculaire excité, l’oxygène atomique et l’ozone et sur les processus dynamiques en présence dans cette région peu étudiée de l’atmosphère vénusienne. Dans un premier temps, les images acquises en mode nadir à 1,27 µm ont été traitées et analysées afin d’étudier la transition atmosphérique infrarouge O2(a1Δg→X3Σg-). Le traitement des données consiste en une correction des effets géométriques dus à l’angle de visée, de la réflexion des nuages et de la contribution thermique. L’analyse des données, qui vise à assurer le suivi des taches d’émission dans des observations individuelles, met en évidence l’extrême variabilité du phénomène, tant en intensité que spatialement. Les pics d’émission peuvent prendre des valeurs allant de 0,5 à 6 MégaRayleigh (MR) et couvrir tout l’hémisphère sud de la planète, domaine observable en mode nadir. En revanche, une fois ces informations individuelles regroupées en une seule et même carte, notre analyse montre que l’émission à 1,27 µm est statistiquement localisée autour du point antisolaire, ce qui confirme la prédominance de la circulation SS-AS. Cette carte est complétée par l’ajout de profils de l’intensité verticale extraits des images acquises au limbe dans l’hémisphère nord de Vénus. Ces profils sont déconvolués pour tenir compte de la résolution spatiale de VIRTIS-M et transformés par l’inversion d’Abel pour obtenir un profil local du taux d’émission volumique. Leur intégration verticale permet de simuler une observation en mode nadir pour compléter la carte statistique bidimensionnelle de l’émission O2(a1Δg). L’intensité atteint alors 1,6 MR au point antisolaire et vaut, en moyenne, 0,5 MR sur la face nocturne de la planète. Cette carte, combinée avec les profils au limbe permet en outre de générer une distribution tridimensionnelle de l’émission, mettant en évidence que la couche émettrice se situe à ~96,5 km. Ces résultats, eux-mêmes combinés avec la distribution tridimensionnelle de la densité de CO2 générée à partir du modèle VTS3 ou des mesures du spectromètre SPICAV à bord de Venus Express, permettent également de créer une représentation 3-D de la densité d’oxygène atomique. La densité moyenne sur la face nocturne vaut 2,0x1011 cm-3 à 103,4 km d’altitude. Cette carte permet de valider le modèle VTGCM puisqu’aucune mesure de la densité d’oxygène atomique n’avait jamais été réalisée dans cette région de l’atmosphère de Vénus. D’autres transitions de l’oxygène ont pu être détectées dans le domaine visible (Migliorini et al., 2012). Il s’agit notamment des transitions de Herzberg II (c1Σu-→X3Σg-) et de Chamberlain (A’3Δu→a1Δg). Leur modélisation à partir des profils de densité de CO2 et O précédemment déduits ont permis d’estimer certains paramètres de réaction dont les mesures en laboratoire sont insuffisantes (voire inexistantes) mais également de simuler la transition de Herzberg I (A3Σu→X3Σg-). D’autres profils d’émission au limbe ont également été extraits de la base de données de VIRTIS-M. Il s’agit des profils d’émission des bandes de Meinel de la molécule hydroxyle : OH(Δv=1) et OH(Δv=2). Dans un premier temps, ces profils ont été traités afin de soustraire le signal parasite. L’étude statistique des profils simultanés révèle que IOH(Δv=1)= 0,60 MR et IOH(Δv=2)=0,23 MR à ~97 km et que les deux émissions sont corrélées. L’analyse de leur spectre grâce à la génération de spectres synthétiques prouve que seuls les niveaux vibrationnels v’≤4 sont peuplés. Pour modéliser ces émissions, la production de OH excité, la désactivation par collisions et les pertes par réaction ou par émission spontanée ont été pris en compte. Les profils de densité utilisés (O et CO2) proviennent de l’étude précédente. Les coefficients de désactivation ont été ajustés en fonction de la température de la couche émettrice et deux mécanismes de désactivation par CO2 ont été implémentés. Cette modélisation a permis de montrer qu’une désactivation collisionnelle par quantum unique (Δv=1) reproduit au mieux les observations. De même, une densité d’ozone de 5,8x106 cm-3 à 96,5 km (pour le meilleur cas de la simulation) est tout à fait compatible avec les récentes détections effectuées à l’aide de SPICAV. Enfin, l’étude des profils au limbe des émissions OH(Δv=1) et O2(a1Δg) acquis simultanément a montré une excellente corrélation spatiale des deux émissions. Ce résultat s’explique par le rôle de l’oxygène atomique, précurseur commun à la formation de l’oxygène moléculaire et de l’hydroxyle

VIRTIS-M-IR nadir and limb observations: variability of the O2(a1∆) nightglow spots

Individual nadir and limb VIRTIS-M-IR at 1.27 μm show that the O2(a1∆) nightglow emission is highly variable. This variability is observed spatially, but also in term of intensity and altitude of the emitting layer over time. Apparent wind velocities have been deduced from the nadir observations, as well as the e-folding times. Limb observations show that an increase of the emitting layer altitude is observed near the cold collar region