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See detailThe O2(a1Δ) Venus nightglow intensity: internal versus solar activity control
Soret, Lauriane ULg; Gérard, Jean-Claude ULg

Conference (2016, April)

Introduction: The O2(a1Δg) Venus nightglow emission at 1.27 μm occurs in the atmospheric region governed by the subsolar to antisolar circulation. Several studies showed that the intensity of this ... [more ▼]

Introduction: The O2(a1Δg) Venus nightglow emission at 1.27 μm occurs in the atmospheric region governed by the subsolar to antisolar circulation. Several studies showed that the intensity of this emission is highly variable on a timescale of hours. Here, we study the possible correlation between the solar flux and the O2 infrared emission using VIRTIS-VEx spectral images at 1.27 μm that has been predicted to exist by theVTGCM model calculations by Bougher and Borucki(1994). VIRTIS data: Using the entire VIRTIS-M-IR nadir database, Soret et al. (2014) generated seven statistical maps of the O2(a1Δg) emission, each containing 500 observations. The purpose was to analyze the location of the brightest spot of the emission and its variations over time. Here, we analyze the intensity of the emission over time. Several methods have been used by Soret et al., (2015) to do so (evolution of the emission maximum, evolution of the average intensity, …) Here we present the results of a new analysis using a masking technique to calculate the time evolution of the nightglow brightness. However, none of them follow the same trend over time. Solar flux data: We now focus on solar flux variations in the time of VIRTIS observations (between May 2006 and October 2008), which were collected during a deep solar minimum. We use the SOHO-CELIAS/SEM (Judge et al., 1998) EUV daily average full solar disk fluxes at 1 AU between 0.1 and 50 nm available from the Space Sciences Center of the University of Southern California. EUV0.1–50 daily average fluxes decrease from 2.6 in May 2006 to 1.9 in October 2008 at the Earth. These values have been adapted to Venus by taking into account the distance from the Sun to the planet, but also the shift in date, considering the difference in solar longitude of the two planets. Values at Venus vary from 4.4 to 3.4, which corresponds to a decrease of 10.4% of the solar flux at Venus compared to a complete solar cycle (ranging from 13.5 to 3.9) Comparison of VIRTIS and SEM datasets: The linear correlation coefficient between the solar flux and the intensity peak is found to be 0.62, which expresses the global decreasing trend for both quantities. This coefficient is not higher because internal variations of the two studied variables do not occur simultaneously. More significantly, the correlation coefficient between the solar flux and the averaged intensities is found to be 0.35, meaning that no relation-ship exists between the O2(a1Δg) brightness and the solar activity. Conclusions: Contrary to the VTGCM calculations, we do not observe here a correlation between the O2(a1Δg) brightness and the solar flux. However, VIRTIS data were acquired during a deep solar minimum and, more importantly, during a relatively stable phase of the solar activity. A high level of variability of the O2(a1Δg) emission has been detected in the same dataset from day to day though (Hueso et al., 2008; Soret et al., 2014). It thus appears that the variability is more controlled by internal than external conditions: transport appears to play a major role in the nightglow emissions than the solar activity eventually does. This conclusion is at least valid for solar minimum conditions. A space mission with global imaging capabilities over an entire solar cycle would definitely allow determining the relative role played by solar activity and internal factors. [less ▲]

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See detailSPICAM observations and modeling of Mars aurorae
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Libert, Ludivine ULg et al

in Icarus (2016), 264

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the ... [more ▼]

Martian aurorae have been detected with the SPICAM instrument on board Mars Express both in the nadir and the limb viewing modes. In this study, we focus on three limb observations to determine both the altitudes and the intensities of the auroral emissions. The CO (a3P–X1R) Cameron bands between 190 and 270 nm, the CO Fourth Positive system (CO 4P) between 135 and 170 nm, the CO2+ doublet at 289 nm, the OI at 297.2 nm and the 130.4 nm OI triplet emissions have been identified in the spectra and in the time variations of the signals. The intensities of these auroral emissions have been quantified and the altitude of the strongest emission of the CO Cameron bands has been estimated to be 137 ± 27 km. The locations of these auroral events have also been determined and correspond to the statistical boundary of open-closed magnetic field lines, in cusp-like structures. The observed altitudes of the auroral emissions are reproduced by a Monte-Carlo model of electron transport in the Martian thermosphere for mono-energetic electrons between 40 and 200 eV. No correlation between electron fluxes measured in the upper thermosphere and nadir auroral intensity has been found. Here, we simulate auroral emissions observed both at the limb and at the nadir using electron energy spectra simultaneously measured with the ASPERA-3/ELS instrument. The simulated altitudes are in very good agreement with the observations. We find that predicted vertically integrated intensities for the various auroral emissions are overestimated, probably as a consequence of the inclination and curvature of the magnetic field line threading the aurora. However, the relative brightness of the CO and CO2+ emissions is in good agreement with the observations. [less ▲]

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See detailIs the O2(a1Δg) Venus nightglow emission controlled by solar activity ?
Soret, Lauriane ULg; Gérard, Jean-Claude ULg

in Icarus (2015), 262

Several past studies showed that the O2(a1Δg) Venus nightglow emission at 1.27 μm is highly variable on a timescale of hours. We examine whether the intensity of this emission shows a more global trend ... [more ▼]

Several past studies showed that the O2(a1Δg) Venus nightglow emission at 1.27 μm is highly variable on a timescale of hours. We examine whether the intensity of this emission shows a more global trend linked to solar activity. [less ▲]

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See detailMars nighttime aurora
European Space Agency, ESA/ATG medialab; Gérard, Jean-Claude ULg; Soret, Lauriane ULg

E-print/Working paper (2015)

Press release by the European Space Agency (ESA) on the occasion of the publication in two journals of the peer-reviewed literature

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See detailObservations of Mars aurorae
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Libert, Ludivine ULg et al

Conference (2015, September)

We present recent results obtained by combining remote sensing observations and in situ measurements of the Martian aurora made from Mars Express.

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See detailTwinkling Lights in the Nightside Upper Atmosphere: How Nightglow Contributes to our Understanding of Global Dynamics
Brecht, Amanda; Bougher, S.; Stiepen, Arnaud ULg et al

Conference (2015, September)

Upper atmospheres of planets continuously emit photons in the UV, Visible, and IR regions of the electromagnetic spectrum. Some of these emissions are classified as airglow, which includes dayglow and ... [more ▼]

Upper atmospheres of planets continuously emit photons in the UV, Visible, and IR regions of the electromagnetic spectrum. Some of these emissions are classified as airglow, which includes dayglow and nightglow. There are several mechanisms to create these emissions, but this presentation will focus on nightglow emissions resulting from photochemistry of neutral components. These neutral components originate on the dayside and are transported from the dayside to the nightside of a planet, where they subsequently undergo chemical reactions yielding nightglow. Nightglow emissions serve as effective tracers for planetary middle and upper atmosphere global wind systems due to their variable peak brightness and spatial distributions. The main planetary focus for this presentation will be on Mars and Venus’ atmospheres, due to the similar chemical constituents which populate their upper atmospheres. Currently, NO UV nightglow has been observed (e.g. Venus Express, Mars Express) on both Venus and Mars, while O2 IR nightglow has only been observed on Venus but is predicted to be seen on Mars. The observations show variations in time and location (latitude, local time, and altitude). The locations of the maximum nightglow intensities on each planet are different, but are supportive of the general picture of these two planet’s global circulation patterns. Model implications for both nightglows on both planets can provide valuable insight and understanding of the dynamical and chemical processes creating the nightglow emission variability. Two three-dimensional general circulation models will be utilized: the Venus Thermospheric General Circulation Model (VTGCM) and the Mars Global Ionosphere-Thermosphere Model (MGITM). The model output will be compared to nightglow datasets for each planet individually and planet to planet, to contrast the variations of the nightglow features and the underlying drivers for those variations. [less ▲]

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See detailThe Hydroxyl Nightglow Emissions on Earth, Venus and Mars
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, G. et al

Poster (2015, August)

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See detailTerrestrial OH nightglow measurements during the Rosetta flyby
Migliorini, A.; Gérard, Jean-Claude ULg; Soret, Lauriane ULg et al

in Geophysical Research Letters (2015), 42

We present a study of the terrestrial hydroxyl nightglow emissions observed with the Visible and Infrared Thermal Imaging Spectrometer on board the Rosetta mission. During these observations, the OH Δv  ... [more ▼]

We present a study of the terrestrial hydroxyl nightglow emissions observed with the Visible and Infrared Thermal Imaging Spectrometer on board the Rosetta mission. During these observations, the OH Δv = 1 and 2 sequences were measured simultaneously. This allowed investigating the relative population of the v = 1 to 9 vibrational levels by using both sequences. In particular, the relative population of the vibrational level v = 1 is determined for the first time from observations. The vibrational population decreases with increasing vibrational quantum number. A good agreement is found with a recent model calculation assuming multiquantum relaxation for OH(v) quenching by O2 and single-quantum relaxation for OH(v) by N2. [less ▲]

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See detailVenus nightglow intensity and solar activity: any correlation?
Soret, Lauriane ULg; Gérard, Jean-Claude ULg

Conference (2015, May 26)

We examine if any correlation is observed between the brightness of the O2 nightside airglow and the EUV solar irradiance using the full database of VIRTIS IR images. We conclude that, as was the case for ... [more ▼]

We examine if any correlation is observed between the brightness of the O2 nightside airglow and the EUV solar irradiance using the full database of VIRTIS IR images. We conclude that, as was the case for the NO airglow observed during the Pioneer Venus mission, no response to solar activity is observed. [less ▲]

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See detailThe EChO science case
Tinetti, Giovanna; Drossart, Pierre; Eccleston, Paul et al

in Experimental Astronomy (2015), 1502

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general ... [more ▼]

The discovery of almost 2000 exoplanets has revealed an unexpectedly diverse planet population. Observations to date have shown that our Solar System is certainly not representative of the general population of planets in our Milky Way. The key science questions that urgently need addressing are therefore: What are exoplanets made of? Why are planets as they are? What causes the exceptional diversity observed as compared to the Solar System? EChO (Exoplanet Characterisation Observatory) has been designed as a dedicated survey mission for transit and eclipse spectroscopy capable of observing a large and diverse planet sample within its four-year mission lifetime. EChO can target the atmospheres of super-Earths, Neptune-like, and Jupiter-like planets, in the very hot to temperate zones (planet temperatures of 300K-3000K) of F to M-type host stars. Over the next ten years, several new ground- and space-based transit surveys will come on-line (e.g. NGTS, CHEOPS, TESS, PLATO), which will specifically focus on finding bright, nearby systems. The current rapid rate of discovery would allow the target list to be further optimised in the years prior to EChO's launch and enable the atmospheric characterisation of hundreds of planets. Placing the satellite at L2 provides a cold and stable thermal environment, as well as a large field of regard to allow efficient time-critical observation of targets randomly distributed over the sky. A 1m class telescope is sufficiently large to achieve the necessary spectro-photometric precision. The spectral coverage (0.5-11 micron, goal 16 micron) and SNR to be achieved by EChO, thanks to its high stability and dedicated design, would enable a very accurate measurement of the atmospheric composition and structure of hundreds of exoplanets. [less ▲]

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See detailConcurrent observations of ultraviolet aurora and energetic electron precipitation with Mars Express
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Libert, Ludivine ULg et al

in Journal of Geophysical Research. Space Physics (2015)

The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures ... [more ▼]

The database of the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) instrument between late January 2004 and Mars 2014 has been searched to identify signatures of CO Cameron and CO<inf>2</inf>+ doublet ultraviolet auroral emissions. This study has almost doubled the number of auroral detections based on SPICAM spectra. Auroral emissions are located in the vicinity of the statistical boundary between open and closed field lines. From a total of 113 nightside orbits with SPICAM pointing to the nadir in the region of residual magnetic field, only nine nightside orbits show confirmed auroral signatures, some with multiple detections along the orbital track, leading to a total of 16 detections. The mean energy of the electron energy spectra measured during concurrent Analyzer of Space Plasma and Energetic Atoms/Electron Spectrometer observations ranges from 150 to 280eV. The ultraviolet aurora may be displaced poleward or equatorward of the region of enhanced downward electron energy flux by several tens of seconds and shows no proportionality with the electron flux at the spacecraft altitude. The absence of further UV auroral detection in regions located along crustal magnetic field structures where occasional aurora has been observed indicates that the Mars aurora is a time-dependent feature. These results are consistent with the scenario of acceleration of electrons by transient parallel electric field along semiopen magnetic field lines. © 2015. American Geophysical Union. All Rights Reserved. [less ▲]

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See detailVIRTIS-M-IR nadir and limb observations: variability of the O2(a1∆) nightglow spots
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

in EPSC2014, Vol. 9 (2014, September)

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 ... [more ▼]

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. [less ▲]

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See detailTerrestrial OH nightglow measurements during the Rosetta flyby
Migliorini, A.; Gérard, Jean-Claude ULg; Soret, Lauriane ULg et al

Conference (2014, August)

During the Rosetta travel to the comet P67, the VIRTIS instrument on board the spacecraft acquired unique images of the Earth in the 1.0-5.1 μm simultaneously. These allowed to investigate the nightglow ... [more ▼]

During the Rosetta travel to the comet P67, the VIRTIS instrument on board the spacecraft acquired unique images of the Earth in the 1.0-5.1 μm simultaneously. These allowed to investigate the nightglow observed spectrum and infer the relative OH populations of levels from 1 to 9. The values are reported in the table. The results are in good agreement with the model from GRANADA code (Kaufmann et al., 2008) for the conditions of a midlatitude night atmosphere. [less ▲]

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See detailTime variations of O2(a1Delta) nightglow spots on the Venus nightside and dynamics of the upper mesosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

in Icarus (2014), 237

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65 km of altitude is governed by the retrograde superrotational zonal (RSZ ... [more ▼]

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65 km of altitude is governed by the retrograde superrotational zonal (RSZ) circulation. The second region above 130 km is dominated by the subsolar to antisolar (SS–AS) circulation. The dynamics of the transition region in between are still not fully understood. However, the O2(a1D) nightglow emission at 1.27 lm, whose emitting layer is located at 96 km, can be used as a tracer of the dynamics in this transition region and the imaging spectrometer VIRTIS-M on board Venus Express, orbiting Venus since April 2006, acquired a large amount of nadir observations at this wavelength. Several previous studies showed that the O2(a1D) nightglow emission is statistically located near the antisolar point. In this study, individual VIRTIS-M nadir observations have been analyzed to investigate the variability of the phenomenon. Bright patches of 1.27 lm airglow have been extracted from every observation. It appears that the location of the bright patch is highly variable, even though the brightest patches occur near the antisolar point. Nadir observations have also been divided into time series, allowing generating animations to follow the intensity and the displacement of bright patches over time. Apparent wind velocities and characteristic decay/rise times and have been deduced from these time series. The speed of the displacements varies from 0 up to 213 m s 1, with a mean value of 54 m s 1. Owing to the high variability of the direction of the displacements both in the short and the long terms, no clear trend of a global motion at 96 km can be deduced from these observations. The mean decay time is 750 min while the mean rise time is 1550 min. The decay time can be explained as a combination of radiative decay and atomic oxygen transport. [less ▲]

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See detailLatitudinal structure of the Venus O2 infrared airglow: A signature of small-scale dynamical processes in the upper atmosphere
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Piccioni, Giuseppe et al

in Icarus (2014), 236

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the ... [more ▼]

Images of the nightside limb of Venus have been obtained in the northern hemisphere with the VIRTIS multispectral infrared imager on board Venus Express between April 2006 and October 2008. We analyze the latitudinal distribution of the O2(a1D) airglow limb profiles at 1.27 lm to characterize its distribution and variability. We show that the instantaneous structure of the emission is very different from the statistical global view of an enhanced emission near the equator, decreasing in brightness and slightly increasing in altitude toward the poles. The peak intensity of the limb profiles varies by a factor up to 50 between the brightest spots and the darkest regions. The bright airglow spots correspond to regions of enhanced downward flow of oxygen atoms originating from the dayside. Considerable variations in brightness and morphology are observed in the altitude–latitudinal distribution over a 24-h period. Analysis of the limb profiles indicates that secondary airglow peaks located at altitudes higher than the mean value of 96 km are observed on about 30% of the latitudinal cuts, but they are concentrated in narrow latitude areas extending over a few hundred kilometers. Most of them occur in transition regions between two altitude regimes in the 50 to 60 N region, possibly associated with the drop of the cloud top altitude observed equatorward of the ‘‘cold collar’’. We interpret these results as an indication that the strength of vertical transport in this mesosphere–thermosphere transition region is very variable both in location and time. This variability, also observed in nadir airglow images and wind measurements, is a key characteristic of the mesosphere–thermosphere transition region. It may be caused by fluctuations of the global day-to-night circulation generated by gravity waves. We show with a one dimensional model that local enhancements of eddy transport is a possibility. This variability is currently not accounted for by global circulation models that predict a single stable region of enhanced airglow in the vicinity of the antisolar point. [less ▲]

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See detailThe time evolution of O2(a1Δ) individual observations acquired by VIRTIS-M on board Venus Express
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

Poster (2013, June 10)

The O2(a1Δ) nightglow emission at 1.27 µm may be used as a tracer of the Venus upper mesosphere dynamics. This emission has been observed with VIRTIS-M-IR on board Venus Express. Previous studies showed ... [more ▼]

The O2(a1Δ) nightglow emission at 1.27 µm may be used as a tracer of the Venus upper mesosphere dynamics. This emission has been observed with VIRTIS-M-IR on board Venus Express. Previous studies showed that the emission maximum is statistically located close to the antisolar point at ∼96 km. This airglow results from the production of oxygen atoms on the Venus dayside by photodissociation and electron impact dissociation of CO2 and CO, which are then transported to the nightside by the subsolar to antisolar general circulation, where they recombine to create metastable O2(a1Δ) molecules. Their radiative deexcitation produces the O2(a1Δ) nightglow with a maximum near the antisolar point. However, VIRTIS individual observations indicate that the O2(a1Δ) nightglow emission is highly variable, both in intensity and location. Individual observations acquired every hour during a short period of time can also be grouped sequentially. Bright emission patches can thus be tracked and both their displacement and intensity variations can be analyzed. The peak intensity can vary from 1 to 6 megaRayleighs. We show that the emission peak moves with a mean value of ~80 m s-1, in good agreement with an earlier study by Hueso et al. (2008). The velocity vector in intensity and direction is evaluated approximately every 40 min. These displacements are highly variable, but some dynamical characteristics can be deduced from the observations. These results will be compared with other results of velocity determination in the upper mesosphere. [less ▲]

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See detailObservation de l’atmosphère de Vénus par le spectromètre imageur VIRTIS-M de Venus-Express : analyse des émissions nocturnes de O2 et OH
Soret, Lauriane ULg

Doctoral thesis (2013)

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 ... [more ▼]

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. [less ▲]

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See detailOxygen nightglow emissions of Venus: Vertical distribution and collisional quenching
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Migliorini, Alessandra et al

in Icarus (2013)

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative ... [more ▼]

We compare the altitude of three O2 night airglow emissions observed at the limb of Venus by the VIRTIS spectral imager with the values predicted by a model accounting for the different radiative lifetimes and collisional deactivation of the upper O2 states. The O and CO2 density profiles are based on remote sensing observations from the Venus Express spacecraft. Effective production efficiencies of the involved O2 metastable states and quenching coefficients by oxygen and carbon dioxide are adjusted to provide the best match with the measured emission limb profiles. We find values in general good agreement with earlier studies for the c1Σ-u state which gives rise to the Herzberg II bands. In particular, we confirm the low net yield of the c state production and the importance of its deactivation by CO2, for which we derive a quenching coefficient of 3x10-16 cm-3 s-1. The ∼4.5 km higher altitude of the Chamberlain band emission also recently detected by VIRTIS and the ratio of the Herzberg II/Chamberlain bands observed with Venera are well reproduced. To reach agreement, we use a 12% yield for the A’3Δu production following O atom association and quenching coefficients by O and CO2 of 1.3x10-11 cm-3 s-1 and 4.5x10-13 cm-3 s-1 respectively. We conclude that the different peak altitudes of the IR Atmospheric, Herzberg II and the Chamberlain bands reflect the relative importance of radiative relaxation and collisional quenching by O and CO2. [less ▲]

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