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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 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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See detailThe OH Venus nightglow spectrum: intensity and vibrational composition from VIRTIS-Venus Express observations
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

in Planetary and Space Science (2012), 73(1),

Limb spectra of the OH nightglow emission corresponding to the ∆v=1 and ∆v=2 sequences have been collected with the VIRTIS infrared imaging spectrograph on board Venus Express between April 2006 and ... [more ▼]

Limb spectra of the OH nightglow emission corresponding to the ∆v=1 and ∆v=2 sequences have been collected with the VIRTIS infrared imaging spectrograph on board Venus Express between April 2006 and October 2008. A detailed statistical analysis shows that the peak intensity and altitude of the two vibrational sequences are significantly correlated, with a mean intensity ratio of the two sequences of 0.38±0.37. The altitude of the maximum of the ∆v=2 emission is located ~1 km lower than ∆v=1. A spectral analysis shows that the Δv=1 sequence is composed at 44.6% by the (1–0) band, 9.3% by the (3–2) band and 7.1% by the (4–3) band. The Δv=2 emission is best fitted if solely including the (2–0) band. A non-LTE model of OH vibrational population by the O3+H reaction including radiative and collisional relaxation has been used to compare the expected spectral distribution, the altitude of the emission peak and the emission rate under different assumptions on the quenching processes to those observed with VIRTIS. The adopted carbon dioxide, atomic oxygen and ozone densities are based on recent Venus Express remote sensing measurements. We find that the “sudden death” quenching scheme by CO2 produces inadequate spectral distribution between the various bands and insufficient airglow brightness. Instead, the observed spectral distribution and the total emission intensity are reasonably well reproduced with the single quantum jump model, a O density profile peaking at 103.5 km with a maximum value of 1.9×1011 cm−3, a O3 density profile peaking at 5.8×106 cm−3 at 96.5 km and a H density profile close to 108 cm−3 between 90 and 120 km, in agreement with several photochemical models. [less ▲]

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See detailVenus O2 night glow observations with VIRTSI/Venus Express
Migliorin, Alessandra; Piccioni, Giuseppe; Gérard, Jean-Claude ULg et al

Conference (2012, September)

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See detailUnderstanding of the Venus upper atmosphere dynamics with O2(a1 ) Venus Express observations
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

Poster (2012, April)

The O2(a1 ) nightglow emission at 1.27 m may be used as a tracer of the dynamics prevailing in the Venusian upper mesosphere. This emission has thus been observed with ground-based telescopes and from ... [more ▼]

The O2(a1 ) nightglow emission at 1.27 m may be used as a tracer of the dynamics prevailing in the Venusian upper mesosphere. This emission has thus been observed with ground-based telescopes and from space with instruments such as VIRTIS on board Venus Express. Observations have shown that the emission maximum is statistically located close to the antisolar point at 96 km. As originally suggested by Connes et al. (1979), such an emission 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 excited O2(a1 ) molecules. Their radiative deexcitation produces the O2(a1 ) nightglow with a maximum near the antisolar point. However, VIRTIS observations indicate that the O2(a1 ) nightglow emission is highly variable, both in intensity and location. Actually, when considering individual observations, the patch of bright emission is rarely located at the antisolar point and the brighter area around this point is the result of statics accumulation. Also, when considering several individual observations acquired in a short period of time, it is possible to follow an individual emission patch and to deduce its displacement and its brightness variation due to activation or deactivation. In this study, we analyze several sequences of VIRTIS observations in order to understand the Venus upper mesosphere dynamics.We show that the intensity can vary by several megaRayleighs in a couple of hours with effective lifetimes on the order of several hours. The horizontal motion of the spots leads to the conclusion that winds in the 95-100 km region are in the range of 25 to 150 m s-1, in good agreement with the study by Hueso et al. (2008). [less ▲]

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See detailThe oxygen nightglow emissions of Venus: vertical distribution and role of collisional quenching
Gérard, Jean-Claude ULg; Soret, Lauriane ULg; Migliorini, Alessandra et al

Conference (2012, April)

The oxygen nightglow emissions of Venus: vertical distribution and role of collisional quenching J.-C. Gérard (1), L. Soret (1), A. Migliorini (2), G. Piccioni (2), and P. Drossart (3) (1) LPAP ... [more ▼]

The oxygen nightglow emissions of Venus: vertical distribution and role of collisional quenching J.-C. Gérard (1), L. Soret (1), A. Migliorini (2), G. Piccioni (2), and P. Drossart (3) (1) LPAP - Université de Liège - Belgium (jc.gerard@ulg.ac.be, 0032 4 366 9711), (2) INAF - Rome, Italy, (3) LESIA, Observatoire de Paris - Meudon, France Three-body recombination of atomic oxygen produces O2 molecules excited in different electronic states such as a 1∆g, b 1 􏰀+g , A 3 􏰀+u , c 1 􏰀uand A’ 3∆u, each with a specific quantum efficiency. When they radiate, optical transitions are observed in a wide range of wavelengths extending from the ultraviolet to the near infrared. In planetary atmospheres, spontaneous radiative deexcitation compete with collisional quenching with ambient molecules and atoms. As a consequence, the corresponding airglow emission profiles may significantly differ from each other in brightness and altitude of the emitting layer. We model the volume emission rates and limb profiles of the O2 Atmospheric Infrared (a 1∆-X 3 􏰀), Herzberg I (A 3 􏰀-X 3 􏰀), Herzberg II (c 1 􏰀-X 3 􏰀), Chamberlain (A’ 3∆-a 1∆) bands expected on the Venus night side. The quenching rates are taken from laboratory and observational planetary data and we apply two different methods to determine the oxygen and CO2 density profiles. One is based on recent analysis of data collected by instruments on board the Venus Express mission. The second one uses a one-dimensional chemical-diffusive model where the free parameters are the strength of turbulent transport and the downward flux of O atoms. Both approaches indicate that the calculated intensities of each transition range over several orders of magnitude and that differences are expected in the altitude of the maximum emission. These predictions will be compared with VIRTIS/Venus Express limb observations, which make it possible to derive the vertical distribution of the O2 emissions in the visible and infrared. These measurements suggest that no difference is observed between the altitude of the peak of the IR Atmospheric and Herzberg II bands. Conclusions will be drawn about the validity of the current set of quenching coefficients used in the model. [less ▲]

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See detailAtomic oxygen on the Venus nightside: Global distribution deduced from airglow mapping
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Montmessin, Franck et al

in Icarus: International Journal of Solar System Studies (2012), 217

The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1[Delta]) nightglow distribution at 1.27 [mu]m in the Venus mesosphere ... [more ▼]

The Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft has measured the O2(a1[Delta]) nightglow distribution at 1.27 [mu]m in the Venus mesosphere for more than two years. Nadir observations have been used to create a statistical map of the emission on Venus nightside. It appears that the statistical 1.6 MR maximum of the emission is located around the antisolar point. Limb observations provide information on the altitude and on the shape of the emission layer. We combine nadir observations essentially covering the southern hemisphere, corrected for the thermal emission of the lower atmosphere, with limb profiles of the northern hemisphere to generate a global map of the Venus nightside emission at 1.27 [mu]m. Given all the O2(a1[Delta]) intensity profiles, O2(a1[Delta]) and O density profiles have been calculated and three-dimensional maps of metastable molecular and atomic oxygen densities have been generated. This global O density nightside distribution improves that available from the VTS3 model, which was based on measurements made above 145 km. The O2(a1[Delta]) hemispheric average density is 2.1 × 109 cm-3, with a maximum value of 6.5 × 109 cm-3 at 99.2 km. The O density profiles have been derived from the nightglow data using CO2 profiles from the empirical VTS3 model or from SPICAV stellar occultations. The O hemispheric average density is 1.9 × 1011 cm-3 in both cases, with a mean altitude of the peak located at 106.1 km and 103.4 km, respectively. These results tend to confirm the modeled values of 2.8 × 1011 cm-3 at 104 km and 2.0 × 1011 cm-3 at 110 km obtained by Brecht et al. [Brecht, A., Bougher, S.W., Gérard, J.-C., Parkinson, C.D., Rafkin, S., Foster, B., 2011a. J. Geophys. Res., in press] and Krasnopolsky [Krasnopolsky, V.A., 2010. Icarus 207, 17-27], respectively. Comparing the oxygen density map derived from the O2(a1[Delta]) nightglow observations, it appears that the morphology is very different and that the densities obtained in this study are about three times higher than those predicted by the VTS3 model. [less ▲]

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See detailThe distributions of the OH (Δv=1) and (Δv=2) emissions on the Venus nightside
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

Conference (2010, September)

The presence of OH was detected in the spectrum of the Venus mesosphere observed at the limb with the VIRTIS instrument on board the Venus Express spacecraft [3]. The (1-0) and (2-1) transitions at 2.80 ... [more ▼]

The presence of OH was detected in the spectrum of the Venus mesosphere observed at the limb with the VIRTIS instrument on board the Venus Express spacecraft [3]. The (1-0) and (2-1) transitions at 2.80 and 2.94 mm, respectively and the (2-0) band at 1.43 mm were clearly identified. The results of this study show that a correlation is observed between the emissions associated to the Δv=1 and the Δv=2 sequences. [less ▲]

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See detailThe Venus oxygen nightglow and density distributions
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Montmessin, Franck et al

Conference (2010, September)

Observing Venus nightglow is a key tool to understand the composition and the dynamics of its atmosphere. Results deduced from observations can be implemented to produce a data model of Venus atmosphere ... [more ▼]

Observing Venus nightglow is a key tool to understand the composition and the dynamics of its atmosphere. Results deduced from observations can be implemented to produce a data model of Venus atmosphere. For instance, the Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft is very useful to analyze the O2(a1Δ) nightglow at 1.27 µm in the Venus mesosphere. Nadir observations can be used to create a statistical map of the emission on Venus nightside. It appears that the maximum of the emission is located near the antisolar point. Limb observations also provide information on the altitude and on the shape of the emission layer. Combining nadir observations and vertically integrated limb observations improves the statistics of the emission map on Venus nightside. An associated limb profile can also be deduced for any point of the nightside. Given all these O2(a1Δ) intensity profiles, O2* density profiles can be calculated. O density profiles can also be calculated as long as CO2 density profiles are available. These can be retrieved either from the VTS3 model or from SPICAV stellar occultation measurements. Finally, three-dimensional maps of excited molecular and atomic oxygen densities can be generated. The oxygen density map shows significant differences from the VTS3 model predictions. [less ▲]

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See detailThe Venus OH Nightglow Distribution
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

Poster (2010, June)

The first identification of the OH airglow in the terrestrial mesosphere was made in 1950 by Meinel [1950]. Recently, the unexpected presence of the OH nightglow was observed in the Venus mesosphere by ... [more ▼]

The first identification of the OH airglow in the terrestrial mesosphere was made in 1950 by Meinel [1950]. Recently, the unexpected presence of the OH nightglow was observed in the Venus mesosphere by Piccioni et al. [2008] using a limb profile from the Visible and Infra-Red Thermal Imaging Spectrometer (VIRTIS) instrument on board the Venus Express spacecraft. They clearly identified the (1-0) and (2-1) transitions at 2.80 and 2.94 µm, respectively and the (2-0) band at 1.43 µm. Additional bands belonging to the Δv=1 sequence also appear to be present longward of the (1-0) band. In a preliminary study of characteristics of the OH emission distribution, Gérard et al. [2010] pointed out a correlation between the OH(Δv=1) and the O2(a1Δ) nightglow intensities. In Soret et al. [2010], the full dataset of VIRTIS-M limb observations of the OH Venus nightglow has been corrected from the thermal emission of the planet and analyzed to determine its characteristics. Based on 3328 limb profiles, the study shows that the emission is highly variable. No clear dependence of the airglow layer altitude versus the antisolar angle is established. The peak brightness appears to decrease away from the antisolar point even if the variability at a given location is very strong. Some correlation between simultaneous observations of the intensity of the OH and the O2(a1∆) emissions has also been detected, presumably because atomic oxygen is a common precursor to the formation of O2(a1∆) and O3, whose reaction with H produces excited OH. A relation given in the one-dimensional photochemical model of Krasnopolsky [2009] has been used to link the OH and the O2(a1∆) airglows through the hydrogen flux at 130 km. It appeared that using a constant flux did not fill well the simultaneous OH and O2 observations. Either the flux has to vary with the distance to the antisolar point or other dimensions have to be involved. [less ▲]

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See detailVenus OH nightglow distribution based on VIRTIS limb observations from Venus Express
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Piccioni, Giuseppe et al

in Geophysical Research Letters (2010), 37

The full set of VIRTIS‐M limb observations of the OH Venus nightglow has been analyzed to determine its characteristics. Based on 3328 limb profiles, we find that the mean peak intensity along the line of ... [more ▼]

The full set of VIRTIS‐M limb observations of the OH Venus nightglow has been analyzed to determine its characteristics. Based on 3328 limb profiles, we find that the mean peak intensity along the line of sight of the OH(deltaV = 1 sequence) is 0.35 MR and is located at 96.4 ± 5 km. The emission is highly variable and no dependence of the airglow layer altitude versus the antisolar angle is observed. The peak brightness appears to decrease away from the antisolar point even if the variability at a given location is very strong. Some correlation between the intensity of the OH and the O2(a1Delta) emissions is also observed, resumably because atomic oxygen is a common precursor to the formation of O2(a1Delta) and O3, whose reaction with H produces excited OH. Comparing our results with predictions from a photochemical model, a constant H flux does not match the simultaneous OH and O2 airglow observations. [less ▲]

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See detailLatitudinal - local time distribution of the O2 and OH infrared nightglows and O density in the Venus lower thermosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Saglam, Adem et al

Conference (2009, September)

Atomic oxygen has been measured in situ only above 145 km on both the day and the night sides of Venus. Limb observations obtained with the Venus Infrared Thermal Imaging Spectrometer (VIRTIS) on board ... [more ▼]

Atomic oxygen has been measured in situ only above 145 km on both the day and the night sides of Venus. Limb observations obtained with the Venus Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express show that the O2 infrared nightglow peaks at ~97 km [1, 2], with a mean intensity value of about 1 MR. Yet, the distribution is largely inhomogeneous, with an enhanced region of ~3 MR statistically located near the midnight meridian at low latitude [3]. The oxygen density can be mapped using the O2 airglow and CO2 density vertical distributions [4]. The O2 vol-ume emission rates are obtained with an Abel inversion of the O2 limb profiles using CO2 vertical distributions taken from the Venus International Reference Atmosphere (VIRA) model. The results show that the O density peak varies in altitude with a mean value of 105 km. It ranges from 1.0x1010 to 14.5x1011 cm-3, with a mean value of 2.2x1011 cm-3. The zonally averaged peak altitude appears to be constant while its amplitude decreases with latitude. Another approach uses the O2 volume emission rates obtained with an Abel inversion of the O2 limb profiles. In-deed, it is then possible to vertically integrate these profiles to simulate nadir observations. The resulting map gives values between 0 and 2.8 MR (with a mean value of 0.6 MR) in the north hemisphere. A statistical map created with actual nadir observations shows intensities ranging from 0 to 2.1 MR, with a mean of 0.5 MR in the south hemisphere. A combination of the two types of observations could cover Venus entire nightside. Statistical mapping of the OH Meinel emission has also been performed using limb profiles. A strong correlation with the O2 emission is revealed. The average altitude of the emission peak is ~95.3 km for the OH(1-0) band and the average intensity is 0.4 MR [5]. [less ▲]

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See detailLatitudinal and local time distribution of the O2 infrared nightglow and O density in the lower thermosphere
Soret, Lauriane ULg; Gérard, Jean-Claude ULg; Saglam, Adem et al

Poster (2009, May)

Atomic oxygen is the major component in the Earth’s upper thermosphere. The O density reaches a maximum of about 3x1010 cm-3 near 100 km. The 1.27 µm emission of the IR Atmospheric bands generated by ... [more ▼]

Atomic oxygen is the major component in the Earth’s upper thermosphere. The O density reaches a maximum of about 3x1010 cm-3 near 100 km. The 1.27 µm emission of the IR Atmospheric bands generated by recombination of O atoms has been observed in the nightglow. On the night side, the O2 airglow peaks at ~95 km with a value of ~10 MR. <br />Atomic oxygen is present in the Martian atmos-phere but that the intensities values are below the cur-rent instrument detectability threshold. The Mars at-mosphere oxygen density is highly variable, depending on the altitude, temperature, latitude and longitude. <br />On Venus, atomic oxygen has been measured in situ only above 145 km on both the day and the night sides. Limb observations obtained with the Venus In-frared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express show that the O2 infrared nightglow peaks at ~97 km, with a mean intensity val-ue of about 1,3 MR [1, 2]. Yet, the distribution is largely inhomogeneous, with an enhanced region of ~3 MR statistically located near the midnight meridian at low latitude [3]. The oxygen density can be mapped using the O2 airglow and CO2 density vertical distribu-tions [4]. The O2 volume emission rates are obtained with an Abel inversion of the O2 limb profiles and CO2 vertical distributions are taken from the Venus International Reference Atmosphere (VIRA) model. The results show that the O density peak is located between 93 and 105 km (with a mean value of 104 km) and ranges from 2.8x1010 to 8.5x1011 cm-3 (with a mean value of 2.2x1011 cm-3). No correlations between the peak altitude and the latitude or the peak altitude and the local time are observed. However, the O density decreases and its variability increases while moving away from the antisolar point. [less ▲]

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See detailAtomic oxygen distribution in the Venus mesosphere from observations of O[SUB]2[/SUB] infrared airglow by VIRTIS-Venus Express
Gérard, Jean-Claude ULg; Saglam, Adem ULg; Piccioni, Giuseppe et al

in Icarus (2009), 199

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the Deltag1-->Sigmag3 infrared atmospheric ... [more ▼]

This VIRTIS instrument on board Venus Express has collected spectrally resolved images of the Venus nightside limb that show the presence of the (0,0) band of the Deltag1-->Sigmag3 infrared atmospheric system of O[SUB]2[/SUB] at 1.27 mum. The emission is produced by three-body recombination of oxygen atoms created by photodissociation of CO[SUB]2[/SUB] on the dayside. It is consistently bright so that emission limb profiles can be extracted from the images. The vertical distribution of O[SUB]2[/SUB](Deltag1) may be derived following Abel inversion of the radiance limb profiles. Assuming photochemical equilibrium, it is combined with the CO[SUB]2[/SUB] vertical distribution to determine the atomic oxygen density. The uncertainties on the O density caused by the Abel inversion reach a few percent at the peak, increasing to about 50% near 120 km. We first analyze a case when the CO[SUB]2[/SUB] density was derived from a stellar occultation observed with the SPICAV spectrometer simultaneously with an image of the O[SUB]2[/SUB] limb airglow. In other cases, an average CO[SUB]2[/SUB] profile deduced from a series of ultraviolet stellar occultations is used to derive the O profile, leading to uncertainties on the O density less than 30%. It is found that the maximum O density is generally located between 94 and 115 km with a mean value of 104 km. It ranges from less than 1×10[SUP][/SUP] to about 5×10[SUP][/SUP] cm[SUP][/SUP] with a global mean of 2.2×10[SUP][/SUP] cm[SUP][/SUP]. These values are in reasonable agreement with the VIRA midnight oxygen profile. The vertical O distribution is generally in good agreement with the oxygen profile calculated with a one-dimensional chemical-diffusive model. No statistical latitudinal dependence of the altitude of the oxygen peak is observed, but the maximum O density tends to decrease with increasing northern latitudes. The latitudinal distribution at a given time exhibits large variations in the O density profile and its vertical structure. The vertical oxygen distribution frequently shows multiple peaks possibly caused by waves or variations in the structure of turbulent transport. It is concluded that the O[SUB]2[/SUB] infrared night airglow is a powerful tool to map the distribution of atomic oxygen in the mesosphere between 90 and 115 km and improve future Venus reference atmosphere models. [less ▲]

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See detailVenus night airglow and implications for thermospheric composition and dynamics
Gérard, Jean-Claude ULg; Saglam, Adem ULg; Cox, Cédric ULg et al

Conference (2008)

Spatially resolved spectra of the NO delta and gamma ultraviolet bands have been obtained from 80 to 130 km on the Venus night side with the SPICAV instrument on board Venus Express. This NO airglow ... [more ▼]

Spatially resolved spectra of the NO delta and gamma ultraviolet bands have been obtained from 80 to 130 km on the Venus night side with the SPICAV instrument on board Venus Express. This NO airglow emission results from radiative recombination of oxygen and nitrogen atoms created on the dayside and transported by the subsolar to antisolar global circulation. Spectral images of the O2 (1 â delta g ) at 1.27 µm have also been made with the VIRTIS-M instrument both at nadir and at the limb. The O2 (1 â g ) emission is produced by three-body recombination of O atoms giving rise to an airglow layer near 96 km. The brightness of both emissions changes by over an order of magnitude. They also show variations in the altitude of the peak emission, with larger variability of the NO airglow. The characteristics of both airglows and their implications on global circulation and vertical transport on the nightside will be discussed. Concurrent observations of both limb airglows will be described. It will be shown that limb observations of the vertical and latitudinal distribution of the 1.27 µm emission make it possible to remotely determine the density of atomic oxygen in the upper mesosphere and improve current atmospheric models. One-dimensional models of the O and N distributions will be presented and global properties of the 1-D parameterization of turbulent transport will be discussed. [less ▲]

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See detailVenus Express observations of the Venus O2 and NO nightglow: distribution and constraints on vertical transport
Gérard, Jean-Claude ULg; Cox, Cédric ULg; Saglam, Adem ULg et al

Conference (2007, April)

Observations have been carried out in the infrared with VIRTIS and the ultraviolet with SPICAV to measure the distribution of the O2 (1 g) nightglow emission at 1.27 μm and the nitric oxide gamma and ... [more ▼]

Observations have been carried out in the infrared with VIRTIS and the ultraviolet with SPICAV to measure the distribution of the O2 (1 g) nightglow emission at 1.27 μm and the nitric oxide gamma and delta bands between 190 and 300 nm. These observations were collected in the tangent limb mode, which maximizes the time period spent by the line of sight through the airglow layer. The O2 (1 g) emission is excited by three-body recombination of O atoms produced on the day side and carried by the general thermospheric circulation to the night side. It is very variable in brightness and has a peak located between 95 and 100 km. The NO airglow is produced by radiative recombination of O atoms with N(4S) resulting from N2 photodissociation and reaches a maximum near 110 km.We combine the altitude and brightness information from the two emissions with simulations of a chemical diffusive model to determine the values of the vertical fluxes of O and N atoms and the strength of the eddy mixing which carries both types of atoms from above the turbopause into the recombination layer.We find that O fluxes on the order of a few 1012 atoms/cm2 s and N fluxes about 1010 atoms/cm2 s can reproduce the observations. The variability of the airglow emissions and the altitude-brightness relation will also be discussed and compared with model predictions. [less ▲]

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