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See detailUnderstanding the variability of nightside temperatures, NO UV and O2 IR nightglow emissions in the Venus upper atmosphere
Brecht, A. S.; Bougher, S. W.; Gérard, Jean-Claude ULg et al

in Journal of Geophysical Research. Planets (2011), 116

Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O[SUB]2[/SUB] IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive ... [more ▼]

Venus Express (VEX) has been monitoring key nightglow emissions and thermal features (O[SUB]2[/SUB] IR nightglow, NO UV nightglow, and nightside temperatures) which contribute to a comprehensive understanding of the global dynamics and circulation patterns above ˜90 km. The nightglow emissions serve as effective tracers of Venus' middle and upper atmosphere global wind system due to their variable peak brightness and horizontal distributions. A statistical map has been created utilizing O[SUB]2[/SUB] IR nightglow VEX observations, and a statistical map for NO UV is being developed. A nightside warm layer near 100 km has been observed by VEX and ground-based observations. The National Center for Atmospheric Research (NCAR) Venus Thermospheric General Circulation Model (VTGCM) has been updated and revised in order to address these key VEX observations and to provide diagnostic interpretation. The VTGCM is first used to capture the statistically averaged mean state of these three key observations. This correspondence implies a weak retrograde superrotating zonal flow (RSZ) from ˜80 km to 110 km and above 110 km the emergence of modest RSZ winds approaching 60 m s[SUP]-1[/SUP] above ˜130 km. Subsequently, VTGCM sensitivity tests are performed using two tuneable parameters (the nightside eddy diffusion coefficient and the wave drag term) to examine corresponding variability within the VTGCM. These tests identified a possible mechanism for the observed noncorrelation of the O[SUB]2[/SUB] and NO emissions. The dynamical explanation requires the nightglow layers to be at least ˜15 km apart and the retrograde zonal wind to increase dramatically over 110 to 130 km. [less ▲]

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See detailMars ultraviolet dayglow variability: SPICAM observations and comparison with airglow model
Cox, Cédric ULg; Gérard, Jean-Claude ULg; Hubert, Benoît ULg et al

in Journal of Geophysical Research. Planets (2010), 115

Dayglow ultraviolet emissions of the CO Cameron bands and the CO[SUB]2[/SUB][SUP]+[/SUP] doublet in the Martian atmosphere have been observed with the Spectroscopy for Investigation of Characteristics of ... [more ▼]

Dayglow ultraviolet emissions of the CO Cameron bands and the CO[SUB]2[/SUB][SUP]+[/SUP] doublet in the Martian atmosphere have been observed with the Spectroscopy for Investigation of Characteristics of the Atmosphere of Mars on board the Mars Express spacecraft. A large amount of limb profiles has been obtained which makes it possible to analyze variability of the brightness as well as of the altitude of the emission peak. Focusing on one specific season (Ls = [90,180] °), we find that the average CO peak brightness is equal to 118 ± 33 kR, with an average peak altitude of 121.1 ± 6.5 km. Similarly, the CO[SUB]2[/SUB][SUP]+[/SUP] emission shows a mean brightness of 21.6 ± 7.2 kR with a peak located at 119.1 ± 7.0 km. We show that the brightness intensity of the airglows is mainly controlled by the solar zenith angle and by solar activity. Moreover, during Martian summer of year 2005, an increase of the airglow peak altitude has been observed between Ls = 120° and 180°. We demonstrate that this variation is due to a change in the thermospheric local CO[SUB]2[/SUB] density, in agreement with observations performed by stellar occultation. Using a Monte Carlo one-dimensional model, we also show that the main features of the emission profiles can be reproduced for the considered set of data. However, we find it necessary to scale the calculated intensities by a fixed factor. [less ▲]

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See detailConcurrent observations of the ultraviolet nitric oxide and infrared O[SUB]2[/SUB] nightglow emissions with Venus Express
Gérard, Jean-Claude ULg; Cox, Cédric ULg; Soret, Lauriane ULg et al

in Journal of Geophysical Research. Planets (2009), 114

Two prominent features of the Venus nightside airglow are the nitric oxide delta and gamma bands produced by radiative association of O and N atoms in the lower thermosphere and the O[SUB]2[/SUB] infrared ... [more ▼]

Two prominent features of the Venus nightside airglow are the nitric oxide delta and gamma bands produced by radiative association of O and N atoms in the lower thermosphere and the O[SUB]2[/SUB] infrared emission generated by three-body recombination of oxygen atoms in the upper mesosphere. The O[SUB]2[/SUB] airglow has been observed from the ground, during the Cassini flyby, and with VIRTIS on board Venus Express. It now appears that the global structure of the two emissions shows some similarities, but the statistical location of the region of strongest emission is not coincident. The Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrograph has collected a large number of spectra of the Venus nitric oxide nightside airglow. Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) images have been obtained at the limb and in the nadir-viewing mode and have provided new information on the horizontal and vertical distribution of the emission. We present the first concurrent observations of the two emissions observed with Venus Express. We show that nadir observations generally indicate a low degree of correlation between the two emissions observed quasi-simultaneously at a common location. A statistical study of limb profiles indicates that the altitude and the brightness of the two airglow layers generally do not covary. We suggest that this lack of correlation is explained by the presence of strong horizontal winds in the mesosphere-thermosphere transition region. They carry the downflowing atoms over large distances in such a way that regions of enhanced NO emission generally do not coincide with zones of bright O[SUB]2[/SUB] airglow. [less ▲]

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See detailLimb observations of the ultraviolet nitric oxide nightglow with SPICAV on board Venus Express
Gérard, Jean-Claude ULg; Cox, Cédric ULg; Saglam, Adem ULg et al

in Journal of Geophysical Research. Planets (2008), 113

Limb observations of the spectrum of nightglow emission in the delta (190-240 nm) and gamma (225-270 nm) bands of nitric oxide have been made with the Spectroscopy for Investigation of Characteristics of ... [more ▼]

Limb observations of the spectrum of nightglow emission in the delta (190-240 nm) and gamma (225-270 nm) bands of nitric oxide have been made with the Spectroscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) ultraviolet spectrometer on board Venus Express. These emissions arise from radiative recombination between O([SUP]3[/SUP]P) and N([SUP]4[/SUP]S) atoms that are produced on the dayside and recombine to form excited NO molecules on the nightside. No other emission feature has been identified. The mean altitude of the emission layer is located at 113 km, but it varies between 95 and 132 km. The mean brightness of the total NO emission at the limb is 32 kR, but it is highly variable with limb intensities as large as 440 kR observed at low latitude and values below 5 kR seen at northern midlatitudes. No systematic dependence of the brightness with latitude is observed, but the mean altitude of the emission maximum statistically drops with increasing latitude between 6° and 72°N. Typical observed limb profiles are compared with simulations based on a one-dimensional chemical-diffusive atmospheric model. From model fits to observed profiles, we find that the downward flux of N atoms at 130 km typically varies between 1 × 10[SUP]8[/SUP] to 4 × 10[SUP]9[/SUP] atoms cm[SUP]-2[/SUP] s[SUP]-1[/SUP]. Comparisons of observed airglow topside scale heights with modeled profiles smoothed by the instrumental field of view indicate that the observations are compatible with a downward flow of O and N atoms by molecular and turbulent transport above the peak of emission. The K coefficient deduced from comparisons to limb profiles is less than that determined from the observations made with the Pioneer Venus UV spectrometer at low latitude during periods of high solar activity. [less ▲]

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See detailMorphology and dynamics of Venus oxygen airglow from Venus Express/Visible and Infrared Thermal Imaging Spectrometer observations
Hueso, R.; Sánchez-Lavega, A.; Piccioni, G. et al

in Journal of Geophysical Research. Planets (2008), 113

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O[SUB]2[/SUB] ([SUP ... [more ▼]

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O[SUB]2[/SUB] ([SUP]1[/SUP]Delta) infrared nightglow on Venus at 1.27 mum. The nightglow distribution is highly inhomogeneous with the regions of brightest emission generally located at low latitudes near the midnight meridian. Unexpectedly some orbits show also intense airglow activity over the south polar region. The spatially resolved airglow is spectacularly variable not only in its morphology and intensity but also in the apparent motions of the airglow small- and large-scale structures. Visual tracking of the bright features allowed to obtain mean zonal and meridional motions related to the subsolar to antisolar circulation. The zonal velocity is dominated by an intense prograde jet (contrary to the retrograde planetary rotation) from dawn to midnight extending up to 22 hours in local time with lower velocities and reversed sign from dusk. Typical zonal velocities range between +60 (prograde) to -50 (retrograde) m/s, whereas most meridional velocities range from -20 (poleward) to +100 m/s (equatorward) with an average meridional circulation of +20 m/s toward low latitudes. The brightest small-scale (~100 km) features appear correlated with locations of apparent convergence which may be a signature of compression and downwelling, whereas this is not evident for the large-scale structures suggesting slow subsidence over large areas mixed with horizontal motions. We argue that part of the tracked motions are representative of real motions at the mesosphere over an altitude range of 95-107 km. [less ▲]

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See detailMonte Carlo model of electron transport for the calculation of Mars dayglow emissions
Shematovich, V. I.; Bisikalo, D. V.; Gérard, Jean-Claude ULg et al

in Journal of Geophysical Research. Planets (2008), 113

A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on ... [more ▼]

A model of the photoelectron collision-induced component of the Mars dayglow using recent cross sections and solar flux is described. The calculation of the photoelectron source of excitation is based on a stochastic solution of the Boltzmann equation using the direct simulation Monte Carlo method. The neutral atmosphere is taken from outputs of a global circulation model, and recent inelastic collision cross sections are adopted. The calculated vertical profiles of the CO Cameron bands and CO[SUB]2[/SUB] [SUP]+[/SUP] doublet emissions integrated along the line of sight compare well with the Spectroscopy for the Investigation of the Characteristics of the Atmosphere of Mars (SPICAM) limb profiles observed with the SPICAM spectrograph on board Mars Express made at Ls = 166° during the summer season at northern midlatitudes. The comparison shows agreement to within the uncertainties of the excitation cross sections. Seasonal changes in the brightness and the altitude of the emission peaks are predicted with intensity variations in the range 15-20%. [less ▲]

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