Abstract We report results from a study of two consecutive Martian years of imaging observations of nitric oxide ultraviolet nightglow by the Imaging Ultraviolet Spectrograph (IUVS) on the Mars Atmosphere and Volatile Evolution (MAVEN) mission spacecraft. The emission arises from recombination of N and O atoms in Mars' nightside mesosphere. The brightness traces the reaction rate as opposed to the abundance of constituents, revealing where circulation patterns concentrate N and O and enhance recombination. Emissions are brightest around the winter poles, with equatorial regions brightening around the equinoxes. These changes offer clear evidence of circulation patterns transitioning from a single cross‐equatorial cell operating during solstice periods to more symmetric equator‐to‐poles circulation around the equinoxes. Prominent atmospheric tides intensify the emissions at different longitudes, latitude ranges, and seasons. We find a strong eastward‐propagating diurnal tide (DE2) near the equator during the equinoxes, with a remarkably bright spot narrowly confined near (0°, 0°). Wave features at the opposite winter poles are dissimilar, reflecting different circulation patterns at perihelion versus aphelion. LMD‐MGCM simulations agree with the patterns of most observed phenomena, confirming that the model captures the dominant physical processes. At the south winter pole, however, the model fails to match a strong wave‐1 spiral feature. Observed brightnesses exceed model predictions by a factor of 1.9 globally, probably due to an underestimation of the dayside production of N and O atoms. Further study of discrepancies between the model and observations offers opportunities to improve our understanding of chemical and transport processes controlling the emission.
Les emissions de monoxyde d'azote (NO) sont observables sur toutes les planetes telluriques entourees d'une atmosphere. Sur Venus, ces emissions ont ete identifiees en 1979. Elles sont classiquement observables dans l'ultraviolet, entre 180 et 300 nm, mais il est egalement possible de les detecter en infrarouge entre 1,2 et 1,3 μm. L'emission aeronomique du NO est due a un processus de recombinaison radiative se produisant du cote nuit de la planete. L'atmosphere de Venus est composee essentiellement de CO2 et de N2 et cote jour, les radiations solaires cassent ces molecules et liberent des atomes d'azote et d'oxygene. Dans la haute atmosphere, au-dessus de 100 km, les vents zonaux transportent ces atomes vers le cote nocturne ou ils se recombinent pour former du NO et emettent alors un rayonnement ultraviolet. L'emission aeronomique du NO est ainsi un bon traceur de cette circulation sub-solaire/anti-solaire. La mission Venus Express, actuellement en orbite autour de Venus, possede a son bord l'instrument SPICAV, un spectrometre capable d'observer les emissions de NO dans l'ultraviolet. Les travaux de cette these se basent sur les observations SPICAV realisees en mode d'occultation stellaire. Ce jeu de donnees, sur lequel les emissions de NO apparaissent, permet d'agrandir la base de donnees sur cette emission aeronomique et permet une approche complementaire des observations au limbe. Le travail a consiste a etablir deux methodes d'inversion de ces emissions de NO. La premiere methode, appelee modele direct, est une simulation de ce que nous devons observer avec SPICAV en occultation stellaire. La seconde methode, nommee algorithme d'inversion, est une inversion matricielle des donnees. Chaque methode aboutit aux caracteristiques de la couche de NO presente dans l'atmosphere de Venus. Nos resultats permettent de mieux contraindre le contexte dynamique de l'atmosphere venusienne, aux altitudes superieures a 100 km.
Abstract We present results from a NO airglow inversion method based on Venus Express data acquired from 2006 to 2010, during the last solar minimum period. We retrieve an altitude of 114 ± 10 km for the emission peak of the NO layer, with an associated scale height of 20 ± 10 km and an average limb brightness of 59.3 kR with a standard deviation of 63 kR. The inversion method allows for the quantification of the horizontal homogeneity of the NO layer. Images of the SPICAV field of view show a great variability of airglow morphologies, with NO layers that can be horizontally homogenous and continuous over distances exceeding 100 km, as well as sporadic patches of NO on a smaller horizontal scale. Frequent secondary emissions seen at lower tangent altitudes are the signatures of the complex dynamics of the upper Venusian atmosphere.
The SPICAV instrument onboard the Venus Express spacecraft is a multi-channel suite covering the far ultraviolet to the mid-infrared. In this presentation, we will focus on the results obtained by the UV channel during stellar occultations observations. Stellar occultation technique possesses well-known advantages: self-calibration, low sensitivity to instrument aging, simple laws of radiative transfer. In addition, occultation with stars permit to cover a broad range of latitudes at any given season and they provide optimal geometrical registration. Since Venus Express orbit insertion, several hundreds of occultations have been performed by SPICAV, yielding profiles of atmospheric constituents between 80 and 140 km. In the SPICAV UV range, CO2 possesses a broad signature shortward of 200 nm which allows one to retrieve CO2 concentration and subsequently to deduce atmospheric pressure and temperature profiles in the upper mesosphere and in the thermosphere. The Venusian thermosphere shows excessive variability, with the equivalent of more than three scale heights change in density in less than a few days. No other spectral signature besides that of CO2 and haze particles was expected to appear in SPICAV ultraviolet spectra at this altitude range but a consistent search was undertaken, revealing the presence of ozone at 100 km (<108 cm-3) and of sulfur dioxide above 90 km at a concentration of 0.1 to 1 ppm.
