[1] SOIR is a high‐resolution spectrometer flying on board the ESA Venus Express mission. It performs solar occultations of the Venus high atmosphere, and so defines unique vertical profiles of many of the Venus key species. In this paper, we focus on the Venus main constituent, carbon dioxide. We explain how the temperature, the total density, and the total pressure are derived from the observed CO 2 density vertical profiles. A striking permanent temperature minimum at 125 km is observed. The data set is processed in order to obtain a Venus Atmosphere from SOIR measurements at the Terminator (VAST) compilation for different latitude regions and extending from 70 up to 170 km in altitude. The results are compared to many literature results obtained from ground‐based observations, previous missions, and the Venus Express mission. The homopause altitude is also determined.

2. Instrument Description and Observation Geometry [5] The SOIR instrument has been extensively described in Bertaux et al. [2007a], Nevejans et al. [2006], and Mahieux et al. [2008, 2009, 2010]. We will summarize here only the most important SOIR characteristics. [6] SOIR is one of the three channels of the SPICAV/SOIR instrument. It is an infrared spectrometer, using an echelle grating as the diffracting element. The accessible wave number range covers the 2200 to 4400 cm−1 region, and is divided into 94 useful diffracting orders, or simply orders, from 101 to 194. The order selection is performed using an acousto‐optic tunable filter (AOTF), which allows us muchto select and transmit only a small wave number range diffracted by the echelle grating. The resolution of SOIR varies from order to order, with a value of about 0.11 cm−1 in order 101 to 0.21 cm−1 in order 194. The SOIR detector is composed of 320 pixels in its spectral direction. The spectral width of a pixel varies from 0.06 to 0.12 cm−1 and the free spectral range (FSR) has a constant value of 22.4 cm−1. The SOIR useful detector pixels are combined into two groups in the spatial direction called bins. Two simultaneous measurements are thus obtained at two slightly different altitudes corresponding to the 2 bins on the detector. The width of the AOTF bandpass transfer function is ∼24 cm−1, while the width of an order varies between 19.3 cm−1 to 37.1 cm−1 with ascending order, which causes an order overlapping on the detector. Moreover, the AOTF transfer function is not zero in the adjacent orders [Mahieux et al., 2009]. To correctly simulate the SOIR measurement, more than one diffraction order have to be taken into account [Mahieux et al., 2010]. Five contiguous diffraction orders are usually simulated, i.e., the scanned one and two adjacent orders on each side. [7] SOIR performs solar occultation observations of the Venus atmosphere from the VEX spacecraft, which is in a polar orbit with its periapsis located above the North Pole. The vertical size of the instantaneously scanned atmosphere at the limb tangent point varies from a few hundreds of meters for the Northern measurements to tens of kilometers for the Southern measurements. The altitude range probed by SOIR, i.e., where measurements are scientifically meaningful, varies from 70 km up to 170 km. The lower boundary corresponds to total absorption of sunlight by Venus' clouds, and the upper boundary to the detection of the strongest CO 2 band in the selected SOIR wave number range. During an occultation, four different diffraction orders are measured quasi‐simultaneously – four sequentially within 1 s, each one lasting 160 ms at maximum. It allows us to study either the same species at different ranges of altitudes, as it will be the case here for CO 2 , or different species to obtain volume mixing ratios [Bertaux et al., 2007b; Fedorova et al., 2008; Vandaele et al., 2008]. The occultations are grouped in occultation seasons (OS), which are time periods when the solar occultations take place relative to the VEX spacecraft orbital configuration. These OS periods occur roughly every three months for one month. The definition of the OS is a parameter linked to the VEX spacecraft orbital characteristics, and has no sense in terms of Venus climatology. [8] The spectroscopic parameters are obtained from the Hitran 2008 database [Rothman et al., 2009], with corrected values for the pressure broadening coefficients and shifts, to take into account that the atmosphere of Venus is mainly CO 2 instead of nitrogen and oxygen [Vandaele et al., 2008]. Carbon dioxide is studied in the range 2400 to 4000 cm−1 (2.5 to 4.16 μm). In this study we essentially concentrate on the vibrational bands of the following five isotopologues 12C16O 2 , 13C16O 2 , 16O12C18O, 16O13C18O and 16O12C17O. The isotopic ratio of the carbon and oxygen atoms is assumed to be the same as on Earth [Bézard et al., 1987; Clancy and Muhleman, 1991]. For 12C16O 2 , the 20011–10002 and 00021–01101 bands (in Hitran notation [Rothman et al., 2009]) are observed in orders 171 (3822 to 3854 cm−1) and 176 to 180 (3933 to 4057 cm−1), respectively. They give information on low altitudes (70 to 90 km). The 21103–00001 band is measured in order 141 (3151 to 3178 cm−1) and the 21102–00001 band in orders 147 to 150 (3285 to 3381 cm−1); they correspond to mid altitudes (80 to 120 km). Finally the 10012–00001 and 10011–00001 bands are measured in the orders 160 to 168 (3576 to 3787 cm−1) and are observed at very high altitudes from 120 to 170 km. The 10012–00001 band of 13C16O 2 is measured in orders 155 and 156 (3464 to 3516 cm−1); it is measured at high altitude from 100 to 140 km. Three bands of 16O12C18O are observed, the 01111–00001 band in order 134 (2995 to 3020 cm−1) at low altitude (from 70 to 90 km), and the 20002–00001 and 20003–00001 bands at midaltitude (90 to 120 km) measured in orders 112 (2503 to 2525 cm−1), and 117 and 118 (2615 to 2637 cm−1) respectively. The 20001–00001 band of the 16O13C18O isotopologue is observed in order 121 (2704 to 2727 cm−1) at low altitude, from 70 to 90 km.

4. Orbit Selection and Localization [19] Although the SOIR data set now contains 478 observations (up to January 2012) from which 465 can be used to retrieve information on CO 2 , only a subset has been considered in this study to define the Venus Atmosphere from SOIR measurements at the Terminator (VAST). Indeed we only considered the orbits for which CO 2 has been measured at least once at very high altitude, i.e., in which the strong 12C16O 2 absorption band at 3 μm is present. This corresponds to SOIR orders 160 to 166. This band is usually observed from an altitude as high as 165 km. Using this criteria, only 59 observations were considered. The selected measurements are obtained on AM or PM terminator sides for a wide range of latitudes. In terms of time coverage, they were obtained during various occultation seasons (from OS 1 to OS 17) between 2006 and 2011. Figure 6 shows the localization in terms of orbit number, latitude and local solar time of the measurements. The fact that the local solar times displayed in Figure 6 are not equal to 6:00 A.M. or 6:00 P.M. when approaching to the pole is an artifact, and comes from the fact that the notion of local solar time becomes meaningless at these latitudes. This means that the only local solar time information that should be used is the terminator side, either AM or PM. Details of the selected orbits are summarized in Table 1. In the future, VAST developed here will be used as starting condition for the resolution of equation (1). The aim is to gradually incorporate new retrievals as they are obtained to refine the model. Figure 6 Open in figure viewer PowerPoint Localization of the orbit data set considered in the study. (top) The local solar time of the measurement as a function of the latitude. (bottom) The orbit number as a function of the latitude of the measurement. The occultation seasons considered are in gray, and their number is also indicated. [20] Due to the VEX orbit, two successive measurements, which occur on a 24 Earth hour basis, are taken at somewhat different latitudes on the same side of the terminator: there is a time‐latitude gap between successive measurements, such that it is not easy to discriminate between time and latitude variations of the density or temperature profiles. In consequence, latitudinal and short‐term trends should be considered with care, as they are intrinsically linked. [21] Short‐term CO 2 density variations for a given latitude region of the terminator are not considered in the current study, since they are removed by the use of a statistically large enough sample of profiles. Systematic long‐term CO 2 density variations are not observed in the subset studied here. These variations could obviously influence the model, and will be investigated further in the future.

5. Carbon Dioxide Density and Temperature Profiles [22] The carbon dioxide density and temperature profiles are presented in Figure 7 for the whole set. The error bars are not displayed to ensure readability of the figure. Figure 7 Open in figure viewer PowerPoint 2 density profiles and (right) CO 2 temperature profiles of the orbit data set considered in this study. The inset panel gives the measurement latitude and the orbit number. The density profiles are given as a function of the altitude, and the temperature profiles are given as a function of the total pressure, with the altitude given on the right side as an indication. The color is the absolute latitude. High‐latitude measurements are reddish, while equatorial measurements are bluish. The black lines are the density and temperature values of the Keating [ Keating et al., 1980 Zasova et al., 2007 2006 (left) COdensity profiles and (right) COtemperature profiles of the orbit data set considered in this study. The inset panel gives the measurement latitude and the orbit number. The density profiles are given as a function of the altitude, and the temperature profiles are given as a function of the total pressure, with the altitude given on the right side as an indication. The color is the absolute latitude. High‐latitude measurements are reddish, while equatorial measurements are bluish. The black lines are the density and temperature values of the Keating [] (plain line) and Zasova models [], for different latitudes (dashed is 0°, dash‐dotted is 45° and dotted is 90°). [23] The density profiles all show the same shape, with a small yet systematic latitudinal dependency. They present a change of slope – a curvature – in the logarithmic scale around 120–140 km of altitude. The steepness of this gradient may change slightly from one orbit to another, and appears to also depend on latitude. Also, Figure 7 indicates that a global variability of the CO 2 density profiles (within a factor 10) is observed as a function of time and/or latitude. At constant density, the variations are equivalent to 2 scale heights (the scale height H is approximately 3 to 5 km). The general relative error on the density profiles is 1% to 10%, except in the region of the curvature change (120–140 km) where larger errors are observed, 10% to 40%. [24] The temperature profiles are given as a function of total pressure to ensure consistent comparison between the different curves, in other terms to remove the influence of the observed local variations in the CO 2 density. The change of slope in the density profiles observed in Figure 7 corresponds to a temperature minimum in the 10−5 mbar region (120–130 km of altitude), with low temperatures between 60 and 110 K. This minimum is surrounded by two temperature maxima, located in the 5·10−7 mbar region or 130–140 km of altitude range (200–350 K) and the 5·10−3 mbar region or 100–110 km of altitude range (180–250 K). For a given pressure level, the temperature profiles also show variability of the order of 50 K, at all altitudes. At constant temperature, the variations are also equivalent to 2 scale heights (around 3 to 5 km). The error bars for the temperature profiles vary between 2 and 10 K below 120 km and between 10 and 60 K above 120 km. These values are directly linked to the error on the density profiles and to the value of the CO 2 VMR obtained from the Keating and Zasova models, through the hydrostatic equilibrium in equation (1). Larger temperature errors are observed at high altitude due to the lower values of the CO 2 VMR. [25] The CO 2 density profiles and the temperature profiles are compared in Figure 7 to the Keating and Zasova models. In terms of density, the agreement is good at altitudes lower than 120 km, but the model does not reproduce the observed CO 2 density drop measured by the SOIR instrument. This is reflected in the temperature profiles, as the measured cold layer in the 10−5 mbar region is not present in the Keating and Zasova models [Keating et al., 1980; Zasova et al., 2007, 2006]. At higher pressure level (lower altitudes), the agreement between the model and the SOIR temperatures is better, but differences can reach up to 50 K.

8. Conclusions [39] An atmospheric model for the Venus mesosphere and lower thermosphere has been constructed based on SOIR observations. Those correspond to solar occultation probing the altitude range from 70 to 170 km with a high vertical resolution. They cover a wide range of latitudes, but correspond all to the 6:00 A.M. and 6:00 P.M. local times, as all observations are performed at the terminator, either on its morning or its evening side. A subset of 59 observations has been considered, from which information on the CO 2 density and the temperature were derived. VAST (Venus Atmosphere from SOIR measurements at the Terminator) is defined on 5 latitudinal zones and provides CO 2 and total densities and temperature as a function of the altitude. VAST has been compared to data available in the literature, and it was shown that, in particular, the temperature profiles are in good agreement with literature data at the lowest altitudes. A striking and never observed before cold layer is always observed at an altitude of 125 km. [40] VAST will be further refined and improved with other CO 2 measurements, already recorded or obtained during future dedicated SOIR observation campaigns. The first step will be to analyze all observations already acquired allowing the determination of the CO 2 density and temperature, but which were not considered in this study because they did not cover a high enough altitude range. The model will also be further expanded with the inclusion of the vertical profiles of various trace gases observed by SOIR, such as H 2 O and HDO [Fedorova et al., 2008], HCl, HF, CO [Vandaele et al., 2008], SO 2 [Belyaev et al., 2008, 2012].

Acknowledgments [41] Venus Express is a planetary mission from the European Space Agency (ESA). We wish to thank all ESA members who participated in the mission, in particular, H. Svedhem and D. Titov. We thank our collaborators at IASB‐BIRA (Belgium), Latmos (France), and IKI (Russia). We thank CNES, CNRS, Roskosmos, and the Russian Academy of Science. The research program was supported by the Belgian Federal Science Policy Office and the European Space Agency (ESA, PRODEX program, contracts C 90268, 90113, and 17645).

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