Abstract
In this work, we analysed nadir observations of atmospheric infrared emissions carried out by VIRTIS, a high-resolution spectrometer on board the European spacecraft Venus Express. We focused on the ro-vibrational band of CO 2 at 4.3 µm on the dayside, whose fluorescence originates in the Venus upper mesosphere and above. This is the first time that a systematic sounding of these non-local thermodynamic equilibrium (NLTE) emissions has been carried out in Venus using this geometry. As many as 143,218 spectra have been analysed on the dayside during the period 14/05/2006 to 14/09/2009. We designed an inversion method to obtain the atmospheric temperature from these non-thermal observations, including a NLTE line-by-line forward model and a pre-computed set of spectra for a set of thermal structures and illumination conditions. Our measurements sound a broad region of the upper mesosphere and lower thermosphere of Venus ranging from 10 −2 –10 −5 mb (which in the Venus International Reference Atmosphere, VIRA, is approximately 100–150 km during the daytime) and show a maximum around 195 ± 10 K in the subsolar region, decreasing with latitude and local time towards the terminator. This is in qualitative agreement with predictions by a Venus Thermospheric General Circulation Model (VTGCM) after a proper averaging of altitudes for meaningful comparisons, although our temperatures are colder than the model by about 25 K throughout. We estimate a thermal gradient of about 35 K between the subsolar and antisolar points when comparing our data with nightside temperatures measured at similar altitudes by SPICAV, another instrument on Venus Express (VEx). Our data show a stable temperature structure through five years of measurements, but we also found episodes of strong heating/cooling to occur in the subsolar region of less than two days.
Highlights
The upper mesosphere and lower thermosphere of Venus are defined in this work as the altitude ranges 90−120 km and 120−150 km above the surface, respectively
The transition from the retrograde superrotating zonal (RSZ) flow to the subsolar-to-antisolar (SS-AS) circulation occurs in this zone (Bougher et al 2006); the CO2 heating and cooling in the IR dominate the radiative balance up to about 120−130 km (Roldán et al 2000), and the absorption/scattering processes of the Venus high-altitude haze play an important role at these altitudes (Wilquet et al 2009)
Since the high thermal contrasts reported between day- and nightside must be responsible for the pressure gradients driving the SS-AS circulation (Bougher et al 2006), The table with numerical data and averaged temperatures displayed in Fig. 7A provided as a CSV data file is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/585/A53 knowledge of the horizontal distribution of neutral gas temperature is essential to understand the general circulation of the Venus atmosphere, to improve numerical models, and to perform aeronomy calculations
Summary
The upper mesosphere and lower thermosphere of Venus (or jointly, UMLT) are defined in this work as the altitude ranges 90−120 km and 120−150 km above the surface, respectively. CO2 is the most abundant molecule in the atmosphere of Venus and its infrared emissions are known to be very strong during the daytime because of solar fluorescence, which is the case in NLTE situations These are important in the upper atmosphere, where pressure and the frequency of molecular collisions are so low that radiation dominates the states’ populations (Dickinson 1972; López-Puertas & Taylor 2001). We carry out systematic analysis and retrieval of these NLTE emissions in this work, similar to a recent study using NLTE limb observations (Gilli et al 2009, 2015): first, the VIRTIS-H spectra are examined and compared with the results from our NLTE model for Venus, and, second, a NLTE retrieval scheme is designed and applied to these data to infer the global horizontal distribution of daytime temperature in the UMLT of Venus with unprecedented detail. Sets of spectra are shown for different intervals of SZA fixing the EA (A), and different intervals of EA fixing the SZA (B)
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