Dayside temperatures in the Venus upper atmosphere from Venus Express/VIRTIS nadir measurements at 4.3 μm

Javier Peralta (1, 2) , Miguel A. López-Valverde (2) , Gabriella Gilli (3) , Arianna Piccialli (4, 5)

1 ISAS - Institute of Space and Astronautical Science
2 IAA - Instituto de Astrofísica de Andalucía
3 LMD - Laboratoire de Météorologie Dynamique (UMR 8539)
4 LESIA - Laboratoire d'études spatiales et d'instrumentation en astrophysique
5 PLANETO - LATMOS

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 CO₂ 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.

Mots clés: Radiative transfer; Radiation mechanisms; Molecular processes; Planets; Spectrographs; Data analysis; Satellites

Domaines: Astrophysique [astro-ph]

Javier Peralta, Miguel A. López-Valverde, Gabriella Gilli, Arianna Piccialli. Dayside temperatures in the Venus upper atmosphere from Venus Express/VIRTIS nadir measurements at 4.3 μm. Astronomy and Astrophysics - A&A, 2016, 585, pp.A53. ⟨10.1051/0004-6361/201527191⟩. ⟨insu-01398155⟩