Abstract :
[en] Atomic oxygen has been measured in situ only above 145 km on both the day and the night sides of Venus. Limb observations obtained with the Venus Infrared Thermal Imaging Spectrometer (VIRTIS) on board Venus Express show that the O2 infrared nightglow peaks at ~97 km [1, 2], with a mean intensity value of about 1 MR. Yet, the distribution is largely inhomogeneous, with an enhanced region of ~3 MR statistically located near the midnight meridian at low latitude [3].
The oxygen density can be mapped using the O2 airglow and CO2 density vertical distributions [4]. The O2 vol-ume emission rates are obtained with an Abel inversion of the O2 limb profiles using CO2 vertical distributions taken from the Venus International Reference Atmosphere (VIRA) model. The results show that the O density peak varies in altitude with a mean value of 105 km. It ranges from 1.0x1010 to 14.5x1011 cm-3, with a mean value of 2.2x1011 cm-3. The zonally averaged peak altitude appears to be constant while its amplitude decreases with latitude.
Another approach uses the O2 volume emission rates obtained with an Abel inversion of the O2 limb profiles. In-deed, it is then possible to vertically integrate these profiles to simulate nadir observations. The resulting map gives values between 0 and 2.8 MR (with a mean value of 0.6 MR) in the north hemisphere. A statistical map created with actual nadir observations shows intensities ranging from 0 to 2.1 MR, with a mean of 0.5 MR in the south hemisphere. A combination of the two types of observations could cover Venus entire nightside.
Statistical mapping of the OH Meinel emission has also been performed using limb profiles. A strong correlation with the O2 emission is revealed. The average altitude of the emission peak is ~95.3 km for the OH(1-0) band and the average intensity is 0.4 MR [5].
Commentary :
References:
[1] Drossart P., et al. (2007), A dynamic upper atmosphere of Venus as revealed by VIRTIS on Venus Express, Nature, 450, 641-645, doi:10.1038/nature 06140
[2] Piccioni G., Zasova L., Migliorini A., Drossart P., Shakun A., García Muñoz A., Mills F. P., Cardesin-Moinelo A., Near-IR oxygen nightglow observed by VIRTIS in the Venus upper atmosphere, J. Geophys. Res., Vol. 114, No. null, E00B38
[3] Gérard J.-C., Saglam A., Piccioni G., Drossart P., Cox C., Erard S., Hueso R. and Sánchez-Lavega A. (2008), Distribution of the O2 infrared nightglow observed with VIRTIS on board Venus Express, Geophysical Research Letters, 35, 2, L02207, doi:10.1029/ 2007GL032021
[4] Gérard J.-C., Saglam A., Piccioni G., Drossart P., Montmessin F. and Bertaux J.-L. (2009), Atomic oxygen distribution in the Venus mesosphere from observations of O2 infrared airglow by VIRTIS-Venus Express, Icarus, 199, 264–272, doi:10.1016/j.icarus. 2008.09.016
[5] Saglam A., Gérard J.-C., Soret L., Piccioni G., Drossart P., The distributions and correlations of the OH Meinel and O2(1Δ) nightglow emissions in the Venus mesosphere based on VIRTIS observations, submitted
