Nightglow investigation around 1.27 µm with VIRTIS/Venus-Express

In this study we report about the investigation of the spectral region 1.22-1.32 µm of the night side of Venus, observed by the VIRTIS instrument (Visible and InfraRed Spectral Imaging Spectrometer) on board the Venus Express spacecraft. This spectral region is characterized by the presence of the extensively studied (a1Δg – X3Σg-) (0,0) O2 nightglow band, the most intense emission observed on the night side of Venus. However, the comparison between data and synthetic spectra from the (0,0) band only, lacks a good match at wavelengths longer than 1.27 μm, especially in the region around 1.28-1.29 μm. The effects of temperature, CO2 and the (8,5) OH emission at 1.28 µm were investigated as a possible cause to the spectral disagreement, but they all are not enough to explain the observed difference. Instrumental effects were also excluded as possible cause of the mismatch. We found that the inclusion of the (1,1) band O2 emission in the synthetic spectra, originating from the upper vibrational level ν=1 of the (a1Δg – X3Σg-) transition centered at 1.28 μm, in addition to the (0,0), significantly improves the agreement between simulated spectra and observed data in the region at 1.28-1.29 μm. A synthetic spectrum, including the (0,0) and the (1,1) is produced and compared to 4 observed VIRTIS spectra, as an example. From the analyzed data, it results that the (1,1) band with an intensity ranging from the 8 to 15% of the (0,0) band is required to best reproduce the observed VIRTIS spectra. This corresponds to a (1,1) band intensity equal to 3.1-5.8 MR, in limb view, in agreement with the upper limit set by Connes et al., (1979), on their ground-based observations of the oxygen nightglow of Venus.