Time evolution of the O2 IR Atmospheric nightglow: VIRTIS observations and comparison with a 2-D model

The O2 (a1∆) emission at 1.27 μm results from three-body recombination of O atoms produced on the day side and transported to the night side by the global solar to antisolar circulation. It is variable in brightness and shows a peak generally located between 95 and 100 km. In this study, we present individual nadir images of the O2 (a1∆) nightside airglow emission [1,2] obtained with VIRTIS IR on board Venus Express [3]. A total of 460 VIRTIS images lasting several hours are used to determine the spatial and temporal variations of regions of enhanced excited O2 (a1∆) density. The intensity may either increase or decrease during the observation period. Intensity drop is expected as a result of three competing processes: (1) the radiative lifetime of the O2 (a1∆) metastable state of 75 min, (2) the effective lifetime of oxygen atoms versus chemical recombination and (3) changes in the localdownward flux of O atoms carried by dynamical processes (diffusion, advection). From the intensity variations, we determine the characteristic rate of change of the bright airglow spots. Their e-folding times have been calculated. The mean decay time is in good agreement with the previous study by [4] who calculated O and CO2density profiles using VIRTIS and SPICAV observations. Indeed, if quenching of O2 (a1∆) is neglected: d O[]dt= −k O[ ]2CO2[ ]= −1τO[ ]⇒τ=1k O[ ]CO2[ ]where k=3.1x10-32 cm6 s-1, [CO2]=7.2x1014 cm-3 and [O]=(2/0.5)/2x2x1011=4x1011 cm-3 at 97 km. In this case, we find a value of τ~1900 min, which is in very good agreement with the value deduced from observations.We compare these results with those of a two-dimensional model of the oxygen chemistry and dynamics [5]. In this model, a time and space varying flux of O atoms is prescribed through the upper boundary and a free boundary condition is used on the lateral boundary. The spot brightness evolutionhas been simulated with a 2-D model by imposing a localized increase of the oxygen flux at the upper boundary. The evolution of the O2(a1∆) volume emission rate is represented in Figure 2. The horizontal motion of the spots is used to determine the wind speed in the 95-100 km region. Values are in the range of 25 to 150 m s-1, in good agreement with an earlier study. A statistical study of the location of the brightest region of emission in each picture frame indicates that their locations on the Venus nightside are very variable. However, a concentration of peak detection is observed at low latitude around the midnight, inagreement with earlier results based on a statistical map.