Time variations of O2(a1Δ) nightglow spots on the Venus nightside and dynamics of the upper mesosphere

Abstract

The dynamical regime of the Venus upper atmosphere is mainly decomposed into three regions. The first one, located below 65km of altitude is governed by the retrograde superrotational zonal (RSZ) circulation. The second region above 130km is dominated by the subsolar to antisolar (SS–AS) circulation. The dynamics of the transition region in between are still not fully understood. However, the O2(a1Δ) nightglow emission at 1.27μm, whose emitting layer is located at ∼96km, can be used as a tracer of the dynamics in this transition region and the imaging spectrometer VIRTIS-M on board Venus Express, orbiting Venus since April 2006, acquired a large amount of nadir observations at this wavelength.Several previous studies showed that the O2(a1Δ) nightglow emission is statistically located near the antisolar point. In this study, individual VIRTIS-M nadir observations have been analyzed to investigate the variability of the phenomenon. Bright patches of 1.27μm airglow have been extracted from every observation. It appears that the location of the bright patch is highly variable, even though the brightest patches occur near the antisolar point.Nadir observations have also been divided into time series, allowing generating animations to follow the intensity and the displacement of bright patches over time. Apparent wind velocities and characteristic decay/rise times and have been deduced from these time series. The speed of the displacements varies from ∼0 up to 213ms−1, with a mean value of 54ms−1. Owing to the high variability of the direction of the displacements both in the short and the long terms, no clear trend of a global motion at ∼96km can be deduced from these observations. The mean decay time is ∼750min while the mean rise time is ∼1550min. The decay time can be explained as a combination of radiative decay and atomic oxygen transport.