The distributions of the OH Meinel and [formula omitted] nightglow emissions in the Venus mesosphere based on VIRTIS observations

Abstract

O 2 ( a 1 Δ ) and recently discovered OH Meinel nightglow emissions have been observed at the limb with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS-M) instrument on board the Venus Express satellite. Hydroxyl bands belonging to Δ v = 1 sequence between 2.60 and 3.14 μ m and to Δ v = 2 sequence at 1.40 – 1.46 μ m have been unambiguously identified. In this study, we analyze the statistical distribution of the Δ v = 1 OH Meinel band sequence and the a 1 Δ g - X 3 Σ (0–0) band of the O 2 Infrared Atmospheric bands at 1.27 μm. We also present an analysis of the correlation between the two emissions. From a statistical point of view, we find that the limb intensity of both emissions reach their maximum value near the antisolar point, while they are significantly dimmer in the vicinity of the terminator. The average altitude of the limb emissions peaks are 95.3 ± 3 km and 96 ± 2.7 km, respectively for the OH Δ v = 1 sequence and O 2 ( a 1 Δ ) emissions. The average intensities are 0.41 ± 0.37 MR and 28 ± 22 MR, respectively, corresponding to a mean ratio of about 70. The altitude of the OH nightglow layer is closely related to that of the O 2 ( a 1 Δ ) emission and some level of co-variation of the maximum intensity along the line of sight is observed. It is suggested that the global subsolar to antisolar circulation plays a key in the control of both airglows by carrying oxygen atoms from the day to the night side of the planet. The O atoms recombine to produce O 2 ( a 1 Δ ) molecules and they also act as precursors of ozone whose reaction with H produces excited hydroxyl.