The OH Venus nightglow spectrum: intensity and vibrational composition from VIRTIS-Venus Express observations

Limb spectra of the OH nightglow emission corresponding to the ∆v=1 and ∆v=2 sequences have been collected with the VIRTIS infrared imaging spectrograph on board Venus Express between April 2006 and October 2008. A detailed statistical analysis shows that the peak intensity and altitude of the two vibrational sequences are significantly correlated, with a mean intensity ratio of the two sequences of 0.38±0.37. The altitude of the maximum of the ∆v=2 emission is located ~1 km lower than ∆v=1. A spectral analysis shows that the Δv=1 sequence is composed at 44.6% by the (1–0) band, 9.3% by the (3–2) band and 7.1% by the (4–3) band. The Δv=2 emission is best fitted if solely including the (2–0) band.

A non-LTE model of OH vibrational population by the O3+H reaction including radiative and collisional relaxation has been used to compare the expected spectral distribution, the altitude of the emission peak and the emission rate under different assumptions on the quenching processes to those observed with VIRTIS. The adopted carbon dioxide, atomic oxygen and ozone densities are based on recent Venus Express remote sensing measurements. We find that the “sudden death” quenching scheme by CO2 produces inadequate spectral distribution between the various bands and insufficient airglow brightness. Instead, the observed spectral distribution and the total emission intensity are reasonably well reproduced with the single quantum jump model, a O density profile peaking at 103.5 km with a maximum value of 1.9×1011 cm−3, a O3 density profile peaking at 5.8×106 cm−3 at 96.5 km and a H density profile close to 108 cm−3 between 90 and 120 km, in agreement with several photochemical models.