The OH Venus nightglow spectrum: Intensity and vibrational composition from VIRTIS—Venus Express observations

Abstract

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 O 3 + 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 2 oxygen and ozone densities are based on recent Venus Express remote sensing measurements.We find that the "sudden death" quenching scheme by CO 2 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 1.9x10 11 cm -3 , a O 3 density profile peaking at 5.8x10 6 cm -3 at 96.5 km and a H density profile close to 10 8 cm -3 between 90 and 120 km, in agreement with several photochemical models.• The infrared OH Meinel bands have been observed by VIRTIS on board Venus Express.• The ∆v=1 and ∆v=2 sequences are significantly correlated in altitude and intensity.• Most of the population is in the lower vibrational levels.• A non-LTE model was developed using O, O 3 and CO 2 densities from VEX measurements.• Observations are reproduced with these densities and a single quantum jump model.

Terrestrial OH nightglow measurements during the Rosetta flyby

“…Soret et al . [] demonstrated that for Venus, compared with these terrestrial results, the paradigm of single vibrational quantum collision deactivation by CO 2 provides a much better agreement with both the spectral structure and the observed total brightness than the “sudden death” model.…”

A new mechanism for OH vibrational relaxation leading to enhanced CO2 emissions in the nocturnal mesosphere

“…[] and Zhu and Yee [] that predicted dramatic differences in the intensity of the calculated OH nightglow emissions in the atmosphere of Mars depending on the choice of limiting case for the mechanism of OH vibrational relaxation by CO 2 (sudden death versus single‐quantum relaxation). More recently, other researchers have also investigated the mechanistic pathways responsible for vibrational relaxation of OH( v ) by CO 2 in the atmospheres of Mars and Venus [ Krasnopolsky , ; Soret et al ., ; Burtt and Sharma , ].…”

Comparative analysis of airglow emissions in terrestrial planets, observed with VIRTIS-M instruments on board Rosetta and Venus Express

“…Other important emissions were recently observed by VIRTIS on the Venus night side, such as the IR hydroxyl emissions Gérard et al, 2010;Soret et al, 2010Soret et al, , 2012bMigliorini et al, 2011), the IR nitric oxide (García-Muñoz et al, 2009b) and the oxygen emissions in the visible range (García-Muñoz et al, 2009a;Migliorini et al, 2013). Combined with the O 2 nightglow emissions, all these species contribute to build up a 3-D picture of the night side of Venus, providing information about dynamics and photochemistry of the upper atmosphere.…”

“…The value of the mean intensity and altitude was investigated on a statistical basis by Soret et al (2012b), reporting a slight difference in the mean peak altitudes for the OH(Dm = 1) and OH(Dm = 2). Limb data reveal a great deal of similarities between the OH Meinel and O 2 (a 1 D g À X 3 R) airglow emissions.…”

“…Considering reaction (c), a density product of [H] Ã [O 3 ] = 4-9 Â 10 15 cm À6 was inferred (Montmessin et al, 2011). This detection of ozone on the Venus night side prompted Soret et al (2012b) to study the spectral distribution of the OH spectra observed with VIRTIS and the expected brightness. They showed that the single vibrational quantum jump model for quenching of OH Ã by CO 2 provides a good fit to the VIRTIS spectra and matches the observed limb intensity with O 3 densities compatible with the SPICAV measurements.…”