Airglow Derived Measurements of Q-Branch Transition Probabilities for Several Hydroxyl Meinel Bands

Christoph Franzen,Niklas Hofmann,Anlaug Amanda Djupvik,Robert Edward Hibbins,Patrick Joseph Espy

doi:10.3390/atmos10100637

Christoph Franzen, Niklas Hofmann + Show 3 more

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https://doi.org/10.3390/atmos10100637

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Abstract

Spectroscopic measurements of the hydroxyl (OH) airglow emissions are often used to infer neutral temperatures near the mesopause. Correct Einstein coefficients for the various transitions in the OH airglow are needed to calculate accurate temperatures. However, studies from some studys showed experimentally and theoretically that the most commonly used Einstein spontaneous emission transition probabilities for the Q-branch of the OH Meinel (6,2) transition are overestimated. Extending their work to several Δv = 2 and 3 transitions from v′ = 3 to 9, we have determined Einstein coefficients for the first four Q-branch rotational lines. These have been derived from high resolution, high signal to noise spectroscopic observations of the OH airglow in the night sky from the Nordic Optical Telescope. The Q-branch Einstein coefficients calculated from these spectra show that values currently tabulated in the HITRAN database overestimate many of the Q-branch transition probabilities. The implications for atmospheric temperatures derived from OH Q-branch measurements are discussed.

Highlights

The reduction of ozone in the upper mesosphere creates hydroxyl (OH) via:H + O3 → OH∗ + O2 (1)This chemical reaction is exothermic by ~3.3 eV, and creates the OH in vibrational levels with6–9 and excited rotational states
Using the Q-branch Einstein coefficients measured here reproduced the P-branch temperatures for both bands. These temperature differences when using the Q-branch transition probabilities from HITRAN and those measured here are consistent with the results shown in Figure 5 for the Nordic Optical Telescope (NOT) data, and the results are shown in Figure 5 as red diamonds with error bars
By evaluating a total of 17.8 h of astronomical background NIR spectroscopic observations of the OH airglow obtained by the Nordic Optical Telescope NOTCam instrument, we have calculated

Summary

Introduction

The reduction of ozone in the upper mesosphere creates hydroxyl (OH) via:H + O3 → OH∗ + O2 (1)This chemical reaction is exothermic by ~3.3 eV, and creates the OH in vibrational levels with6–9 and excited rotational states. This chemical reaction is exothermic by ~3.3 eV, and creates the OH in vibrational levels with. Relaxation of the vibrationally excited OH molecule happens through radiative and collisional relaxation. Radiative relaxation of this excited OH in the Meinel system results in the bright near-infrared (NIR) radiation known as OH nightglow or airglow. Meinel emission occurs over an approximately 8 km thick layer [1], and spectroscopic observations of the nightglow have been used to infer the atmospheric conditions at the peak of the layer near 87 km altitude [2,3,4,5,6,7,8].

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