New insights for mesospheric OH: multi-quantum vibrational relaxation as a driver for non-local thermodynamic equilibrium.

Jérôme E. Thiebaud,Constantin Romanescu,Daniel Matsiev,Alexander A. Kutepov,Philip C. Cosby,Stefano Falcinelli,Stefan Noll,James A. Dodd,Peter A. Panka,Jonas Hedin,Konstantinos S. Kalogerakis

doi:10.5194/angeo-36-13-2018

Abstract

The question of whether mesospheric OH(υ) rotational population distributions are in equilibrium with the local kinetic temperature has been debated over several decades. Despite several indications for the existence of non-equilibrium effects, the general consensus has been that emissions originating from low rotational levels are thermalized. Sky spectra simultaneously observing several vibrational levels demonstrated reproducible trends in the extracted OH(υ) rotational temperatures as a function of vibrational excitation. Laboratory experiments provided information on rotational energy transfer and direct evidence for fast multi-quantum OH(high-υ) vibrational relaxation by O atoms. We examine the relationship of the new relaxation pathways with the behavior exhibited by OH(υ) rotational population distributions. Rapid OH(high-υ) + O multi-quantum vibrational relaxation connects high and low vibrational levels and enhances the hot tail of the OH(low-υ) rotational distributions. The effective rotational temperatures of mesospheric OH(υ) are found to deviate from local thermodynamic equilibrium for all observed vibrational levels.

Highlights

The emission of radiation from vibrationally excited OH is an important observable in the Earth’s upper atmosphere and has been the topic of numerous studies over the past several decades
No information is available on how collisions of OH with atomic oxygen may affect the hydroxyl rotational excitation, for example in the case that reactive or energy transfer processes have cross sections that vary as a function of the initial OH rotational level
Another important piece of evidence for the characterization of the OH rotational temperatures as non-local thermodynamic equilibrium (LTE) for all observed levels stems from the fact that in the studies by Noll et al (2016) and Cosby and Slanger (2007), consideration of additional rotational levels beyond the lowest three resulted in gradually different results for the OH rotational temperature

Summary

Introduction

The emission of radiation from vibrationally excited OH is an important observable in the Earth’s upper atmosphere and has been the topic of numerous studies over the past several decades. Kalogerakis et al.: New insights for mesospheric OH have investigated the relevant collisional energy transfer processes because they play a key role in determining the observed vibrational population distributions and their relative emission intensities. Modeling calculations of these emissions have attracted considerable interest and are an essential part of the synergistic interplay between observations, laboratory experiments, and theoretical calculations. Meinel observed similar intense emissions at the Yerkes Observatory and, following the suggestion of Gerhard Herzberg (Herzberg, 1951), attributed them to OH rovibrational transitions within the electronic ground state (Meinel, 1950b, c, d) These emissions, known as the OH Meinel bands, represent some of the most prominent features in the visible and infrared regions of the nightglow. We briefly discuss the implications of the new insights for our understanding of the mesosphere and future needs for relevant observations, laboratory measurements, and modeling calculations

Mesospheric OH sources and sinks

Removal by radiative decay and rotational–vibrational relaxation

The long-standing debate on whether OH rotational temperatures are in LTE

OH rotational temperatures and multi-quantum relaxation

Findings

Discussion

Conclusions