Title: Hydroxyl infrared emission modelling from space observations of the Earth\

Abstract:
During the Rosetta mission flyby with the Earth in November 2009, hydroxyl nightglow 
emissions were observed with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) 
on board the Rosetta mission. Data are analyzed to investigate the relative population of the 
ν=1 to 9 OH vibrational levels. These observations are unique because the instrument could 
simultaneously acquire an image at each wavelength in the range 1-5.1 μm. This allows one to 
simultaneously study the emissions of the OH (Δν=1,2) sequences, while observing the same 
region of the planet. In addition, observations from space allow overcoming the difficulties 
associated with corrections for atmospheric absorption.
We obtained a mean spectrum of the nightside of the Earth covering the altitude range 85-95 
km. Following correction for background, we compared the observations with a synthetic 
spectra model, based on the PGOPHER code. This allowed us to infer the relative population 
from levels from 1 to 9. In particular, the relative population of level 1 has been determined for the 
first time.
Our results are in general satisfactory agreement with previous observations and models 
developed for mid-latitudes conditions. They favor models where sudden death deactivation by 
atomic oxygen is a major process controlling the vibrational population.