Neutral temperature and atmospheric water vapour retrieval from spectral fitting of auroral and airglow emissions

Abstract

We have developed a spectral fitting method to retrieve upper atmospheric parameters at multiple altitudes simultaneously during times of aurora, allowing us to measure neutral temperatures and column densities of water vapour. We use the method to separate airglow OH emissions from auroral O+ and N2 in observations between 725 and 740 nm using the High Throughput Imaging Echelle Spectrograph (HiTIES) located on Svalbard. In this paper, we describe our new method and show the results of Monte Carlo simulations using synthetic spectra which demonstrate the validity of the spectral fitting method and provide an indication of uncertainties on the retrieval of each atmospheric parameter. We show that the method allows for the retrieval of OH temperatures with an uncertainty of 6% when contamination by N2 emission is small. N2 temperatures can be retrieved with uncertainties down to 3 %-5% when N2 emission intensity is high. We can determine the intensity ratio between the O+ doublets at 732 and 733 nm (which is a function of temperature) with an uncertainty of 5 %.