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 N 2 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 N 2 emission is small. N 2 temperatures can be retrieved with uncertainties down to 3 %-5 % when N 2 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 %.Published by Copernicus Publications on behalf of the European Geosciences Union.

Multiscale Observation of Two Polar Cap Arcs Occurring on Different Magnetic Field Topologies

“…The black line in Figure 5 shows the scaled‐residual spectra measured by the H‐ α panel of the HiTIES instrument, which have been integrated between 18:48:48 and 18:49:04 UT during this event. The contributions from OH, N 2 , N + , and emissions have been fitted and removed using methods outlined in Chadney and Whiter (2018) and Price et al. (2019), leaving just the H‐ α emission at 6563 Å (indicated by a vertical dashed line).…”

“…The red line in Figure 5 shows the scaled‐residual spectra measured by the H‐ α panel of the HiTIES instrument during the “open” event, integrated between 18:35:00 and 18:36:10 UT. As before, a fit for the spectra (excluding H‐ α ) has been obtained using methods outlined in Chadney and Whiter (2018) and Price et al. (2019), which has then been taken away from the integrated spectra.…”

Method for retrieval of atmospheric water vapor using OH airglow for correction of astronomical observations

“…OH airglow originates in the mesopause region within a layer located at a peak altitude of about 87 km (Xu et al 2012;von Savigny et al 2012). Chadney & Whiter (2018) note the wavelength coincidences of some OH emission lines, such as the P1(4.5) double lines in the OH(8-3) band and water vapor absorption lines. Therefore, these ground-based observed OH airglow emission lines originating in the mesopause region are attenuated by the presence of water vapor located in the troposphere.…”

“…In this work, the PWV retrieval process is similar to that from Chadney & Whiter (2018). The OH airglow emission that passes through the atmosphere and reaches the ground can be given by the Beer-Lambert law (Chadney et al 2017) as follows:…”

“…This effect may be the cause of the only partial success of methods of OH sky subtraction developed by many researchers (Davies 2007;Noll et al 2014;Soto et al 2016). However, Chadney & Whiter (2018) took advantage of these coincidences, for an atmospheric study, and developed a method to determine the amount of water vapor using auroral and OH airglow emissions in the polar region and evaluated the uncertainties of the retrieval method theoretically. In this paper, we propose a method, based on the Chadney & Whiter (2018) method, to retrieve the PWV from astronomical spectra.…”