Optimizing hydroxyl airglow retrievals from long-slit astronomical spectroscopic observations

Abstract

Abstract. Astronomical spectroscopic observations from ground-based telescopes contain background emission lines from the terrestrial atmosphere's airglow. In the near infrared, this background is composed mainly of emission from Meinel bands of hydroxyl (OH), which is produced in highly excited vibrational states by reduction of ozone near 90 km. This emission contains a wealth of information on the chemical and dynamical state of the Earth's atmosphere. However, observation strategies and data reduction processes are usually optimized to minimize the influence of these features on the astronomical spectrum. Here we discuss a measurement technique to optimize the extraction of the OH airglow signal itself from routine J-, H-, and K-band long-slit astronomical spectroscopic observations. As an example, we use data recorded from a point-source observation by the Nordic Optical Telescope's intermediate-resolution spectrograph, which has a spatial resolution of approximately 100 m at the airglow layer. Emission spectra from the OH vibrational manifold from v = 9 down to v = 3, with signal-to-noise ratios up to 280, have been extracted from 10.8 s integrations. Rotational temperatures representative of the background atmospheric temperature near 90 km, the mesosphere and lower thermosphere region, can be fitted to the OH rotational lines with an accuracy of around 0.7 K. Using this measurement and analysis technique, we derive a rotational temperature distribution with v that agrees with atmospheric model conditions and the preponderance of previous work. We discuss the derived rotational temperatures from the different vibrational bands and highlight the potential for both the archived and future observations, which are at unprecedented spatial and temporal resolutions, to contribute toward the resolution of long-standing problems in atmospheric physics.

Modelled effects of temperature gradients and waves on the hydroxyl rotational distribution in ground-based airglow measurements

“…4. It may be seen that the lowest three rotational levels are well characterized by a single, Boltzmann rotational temperature, T 1,3 = 155.8 ± 0.8 K, as has been observed in nightglow spectra (Espy and Hammond, 1995;Franzen et al, 2017;Harrison et al, 1970;Noll et al, 2015). However, there is excess emission in the higher rotational lines which could be interpreted as populations exceeding that expected from a thermalized Boltzmann distribution.…”

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

“…To obtain the intensities of all lines, Gaussian functions that characterize the instrumental line-shape [23] were centered at the HITRAN positions of the rotational lines and their integrated intensities fitted to the spectrum using a least-squares method. The line width of the Gaussian line-shapes was allowed to vary by up to 2%, to account for small noise variations.…”

“…The data reduction employed here follows that developed and described in Franzen et al [23]. The NOTCam spectra of astronomical point sources were chosen as described above.…”

“…Since point-source astronomical objects only extend over a few spatial pixels (typically less than 60), these regions were masked leaving the spectrum of the extended-source atmosphere in the background. Warping effects along the slit from the telescope optics were corrected [23], and the final OH spectrum was integrated along the slit dimension to yield a conventional intensity versus wavelength spectrum. These steps were executed for each individual spectrum of a given astronomical target and then integrated to yield a single, high signal-to-noise spectrum of the OH airglow.…”

Observation of Quasiperiodic Structures in the Hydroxyl Airglow on Scales Below 100 m

“…Figure below is a typical example of a single 20‐s integration recorded during May that shows only the (9,7) transition of OH and describes the techniques used to process the data. The basic data processing for telescope data of the OH is described in Franzen et al (). Briefly, the effects of the known bad pixels on the detector and the curvature of the image due to focal plane orientation were corrected.…”