Laboratory Characterization of a UV Spectrograph for Polar Night Nitric Oxide Measurements

The polar night provides a unique environment in which Nitric Oxide (NO) concentrations are enhanced due to auroral energy deposition and the absence of sunlight. The most widely used techniques for measuring NO require sunlight, so observations of NO in the polar night are limited. The PolarNOx sounding rocket mission in 2020 provided a significant advancement by detecting a large abundance of NO peaking at a lower altitude than is observed outside the polar night. The PolarNOx 2026 flight will improve our understanding of NO in the polar night by making observations under elevated geomagnetic conditions (Solar Maximum) when much larger NO abundances are likely to descend into the lower atmosphere. This study details the laboratory characterization of a UV Spectrograph which is designed to measure NO in the polar night upper atmosphere with moderately high spectral resolution of 0.03 nm. The spectrograph performance was evaluated under various controlled conditions to ascertain its suitability for deployment for the PolarNOx 2026 mission. The instrument is based on classical optics, a plane grating with 3600 lines per mm, and a Teledyne e2V SIRIUS – CIS115 detector. Characterizations include the sensor’s radiometric and spectral response, particularly in relation to detector temperature fluctuations over time. The instrument’s efficiency was quantitatively assessed using a deuterium arc lamp with a calibration traceable to the National Institute for Standards and Technology. Spectral resolution was verified using a Tellurium lamp, the application of a bright deuterium lamp facilitated the observation of multiple NO absorption spectra, employing NO cells of varying column densities to simulate different atmospheric conditions. The calibration results validate the instrument’s ability to accurately detect and resolve NO absorption features within the ultraviolet spectrum. The measured transmission spectrum through the NO cell, when compared to a model spectrum of NO absorption, confirms the instrument's sensitivity to NO near 215 nm. The findings from this laboratory characterization ensure the spectrograph’s capabilities for the forthcoming PolanNOx 2026 flight to provide critical insights into the distribution of NO in polar night.