Neutral Composition Information in ICON EUV Dayglow Observations. Issue 8 (24th August 2022) - Tuminello, Richard M.
Abstract: Since the earliest space‐based observations of Earth's atmosphere, ultraviolet (UV) airglow has proven a useful resource for remote sensing of the ionosphere and thermosphere. The NASA Ionospheric Connection Explorer (ICON) spacecraft, whose mission is to explore the connections between ionosphere and thermosphere utilizes UV airglow in the typical way: an extreme‐UV (EUV) spectrometer uses dayglow between 54 and 88 nm to measure the density of O +, and a far‐UV spectrograph uses the O 135.6 nm doublet and N2 Lyman‐Birge‐Hopfield band dayglow to measure the column ratio of O to N2 in the upper thermosphere. Two EUV emission features, O + 61.6 and 83.4 nm, are used for the O + retrieval; however, many other features are captured along the EUV instrument's spectral dimension. In this study, we examine the other dayglow features observed by ICON EUV and demonstrate that it measures a nitrogen feature around 87.8 nm which can be used to observe the neutral thermosphere. Plain Language Summary: The ionosphere is a region of near‐Earth space made up of plasma. NASA's Ionospheric Connection Explorer (ICON) mission seeks explore the factors influencing formation of the ionosphere and how it interacts with Earth and its atmosphere. One of the ways ICON does this is by measuring airglow: light released by the air in the upper atmosphere. This occurs with visible light, with the same shades seen in the aurora; it also occurs in the ultraviolet range, invisible to the human eye but visible to ICON instruments. An imager is included on ICON to measure extreme‐ultraviolet light, almost as energetic as X‐rays. Certain atoms and molecules in the atmosphere are known to glow at specific wavelengths. By measuring the brightness of airglow at certain wavelengths, ICON is able see the structure of ionospheric oxygen. The instrument also measures dimmer emissions at other wavelengths, some of which are known to come from certain atmospheric species and others which are unknown or uncertain. Here we look at the other wavelengths and attempt to find their origins. We find that most are likely coming from oxygen. Interestingly, we find one that we think comes from nitrogen. This could be useful for measuring the abundance of molecular nitrogen in the upper atmosphere, a task currently performed by another instrument on the ICON spacecraft. We make a case for the practicality of this approach. Key Points: A comprehensive introduction to the Ionospheric Connection Explorer Extreme ultraviolet dayglow observations is presented Some dim emission features are identified as originating from O + from similarity to known features Emission near 87.8 nm follows N2 and, combined with 61.6 nm data, contains information about ΣO/N2