The Near-Infrared Airglow Camera on the International Space Station

Abstract

Abstract The OH airglow layer is a persistent feature of Earth’s upper mesosphere, centered near 87 km altitude, that can be perturbed by atmospheric gravity waves (AGWs) and instabilities. While ground-based airglow imaging has been used to study these perturbations locally, this technique is limited by tropospheric weather. Space-based remote sensing provides a platform to measure these processes globally. In addition, portions of the OH airglow band span an atmospheric window, allowing airglow illumination of the ground for imaging of nighttime clouds and Earth terrain features. The Near-Infrared Airglow Camera (NIRAC) images the airglow at 1.6 μm and while deployed to the International Space Station (ISS) from May 2019 to November 2021 demonstrated these applications. The camera uses a patented motion-compensation system with a custom rectilinear lens that allows multisecond, nearly smear-free imaging (∼<1.5 pixels) at a ground pixel resolution of ∼83 m. With a ∼170 km × 170 km ground swath, NIRAC acquires overlapping images at a 7–10-s cadence. Parallax considerations enable detection of both AGWs and instabilities in the airglow, and scenes can be analyzed for terrain and cloud height. NIRAC also has a short-exposure daytime mode for cloud and ground imagery. This study describes NIRAC and its operations on the ISS and presents imagery examples of Earth terrain and surface phenomenology (such as fires), cloud imagery at all moon phases day and night, and the nighttime detection of AGWs and instabilities above 80 km altitude. Significance Statement The Near-Infrared Airglow Camera (NIRAC) is the first space-based instrument to exploit the bright 1.6 μm OH Meinel airglow emission band for Earth surface imager at resolution of ∼83 m. During its 2.5-yr deployment on the International Space Station (ISS), NIRAC obtained over a half million images of Earth’s surface and OH airglow layer. NIRAC has been able to capture images of the very small-scale (<30 km) AGWs and instabilities under a wide range of viewing conditions, including (i) in the vicinity of city lights, (ii) over complex cloud scenes, and (iii) under both moondown and moonup illumination. NIRAC also acquired daytime and nighttime images of clouds, hurricanes and typhoons, human lighting, and forest fires in the 1.6 μm band.