Atmospheric Gravity Wave Observations from the International Space Station using the Near InfraRed Airglow Camera (NIRAC)

Abstract:

The Near Infrared Airglow Camera (NIRAC) is a space based infrared imager deployed to the International Space Station (ISS) in May 2019. The camera bandpass is 1.5 to 1.72 microns (H band), which is ideal for observations of the bright OH Meinel airglow emission. NIRAC has a 23° x 23° FOV, sweeping out an approximate 170 km swath from 420 km altitude, or a smaller swath when the ISS is at a lower altitude. The camera utilizes a state-of-the-art 4-megapixel (2K x 2K, 18-micron pixel) Teledyne H2RG detector, a custom-designed f/2 rectilinear lens, and a newly developed unique motion compensation system to obtain a high native spatial resolution, depending on the ISS altitude, of about 85 m (70 m) at the ground (airglow layer) per pixel. This design also allows smear free, high (> 50) signal to noise (S/N) imaging (of the ground or airglow layer) over the whole FPA during multi-second image acquisition times, even though the ISS is moving greater than 7 km/s. Because of the intrinsically bright H band airglow and the long exposure times needed to observe airglow structures (typically ~ 1.5 s) this system is quite sensitive, with airglow S/N observations under Moon-down nighttime conditions exceeding the VIIRS DNB capability by a substantial margin.

NIRACs nominal acquires imagery at ~10 s cadence, which results in image sequences with an approximate 50% overlap between images. Because of NIRAC’s high spatial resolution, overlapping image sequences can be analyzed for atmospheric gravity wave (AGW) features using the relative displacement of the airglow AGW features as viewed from the two different viewing angles. NIRAC operates in an atmospheric window with a see-to-the-ground capability and thus the airglow images also contain ground and low atmosphere cloud features. Image differencing removes these cloud backgrounds and Earth surface features that otherwise would make AGW identification difficult in this band. In this presentation we show examples of AGWs observed in the more than 100,000 nighttime images taken to date. We explore AGW occurrence in several well-sampled geographic locations under varying conditions, and consider the spatial extent and characteristics of these wave features. Where possible, we also make comparisons between wave features observed by NIRAC and ground-based or other space-based instrumentation.