This project aims to develop an accurate technique for determining the atomic oxygen density in the mid-latitude thermosphere by combining modern instrumentation with state of the art modeling. The method will combine optical and radar measurements to constrain a forward model of a key thermospheric O-atom emission, the twilight airglow at 8446 A, and in turn use the constrained model to develop an [O] estimation scheme suitable for application at other mid-latitude locations. The 8446 A emission has long been considered an ideal candidate for [O] remote sensing owing to its relatively simple emission model, and the project will acquire an unprecedented set of 8446 A spectral data under various observational conditions at two distinct mid-latitude facilities: Millstone Hill (MH) Observatory in Massachusetts and Arecibo Observatory (AO) in Puerto Rico. Additional parameters derived from nested incoherent scatter radar, Fabry-Perot interferometer, and photometer measurements will not only serve as additional forward model constraints, but also help assess the validity of current model assumptions, specifically with regard to the role of secondary sources of 8446 A production. Together with a thorough quantification of model parameter dependencies, the constrained forward model will be used to develop a novel inverse-theoretical technique to estimate thermospheric [O] from measured 8446 A brightness at mid-latitudes. Quantification of neutral atomic oxygen, the dominant constituent in the Earth's thermosphere between 200 - 600 km, is important for several reasons. In this region, its resonant charge exchange with O+, the principle ion in the F-region ionosphere, plays a vital role in both the momentum and energy exchange between the thermosphere and ionosphere. Similarly, its charge exchange with H+ has long been recognized as an important influence on ion transport between the ionosphere and plasmasphere. Owing to this strong chemical coupling, the accuracy of many fundamental aeronomical calculations -- such as the derivation of transport coefficients, neutral wind speeds, energy deposition rates, chemical reaction rates, or photochemical emission brightnesses -- hinges on accurate specification of [O]. Thus, current uncertainties in thermospheric composition and density limit the understanding of the coupled thermosphere-ionosphere system, both with regard to its climatological variability as well as its response to impulsive forcing from above and below. The development of a new, ground-based capability of measuring thermospheric [O] will benefit both of these central priorities of the NSF CEDAR program. One graduate student will be trained in this Aeronomy-related area with support from the project. The student will gain familiarity with acquisition and analysis of ISR spectra as well as that of optical SHS, FPI, and photometer data. Available internal funds for undergraduate research support, together with the strong involvement of AO in the Research Experiences for Undergraduates (REU) program, presents another opportunity to introduce undergraduate students to aeronomy as well.
This project, led by an early-career female scientist, culminated in the development of two new algorithms to estimate neutral atomic oxygen, O, density in the earth's upper atmosphere near 400 km, where it generates frictional drag and oxidation of satellites in low earth orbit. Knowledge of the spatial and temporal dependence of this fundamental state parameter is necessary not only for spacecraft operations in this region but also for the modeling and prediction of secular evolution of the upper atmosphere and its transient response to solar storms. Both algorithms developed through this award incorporate measurements of atmospheric emissions at visible wavelengths ("airglow"), together with simultaneous radar remote sensing of the region, in order to constrain physical models of atmospheric processes. The development of the algorithms was based on analysis of nearly a decade's worth of data acquired from Arecibo Observatory, an NSF-supported facility located in Puerto Rico, as well as from the Global Ultraviolet Imager onboard TIMED satellite orbiting earth near 600 km. As the world's largest single dish radio telescope, the Arecibo radar is typically used to obtain measurements of the ionized atmosphere at unrivaled sensitivity; this project extended these capabilities toward neutral atmospheric sensing as well. This work demonstrated that the fusion of such multi-platform data sources not only enables the reliable derivation of a fundamental, and poorly constrained historically, atmospheric state parameter, but also provides unprecedented constraints on a key physical mechanism, recombination of ionized oxygen atoms, that governs atmospheric behavior itself. These results were dissiminated through peer-reviewed journal articles, conference presentations, and invited lectures at several universities around the country. Several undergraduate students in the Department of Electrical and Computer Engineering at the University of Illinois at Urbana-Champaign, participated in several aspects of the research, and the tools, techniques and results of the research formed the foundation for several outreach activities involving elementary, middle, and high-school aged girls interested in pursuing careers in STEM fields such as this one.
