CEDAR: A Robust Interferometer for Studies of Thermospheric Composition and Dynamics

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The investigators will develop a procedure for measuring atomic oxygen concentrations in the upper atmosphere using ground-based observations of 844.6 nanometer Bowen fluorescence emissions, combined with incoherent scatter radar measurements and a numerical model. The neutral atmosphere plays a significant role in almost every thermospheric process, and oxygen is the dominant neutral species between 250 and 500 km. It is important in contexts ranging from thermospheric dynamics, to thermospheric-ionospheric interaction, to spacecraft operation. Yet, for the last two decades, determination of neutral oxygen density, [O], has relied heavily on semi-empirical atmospheric models like the Mass Spectrometer Incoherent Scatter (MSIS) theory, supported by 20-year-old satellite mass spectrometer data. While such methods are rather useful, their reliance on statistical averages leads to substantial uncertainties in their predictions, especially during variable phenomena such as magnetic storms. This study will measure atomic oxygen using a forward model and ground-based incoherent scatter radar and optical airglow measurements. The optical measurements will be made with a Spatial Heterodyne Spectrometer (SHS), which is a novel Fourier transform spectrometer that provides outstanding interferometric performance over a broad range of wavelengths without scanning. The SHS is like conventional interferometers (Michelson, Fabry- Perot), in that it is more compact with higher etendue at a given resolving power than grating spectrometers. However, being unscanned, it can be built far more robust than conventional interferometers, with relaxed flatness tolerances on critical optics. Moreover, unlike Michelsons, the SHS can be adjusted from one observation to the next so that the center of its transformed spectrum always falls near the science lines. A single SHS may thus produce easy-to-sample fringes in many wavelength regions. And, with a few other minor adjustments, one SHS can observe the airglow emissions mentioned above far more efficiently than a single Fabry-Perot system. It can also be used in an imaging mode. The same instrument can also measure other airglow emissions near 630, 732, and 1129 nanometer, providing even more information about crucial thermospheric processes and global winds.