Thermospheric Neutral Densities Derived by Tomographic Inversion of Ultraviolet Dayglow Observations Made by the SSULI Instruments

Abstract:

Inversion of ultraviolet airglow measurements using non-linear modeling of the dayglow production mechanisms has been demonstrated as a means of inferring daytime thermospheric densities. However, the prior results have all relied on inversions of single altitude profiles for their inferences. While this is acceptable, in some viewing geometries, we find that it can be inaccurate when the instrument is viewing primarily in the North-South direction where both density and illumination gradients can adversely affect the inversion accuracy. Recently, we have developed and published a new inversion approach called Volume Emission Rate Tomography, which handles the radiation transfer as part of the tomography process. The VERT approach was validated using satellite-based measurements of the nighttime ionosphere, which were used to infer the electron densities. These retrievals that were shown to be in good agreement with coincident incoherent scatter radar measurements. We apply the VERT technique to ultraviolet dayglow measurements made by the Special Sensor Ultraviolet Limb Imager (SSULI) instruments aboard the Defense Meteorological Satellite Program satellites made during 2010 and 2014. We determine the density distributions of O and N₂ in the orbit plane, including the effects of transfer of the radiation from its point of origin to the observer, the redistribution of the photons by multiple resonant scattering (radiation transport), Mutual Neutralization and Radiative Recombination contributions to the volume emission rate at 135.6 nm, as well as the photoelectron impact excitation of the emissions. In this presentation, we concentrate on the technique used for the inversions and comparisons to single limb-scan inversions. A companion presentation, (see Fritz et al. [2020]) compares the results to the NRLMSIS model and to the TIME-GCM physics-based model with tropospheric nudging by NAVGEM-HA.