Observations of Non-Migrating Tides in the Middle Thermosphere

Abstract

Non-migrating tides are understood to be an important coupling mechanism through which lower atmospheric conditions influence the variability of the near-Earth space environment. Tidal variations have been exhaustively characterized in the mesosphere/lower thermosphere and the upper thermosphere, yet it is not quite known to what extent tides appear in the middle thermosphere due to a lack of global-scale systematic observations. This dissertation addresses the middle thermospheric "gap" of tidal observations. Using far ultraviolet (FUV) observations of the Earth, the recent NASA GOLD and ICON missions retrieve thermospheric parameters, the former from geostationary orbit (GEO) and the latter from low Earth orbit. Both missions infer the daytime column O/N2 ratio, while GOLD also retrieves the daytime thermospheric disk temperature. Traditional spectral tidal decomposition cannot be employed on GOLD data due its incomplete longitude/local solar time sampling. The known limitation introduced by ionospheric contamination of emissions at 135.6 nm and the dependence of the GOLD disk temperature on solar zenith angle both serve as additional obstacles. This dissertation proposes a pair of algorithms, based on the local time sum and difference approaches, that retrieves diurnal and semidiurnal tidal parameters for temperature and O/N2 in a single inversion, while constraining the tidal phase differences in temperature and O/N2 using a general circulation model. The first estimates of non-migrating tidal amplitudes and phases in middle thermospheric temperature and O/N2 using a geostationary observational platform are presented. Comparisons to physics-based and empirical models reveal that modeled amplitudes are generally weaker than the amplitudes retrieved from GOLD data, highlighting the need for better representation of tidal dynamics. In addition, this dissertation proposes a universal approach to systematically reduce the ionospheric contamination of O/N2 and thus determine the true thermospheric tidal variations. The approach considers biases between the remote sensing and ionospheric data sets, and due to its universality can be adapted for any retrieval of O/N2 from far ultraviolet emissions. The modified O/N2 shows a wavenumber-3 pattern indicative of the diurnal eastward propagating zonal wavenumber 2 tide (DE2), where it was not present in the original O/N2. This DE2 presence as well as the finding of DE3 and SE2 signatures in wavenumber-4 patterns is consistent with the theory that upward propagating tides influence the longitudinal structure of the thermospheric composition.