A Search For Thermospheric Composition Perturbations Due To Vertical Winds

Thesis (Ph.D.) University of Alaska Fairbanks, 2006The thermosphere is generally in hydrostatic equilibrium, with winds blowing horizontally along stratified constant-pressure surfaces, driven by the dayside-to-nightside pressure gradient. A marked change in this paradigm resulted after Spencer et al. [1976] reported vertical wind measurements of 80 m·s-1 from analyses of AE-C satellite data. It is now established that the thermosphere routinely supports large-magnitude (~30-150 m·s-1) vertical winds at auroral latitudes. These vertical winds represent significant departure from hydrostatic and diffusive equilibrium, altering locally---and potentially globally---the thermosphere's and ionosphere's composition, chemistry, thermodynamics and energy budget. Because of their localized nature, large-magnitude vertical wind effects are not entirely known. This thesis presents ground-based Fabry-Perot Spectrometer OI(630.0)-nm observations of upper-thermospheric vertical winds obtained at Inuvik, NT, Canada and Poker Flat, AK. The wind measurements are compared with vertical displacement estimates at ~104 km2 horizontal spatial scales determined from a new modification to the electron transport code of Lummerzheim and Lilensten [1994] as applied to FUV-wavelength observations by POLAR spacecraft's Ultraviolet Imager [Torr et al. , 1995]. The modification, referred to as the column shift, simulates vertical wind effects such as neutral transport and disruption of diffusive equilibrium by vertically displacing the Hedin [1991] MSIS-90 [O2]/[N2] and [O]/([N2]+[O2]) mixing ratios and subsequently redistributing the O, O2, and N 2 densities used in the transport code. Column shift estimates are inferred from comparisons of UVI OI(135.6)-nm auroral observations to their corresponding modeled emission. The modeled OI(135.6)-nm brightness is determined from the modeled thermospheric response to electron precipitation and estimations of the energy flux and characteristic energy of the precipitation, which are inferred from UVI-observed Lyman-Birge-Hopfield N2 emissions in two wavelength ranges. Two-dimensional column shift maps identify the spatial morphology of thermospheric composition perturbations associated with auroral forms relative to the model thermosphere. Case-study examples and statistical analyses of the column shift data sets indicate that column shifts can be attributed to vertical winds. Unanticipated limitations associated with modeling of the OI(135.6)-nm auroral emission make absolute column shift estimates indeterminate. Insufficient knowledge of thermospheric air-parcel time histories hinders interpretations of point-to-point time series comparisons between column shifts and vertical winds

A comparison between vertical winds in the lower thermosphere and magnetic field perturbations on the ground

Vertical winds in the lower thermosphere are estimated from OI557.7-nm Doppler shifts obtained with a Fabry-Perot interferometer at the Poker Flat Research Range (65.12N, 147.43W in geographic coordinate), Alaska. The temporal variation of vertical winds was compared with the horizontal component of the magnetic field obtained at Poker Flat and two other sites, Gakona (62.12N, 145.14W) and Fort Yukon (66.36N, 145.22W). Two nights of observations were examined and the results were shown here. The results showed that temporal variations of vertical winds were similar to that of magnetic field variation during each substorm. In some cases the results of cross correlation between these two parameters showed that the magnetic field perturbation leads vertical winds in the earlier period of the substorm. The difference increased gradually and reached a maximum at around the center of the recovery phase. From there, the differences decreased. The mechanism for the relation between the two parameters is still unclear, but this result suggests an intimate relation between ionospheric currents and vertical wind in the thermosphere