Optical remote sensing of local-scale thermospheric dynamics above Antarctica

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Engineering and Mathematical Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

The aim of this project was to test the hypothesis that small spatial scales (on the order of 500 km or less) play an important role in thermospheric dynamics. To achieve this, a new all-sky scanning, imaging Fabry-Perot spectrometer was built at La Trobe University and installed at Mawson station, Antarctica (67◦360 S, 62◦520 E, Inv 70◦300 S, Magnetic Midnight: 2240 UT). This instrument was capable of recording independent spectra from 61 locations simultaneously across a 144◦ full-angle field-of-view, with a maximum time resolution of ∼2 minutes. Data from this instrument was combined with that from a narrow-field (6◦ full-angle), imaging Fabry-Perot spectrometer operating at Davis station (68◦350 S, 77◦580 E, Inv 74◦360 S, Magnetic Midnight: 2200 UT). A superposed epoch analysis of spatially-resolved wind and temperature showed predominantly pressure-gradient driven winds during quiet conditions, and convection driven winds during active conditions. A wind abatement in the pre-magnetic midnight sector was frequently observed under both levels of activity, while sunward flow was often observed equatorward of Mawson in the early magnetic morning sector. Bistatic measurements between Mawson and Davis allowed for vertical wind measurements at three locations along the line joining the two stations in addition to the vertical wind measured routinely above each station. These vertical winds were at times correlated over horizontal scales of ∼ 160-480 km, while at other times there was little correlation over the smallest separations of ∼ 160 km. Vertical winds were positively correlated with the large-scale horizontal divergence, in agreement with the relation predicted by Burnside. However, at small scales (∼ 100 km), the Burnside relation was not a reliable predictor of vertical winds. Strong wind shears were observed above Mawson in the presence of bright auroral arcs, and the heating rates due to these shears were found to make appreciable contributions to the energy balance. These results indicate that small spatial scales do indeed play an important role in thermospheric dynamics.