Computational modelling of high-latitude local-scale thermospheric dynamics

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.

A simplified three-dimensional local scale model has been developed to investigate high latitude thermospheric dynamics. The model’s spatial extent (resolution) is 1000 (20) km zonally, 600 (15) km meridionally and 300 km vertically, from 100-400 km altitude (750 m); a resolution that is higher than global models of the thermosphere. The cost for this increase is a smaller extent in the spatial domain. Unlike most other numerical models of the atmosphere, hydrostatic equilibrium is not enforced, which opens up a range of phenomena not usually modelled. The model was used to numerically evaluate a commonly used approximate relationship for deriving vertical winds from observations of horizontal divergence, called the Burnside Condition. The results show that the vertical wind response is more closely related to the total divergence field, rather than purely horizontal divergence. Additionally, the atmospheric response to heating of the ionosphere via ground based radio transmission instruments such as the High Frequency Active Auroral Research Program’s (HAARP) Ionospheric Research Instrument (IRI), has been investigated to determine the magnitude of any atmospheric waves potentially excited. Waves were indeed generated in the model for realistic heating scenarios, however the magnitudes of these waves are at the limits of detection for current ground based instruments. As the heating efficiency of the IRI is not accurately known it may be possible that the atmospheric response is large enough for waves to be extracted from observational data. Finally, the spatial extent (resolution) of the model was decreased (increased) to 500 (2) km zonally, 300 (2) km meridionally and 100.5 km (750 m) vertically, from 75-175.5 km altitude. The energy deposition from pulsating aurora was then simulated to examine whether resulting pressure perturbations included infrasonic waves. Such waves have previously been observed at ground level, and are postulated to have propagated down from an auroral source. Simulations show that perturbations of sufficient magnitude can be produced that, with the results from de Larquier and Pasko [2009], suggest a possible mechanism for the propagation of such waves may exist.