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Title: A new coupled middle atmosphere and thermosphere general circulation model : studies of dynamic, energetic, and photochemical coupling in the middle and upper atmosphere
Author: Harris, Matthew James
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 2001
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A new Coupled Middle Atmosphere-Thermosphere (CMAT) general circulation model has been developed as an extension to the UCL Coupled Thermosphere-Ionosphere-Plasmasphere (CTIP) model. As well as updates and improvements to the original thermospheric code, it includes mesospheric energetics, dynamics, and composition. The aim in creating this model is to provide a tool with which to investigate two-way coupling between the Earth's lower and upper atmosphere through the Mesosphere-Lower Thermosphere (MLT) region. CMAT steady state results are presented and compared to various data sources. CMAT is able to reasonably simulate the mesosphere, with improved modelling of the MLT and upper thermosphere regions. Two studies were carried out, the first of which was an investigation into the effects of the diurnal tide upon mean state of the mesosphere and thermosphere. Previous studies have shown that tides act to deplete mean atomic oxygen in the MLT region due to increased recombination in tidally displaced air parcels. The model runs presented suggest that mean residual circulation associated with tidal dissipation also plays an important role. Stronger lower boundary tidal forcing was seen to increase the equatorial local diurnal maximum of atomic oxygen and associated O(1S) 557.7 nm green line volume emission rates. The changes in mean temperature were found to correspond to changes in mean circulation and exothermic chemical heating. The second study presented is a simulation of geomagnetic and solar cycle variation of auroral [OI] 630 nm nighttime emission. CMAT was able to simulate winter solstice trends in [OI] 630 nm integrated column brightness, as observed by the University College London Fabry-Perot Interferometers at Kiruna (Sweden), and Longyearbyen (Svalbard). The simulated 630 nm emission at these sites is mainly due to the two classical production mechanisms, the impact of energetic electrons and the dissociative recombination of O2+. The relative importance of these mechanisms is controlled by both direct particle precipitation and background electron density, which is in turn influenced by thermospheric dynamics. The UT maximum of integrated column brightness did not necessarily correspond to the maximum in electron precipitation. A parameterisation for the nighttime activation of O(3P) to O(1D) due to energetic precipitating electrons has been formulated using the GLOW electron transport and auroral model. This parameterisation is presented for possible application in other thermospheric general circulation models.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Thermospheric; Mesospheric