Mueller-Wodarg, Ingo; (1997) Modelling perturbations propagating through the mesopause into the earth's upper atmosphere. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Global oscillations formed in the terrestrial troposphere, stratosphere and mesosphere propagate into the thermosphere and ionosphere where they change the dynamics, energy and composition. This thesis presents a series of studies which examine in detail the nature and influence of solar tides and the planetary 2-day wave above 80 km altitude. The Coupled Thermosphere-Ionosphere Model (CTIM) calculates self-consistently the dynamics, energy and composition of the terrestrial thermosphere and ionosphere in three dimensions and is used as the main tool in these studies. In order to simulate the upwardly propagating perturbations which are formed outside the height range of the model, the lower boundary of the CTIM at 80 km height was modified to allow the global profiles of pressure-, wind- and temperature oscillations to be specified. In principle, following the modification, any such profile can be used for the external forcing as long the parameters at the lower boundary' are self-consistent. One effective method of achieving this is to specify global perturbations of geopotential height, using Hough functions for the latitudinal structure, and calculating the simultaneous wind- and temperature oscillations at the lower boundary analytically with expressions from Classical Tidal Theory. The necessary formalism for this has been fully implemented. For validation of the new code a series of comparisons with other numerical models and Incoherent Scatter Radar measurements at equinox and solstice are presented and show that CTIM is capable of reproducing many tidal features found in the "real" thermosphere. A further study is presented which investigates processes causing planetary' wave signatures in the ionosphere. It is found not only that CTIM reproduces some key properties of upwards propagating planetary waves found in other theoretical and modelling studies, but also that upwards propagating tides may, through modulation of their amplitudes, carry planetary wave signatures into the 200 km height regime where they are transferred into the ionosphere by chemical processes. The new CTIM thus offers the possibility of carrying out many unprecedented studies exploring the nature of the Earth's upper atmosphere.

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