Abstract The thermosphere/ionosphere system is forced by three main drivers. Direct solar heating, propagating lower atmosphere tides, and magnetospheric energy and momentum inputs mainly via the polar and high latitude regions. The heating and ionizing UV and EUV solar radiation may be considered to have a stable background, which changes slowly during the solar cycle, a 27 day periodicity associated with active regions on the sun (the strength and activity of which also vary throughout the solar cycle) and brief enhancements during major solar flares. The effects of solar radiation inputs can be modelled fairly reliably, given the solar UV / EUV spectrum and its history over a period of the order of 3 months. Lower atmosphere tidal forcing is predictable in the driving region (troposphere and stratosphere), but tidal influences on the thermosphere are more variable due to the variable propagation characteristics of the mesosphere. Some important variations, such as the semi-annual oscillation in thermospheric density have not yet been modelled successfully, using global 3-D finite-difference techniques. Magnetospheric sources have strong spatial and temporal variability, and dealing with this variability poses considerable problems. Most numerical simulations of the thermosphere / ionosphere system have been carried out with “steady-state” solar and geomagnetic inputs, on the assumption that the average response at given activity level will emerge. Inherent time delays in the response and recovery times of the thermosphere/ionosphere system imply that simulations using average inputs may be misleading. To assess the differences between the ‘steady-state’ and more realistic time-dependent simulations, the UCL-TIGCM has been used to investigate the differences between the response to steady and 3-hr varying indices (i.e. Kp). The simulations using the coupled ionosphere — thermosphere model, have used three distinct means of parameterising the time-dependent geomagnetic input: the Global Kp Index; the NOAA / TIROS Precipitation Activity Index; and the AE index. Daily mean global predictions of thermospheric temperature, density and composition are relatively similar, using each of these indices. However, at progressively shorter time intervals, there are significant discrepancies in predicted energy input, and thus the simulated ionospheric and thermospheric response, according to which index is chosen. The data obtained from recent space-borne global auroral imaging shows that local and regional variations are poorly described by global geomagnetic parameters, so that prediction of small scale and short-period variations require considerably improved descriptions and indices of the geomagnetic inputs and their variations.


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    Title :

    Modelling the response of the thermosphere/ionosphere system to time dependent forcing


    Contributors:

    Published in:

    Publication date :

    1991-01-01


    Size :

    19 pages




    Type of media :

    Article (Journal)


    Type of material :

    Electronic Resource


    Language :

    English





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