Thermospheric dynamics: A system theory approach

To gain insight into the important interactions, a simple numerical model of the horizontal neutral winds in the thermosphere was developed and used to study the effect of changes in ion densities on the general circulation. The results obtained from the simple model show that changes in ion densities of 30-40%, which occur frequently during disturbed geomagnetic conditions, can produce appreciable effects in the neutral wind pattern. The conclusion from this simulation is that during disturbed geomagnetic conditions one needs an interactive ionosphere in order to have a realistically modeled thermosphere.;The results from the simple model were used to model in a mechanistic way the coupling between the ionosphere and the neutral atmosphere in the TGCM of NCAR. The two versions of the TGCM, interactive and noninteractive, were used to simulate a magnetic storm during equinox solar maximum conditions. Comparison of the two simulations shows large, well-structured differences produced by the feed-back effects present in the interactive version of the TGCM. The interactive TGCM was then analyzed using a system theory approach. The equivalent systems for meridional winds at two locations, Millstone Hill and Arecibo, were studied. It is shown that the height profile of the meridional velocity at each location can be characterized by a set of corresponding steady state and unit sample response functions (SRF). The SRF can be used to approximate TGCM solutions for the meridional winds for arbitrary geomagnetic conditions. Comparisons of SRF approximated and TGCM computed solutions at Millstone Hill and Arecibo show good agreement. The SRF approximated solutions can thus provide a very efficient way to perform a first-order comparison between data and theoretical simulations and may provide real time prediction capabilities for satellite ephemeris and reentry trajectories during disturbed geomagnetic conditions.;The 18-28 March, 1979 period was also modeled with the new Thermosphere-Ionosphere General Circulation Model (TIGCM) of NCAR. The dynamical behavior of the TIGCM ionosphere is compared with data derived from ionosonde stations. It is shown that the TIGCM is able to successfully predict features of the observed response of the ionosphere and thermosphere to geomagnetic activity. The comparison also show that the forcing on the thermosphere was too large, for this particular simulation. A second simulation for the same period but with 60% reduced forcing is planned and is expected to show improved agreement with data. This indicates the importance of analyzing and comparing independent data sets with model simulations.