COUPLING MECHANISMS IN THE LOWER IONOSPHERIC-THERMOSPHERIC SYSTEM AND MANIFESTATIONS IN THE FORMATION AND DYNAMICS OF INTERMEDIATE AND DESCENDING LAYERS

We present results from a first attempt at developing a broad database relevant to the determination of electron densities, electrodynamics, and tidal structure in the lower ionospheric-thermospheric domain. The focus is on intermediate and descending layers, their diurnal and latitudinal variations, averaged behavior, day-to-day variabilities, and cause-effect relationships. Working with an ionosonde database of 30-day around-the-clock observations in September 1989, we found that the layers appeared more regularly than not and manifested characteristics which showed their formation at high altitudes (sometimes higher than 170 km) followed by a monotonic descent to the 100-110 km region at rates as high as 8.5 km/h. Descending layers were observed throughout the day at all sites without obvious bias in daytime or nighttime occurrence probabilities. They appeared at all latitudes in the northern and southern hemispheres, and for the same UT were observed in all local time zones. Using simulations from the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) we identified diurnal, semi-diurnal, and (possibly) terdiurnal tidal modes as the causal mechanisms for layer formation and transport with primary controls driven by meridional and zonal wind-shear forces. Poorest model-measurement correlations were at high latitude stations (dipole latitudes > 49 degrees) and when quantifying the relative magnitude of electric field controls in the overall process of layer convergence and transport.