COUPLING MECHANISMS IN THE LOWER IONOSPHERIC-THERMOSPHERIC SYSTEM ANDMANIFESTATIONS 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. Th e focus is on intermediate and descending layers, their diurnal and la titudinal 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 characteristic s which showed their formation at high altitudes (sometimes higher tha n 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 occu rrence probabilities. They appeared at all latitudes in the northern a nd southern hemispheres, and for the same UT were observed in all loca l time zones. Using simulations from the NCAR Thermosphere-Ionosphere- Electrodynamics General Circulation Model (TIE-GCM) we identified diur nal, semi-diurnal, and (possibly) terdiurnal tidal modes as the causal mechanisms for layer formation and transport with primary controls dr iven by meridional and zonal wind-shear forces. Poorest model-measurem ent correlations were at high latitude stations (dipole latitudes > 49 degrees) and when quantifying the relative magnitude of electric fiel d controls in the overall process of layer convergence and transport.