Measurements and modelling of intermediate, descending, and sporadic layers in the lower ionosphere: Results and implications for global-scale ionospheric-thermospheric studies

We demonstrate that conventional ionosondes can provide long‐term observations of intermediate, descending, and transitional layers in the 100–200 km altitude region of the ionosphere. Using 15 consecutive days of observations at Townsville, Australia, during the SUNDIAL campaign of September 1989, we tracked the “birth” of the layers at altitudes above 150 km and their systematic downward motion to the 110 km region at rates between 4 and 5 km/hr. The observations are compared with NCAR TIGCM simulations, and the results show: (i) that the layering process is identifiable with meridional wind‐shear‐node convergence of ions; (ii) that zonal wind controls of the layers are insignificant under the prevailing conditions; and (iii) that electric fields play an important role in the effectiveness of the ion‐convergence and downward transport processes at altitudes above 125 km. The measurement and modelling comparisons are the first of a kind, providing insight into the relative roles of winds and electric fields, and opening possibilities for determining the global characteristics of the layers and their cause‐effect roles in the dynamics of the lower ionospheric‐thermospheric domain.