Abstract. We report on a study of three intense ionospheric storms that occurred in September 1989. Using Dst as a reference for storm onset and subsequent main and recovery phases, we analyze the observed worldwide responses of F region heights hmF 2 and densities NmF 2 as a function of universal and local times, latitudinal domains, and storm onset-times; and we compare the characteristics of all three storms. The following points are among the major findings: (1) The negative phase storm was the dominant characteristic, with the greatest intensity occurring in the regions which were in the nighttime hemisphere during the main phase; (2) at middle and low latitudes negative phase characteristics were observed first in the nighttime hemisphere and then corotated with the Earth into the dayside; (3) the most intense negative response occurred in the recovery phase; (4) observations of the negative phase characteristics supported thermospheric upwelling, increased mean molecular mass, and an associated enhancement in dissociative recombination as the principal cause-effect chain; but the observations suggest greater ion-neutral chemistry effects than accounted for in current models; (5) hmF2 was observed to respond quickly to the storm onset (pointing to the importance of electric fields) with enhanced values in all latitudinal and local time domains; (6) positive storm characteristics were among the issues most difficult to reconcile with current descriptions of cause-effect relationships; and (7) the analysis of all storm phases and comparisons with several modeling efforts show that future advances in understanding require a more accurate accounting of the influences of magnetospherically-imposed and dynamo-driven electric fields, plasmaspheric fluxes, and vibrationally excited N 2.

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.

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The nonlinear evolution of the local collisional Rayleigh‐Taylor instability in downward‐moving equatorial F layers has been studied in a coordinated theoretical and experimental program dealing with actual conditions of bottomside spread F. For ambient bottomside electron density gradient scale lengths L=8 and 25 km, we find large percentage depletions and inverse power law spatial power spectra over the intermediate wavelength range λ=25 m‐1 km. In addition we outline a nonlinear theory of the collisional Rayleigh‐Taylor instability applicable to an upward‐ or downward‐moving equatorial F region ionosphere. The agreement between the experimental and theoretical results of this study lends further support to the belief that the collisional Rayleigh‐Taylor instability is responsible for large‐scale size irregularities that occur under conditions of equatorial spread F.