A comparative study of global ionospheric responses to intense magnetic storm conditions

<jats:p>We report on a study of three intense ionospheric storms that occurred in September 1989. Using <jats:italic>Dst</jats:italic> as a reference for storm onset and subsequent main and recovery phases, we analyze the observed worldwide responses of <jats:italic>F</jats:italic> region heights <jats:italic>h<jats:sub>m</jats:sub>F</jats:italic><jats:sub>2</jats:sub> and densities <jats:italic>N<jats:sub>m</jats:sub>F</jats:italic><jats:sub>2</jats:sub> 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) <jats:italic>h<jats:sub>m</jats:sub>F</jats:italic><jats:sub>2</jats:sub> 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<jats:sub>2</jats:sub>.</jats:p>