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

Abstract

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.

On the nature of nighttime ionisation enhancements observed with the Athens Digisonde

“…Within the last few years, it was well confirmed that positive storm effects are also observed in the night-side hemisphere (Oliver and Hagan, 1991;Burnside et al, 1991;Szuszczewicz et al, 1998;Tsagouri et al, 2000;Belehaki and Tsagouri, 2002). They are also predicted by numerical simulations (Mansilla and Manzano, 1998) and many responsible mechanisms have been occasionally proposed.…”

“…They also maintain that if a positive phase is driven by winds before dusk, it will rotate into the nightside and nighttime positive effects of this type have been observed (Szuszczewicz et al, 1998;Tsagouri et al, 2000). However, the understanding of positive phase observations is not always so simple, such as in cases of nighttime initiated positive effects.…”

“…This could also be the case in the expansion phase. Downdwelling could be a candidate mechanism for positive phase storms commencing at night (Szuszczewicz et al, 1998). In a recent work, Belehaki and Tsagouri (2002) demonstrated that the existence of a new source of ionisation is necessary to explain nighttime positive effects and, therefore, the increased nighttime density can only be speculated to be due to increased downward fluxes from the plasmasphere (Rishbeth, 1987;Richards, 1994).…”

“…In a recent work, Belehaki and Tsagouri (2002) demonstrated that the existence of a new source of ionisation is necessary to explain nighttime positive effects and, therefore, the increased nighttime density can only be speculated to be due to increased downward fluxes from the plasmasphere (Rishbeth, 1987;Richards, 1994). In fact, the plasmaspheric flux term can also modify the value of hmF2 and contribute to the height increase (Szuszczewicz et al, 1998).…”

“…For example, Oliver and Hagan (1991), in a study of storm-induced gravity waves, invoked the assumption of downward plasmaspheric fluxes as a source of enhanced nighttime densities. This mechanism is an open issue even in normal conditions (Szuszczewicz et al, 1998). It is not ascribed to any storm mechanism, although there is observational and theoretical evidence for the existence of storm time gravity-waveinduced perturbations in atmospheric densities (Trinks and Mayr, 1976;Mayr and Volland, 1976;Potter et al, 1976).…”