On the occurrence of storm-induced nighttime ionization enhancements at ionospheric middle latitudes

Ionospheric observations from nine middle-latitude stations are studied for five magnetic storms that occurred during September and October 2000. The correlation between various solar wind, magnetospheric and ionospheric parameters shows that the nighttime ionospheric response is strongly dependent on the conditions during which solar wind-magnetosphere coupling occurred. Storms with initial compressive phase and rapidly evolving main phase have as a global effect the ionization depletion in the nightside at middle latitudes, independent of the storm intensity. These storms are caused by the abrupt dissipation of a large amount of energy input, resulting in the rapid expansion of the neutral composition disturbance zone equatorward, producing the observed negative effects in all middle latitude stations presented here. Gradually evolving geomagnetic storms, driven by slowing increasing southward IMF, result in the observation of positive effects at night in low to middle latitude stations. The weaker the intensity of the storm is, according to the Dst index, the more likely it is that one will observe nighttime ionization enhancements in subauroral latitudes as well. There are two competing mechanisms causing the observed effects; the expansion of the neutral composition disturbance zone results in negative effects, while downward plasmaspheric fluxes produce ionization enhancements at night. Gradually evolving storms are characterized by the restricted development of the neutral composition disturbance zone to higher latitudes, and the extent of its equatorward boundary depends on the intensity of the storm. During storms of this type, the role of plasmaspheric fluxes dominates at middle to low latitudes. Their effects are observable up to subauroral latitudes given that the neutral composition disturbance zone development is restricted to higher latitudes, as happens when the geomagnetic activity is of low or moderate intensity.