DAYTIME F2-LAYER POSITIVE STORM EFFECT AT MIDDLE AND LOWER LATITUDES

Daytime F2-layer positive storm effects at middle and lower latitudes in the winter thermosphere are analyzed using AE-C, ESRO-4 neutral gas composition data, ground-based ionosonde observations and model calculations. Different longitudinal sectors marked by the storm onset as night-time' and day-time' demonstrate different. F2-layer positive storm mechanisms. Neutral composition changes in the 'night-time' sector with increased [O] and [N-2] absolute concentrations, while (N-2/O)(storm)/(N-2/O)(quiet) approximate to 1 at F2-layer heights are shown to contribute largely to the background N(m)F2 increase at lower latitudes lasting during daytime hours. Storm-induced surges of the equatorward wind give rise to an additional N(m)F2 increase above this background level. The mid-latitude F2-layer positive storm effect in the 'daytime' sector is due to the vertical plasma drift increase, resulting from the interaction of background (poleward) and storm-induced (equatorward) thermospheric winds, but not to changes of [O] and [N-2] concentrations.