F2-region response to geomagnetic storms at the equatorial anomaly during 1989-2001

The storm-time effects in the F2-region during the period of 1989-2001, at Ahmedabad (23.2 degrees N, 72.4 degrees E). a station at the ionization anomaly crest in the Indian region, are studied. In general, the negative storm effects (decrease in f(o)F2) are more pronounced than the positive storm effects. Seasonally, the occurrence of negative storm effects is more in the summer and equinox as compared to that in the winter. Diurnal variation of storm effects shows that negative storm effects occur more often in the post-midnight period and less in the daytime. The negative storm effects associated with strong storms (Dst < -100 nT) and that occur in the post-midnight are mostly accompanied by a substantial increase in h'F. The complicated diurnal and seasonal variation of storm effects is explained in terms of (a) changes in E x B drifts (Fountain effect) associated with the zonal ionsopheric electric field perturbations due to prompt penetration of the magnetospheic electric fields to the low latitudes and the electric fields generated by the disturbance dynamo and (b) the downwelling of the thermospheric circulation due to storm-induced circulation. A possibility of changes in the neutral gas composition due to storm-induced circulation that may bring the gas with depleted [O]/[N-2] ratio up to the latitude of Ahmedabad during strong storm events, is also expressed to explain the post-midnight negative storm effects accompanied by a substantial increase in h'F.