Interplanetary origin of magnetic storms and their F2-region responses at the equatorial anomaly

The interplanetary origin of 102 storms (Dst ⩽ −50 nT) of varying strength that occurred during the period of 1997–2001, have been studied. The shock compression, magnetic clouds, and shock compression followed by magnetic clouds are most probable sources of intense southward component BS of IMF in the interplanetary medium causing the magnetic storms. Thirty five percent of magnetic storms were associated with the magnetic clouds. The main phase of 60% intense and very intense magnetic storms associated with the magnetic clouds, occurred through the multi-step growth in the ring current during the main phase evolution of the storms. The storms were associated with enhanced IMF intensity, proton bulk velocity, ion dynamic pressure and ion density, in the solar wind, in general, with increase in strength of the storms, but not linearly. The storm-time effects in the ionospheric F2-region during 51 magnetic storms during 1997–2001, at Ahmedabad (23.2°N, 72.4°E, dip angle 30.7° at sub-ionospheric points), a station at the ionization anomaly crest in the Indian region, are also studied. The negative ionospheric effects are more pronounced than the positive ionospheric effects. While the prompt penetration of high-latitude electric fields and the electric fields generated by the disturbance dynamo play important role in F2-region response by changing E × B drifts at low latitude, the storm-induced transport of gas with depleted ratio of [O]/[N2] may also be responsible for the post-midnight large decrease in foF2, during strong magnetic storms (Dst < 100 nT).