Geomagnetic storms and their ionospheric effects observed at the equatorial anomaly crest in the Indian Region

This paper reports the interplanetary causes of some selected magnetic storms that occurred during the period of 1999-2001. The storm-time (st) effects in the ionospheric F2-region during the periods of 1989-1991 and 1999-2001, and VHF scintillations during the period of 1999-2001, at Ahmedabad, a low latitude station at the ionization anomaly crest in the Indian region, are also studied. A southward interplanetary magnetic field (IMF BS) is the precondition for the formation of the storms. The main phase of 60% of intense and very intense storms occurs by a multi-step growth in the ring current. First-step growth in the ring current is due to the IMF BS in the sheath region behind the fast forward shock. Second-step growth in the ring current is possibly due to ionospheric origin ions (particularly O+) which can be identified by the initial fast recovery of the storms. Magnetic clouds are also probable sources of the growth in the ring current. The recovery phase begins with northward turning of IMF with a delay ≈1-3 h. Ionosonde data show both positive and negative effects in foF2; the negative storm effects are more pronounced with a maximum occurrence in the post-midnight period. The interplay between the st thermospheric circulation and the undisturbed circulation determines the occurrence of these effects. The negative storm effects that occur in the post-midnight period are mostly accompanied by a substantial increase in h′F. A decrease in foF2, during intense and very intense magnetic storm, due to storm-induced circulation that brings high latitude gas with depleted [O]/[N2] ratio to low and equatorial latitudes, dominates an increase in foF2 due to an increase in h′F, and results in a net decrease in foF2 in the post-midnight period. The penetrating strong magnetospheric electric fields and the electric fields generated by the disturbance dynamo play important roles. The probability of occurrence of scintillations in the post-midnight sector is enhanced for storms in which Dst⩽-75 nT occurs in the post-midnight to dawn sector. Magnetic storms with Dst<−100 nT have been found to be most effective in producing such scintillations. Reversal of the equatorial ionospheric electric field due to the penetration of st magnetospheric electric fields explains the occurrence of such scintillations.