Ionospheric storm of early November 1993 in the Indian equatorial region

An investigation is made of the response of equatorial ionosphere in the In dian (75 degrees E) sector to the major magnetic storm of November 3, 1993, using data from the ionosonde and magnetometer networks spanning the regio n 0.3-34.5 degrees N dip. Some outstanding and new aspects of the storm tim e ionospheric behaviour are revealed. hn anomalous and striking positive gr adient in f0F2 from the magnetic equator to 34.5 degrees dip developed unde r counter electrojet (CEJ) condition in the morning on November 4, correspo nding to the early stage of the storm main phase. This storm effect is attr ibuted to plasma transport by a poleward surge in transequatorial winds due to large scale atmospheric gravity waves (AGWs) launched by auroral heatin g. Remarkable wave-like variations in hpF2 and f0F2 immediately followed at locations away from dip equator till local sunset, with concomitant disrup tions in the development of the equatorial ionization anomaly (EIA). Rapid variations in meridional neutral winds due to large-scale AGWs are assessed as the cause of the oscillations in hpF2 and the associated cyclic sequenc e of development and inhibition of EIA as the outcome of the combined effec ts of plasma transport due to meridional winds and the plasma "fountain" dr iven by EXB drift. Just after the onset of the storm recovery phase at 1800 LT, a sudden and anomalous drop (54-57%) in f(0)F(2) prevailed throughout the anomaly region over the interval 1915-2330 LT, with an apparent time de lay in occurrence toward the magnetic equator. This premidnight collapse of equatorial F region is interpreted in terms of horizontal transport of pla sma across the equator toward the opposite hemisphere by an equatorward sur ge in meridional winds. The case study showed that besides disturbances in the zonal electric field due to prompt penetration and ionospheric disturba nce dynamo effects, perturbations in neutral meridional winds played a prom inent role in the ionospheric storm of November 4, 1993, in the Indian equa torial region.