Response of the equatorial and low-latitude ionosphere in the Indian sector to the geomagnetic storms of January 2005

The equatorial and low-latitude ionospheric response to three moderate geomagnetic storms ( 17, 18, and 22 January) during the period from 16 to 23 January 2005 is investigated in the context of development/inhibition of the Equatorial Ionization Anomaly (EIA) and the subsequent occurrence/nonoccurrence of Equatorial Spread F (ESF) irregularities on these days. The study is carried out using the Total Electron Content (TEC) measured with the GPS receivers along the similar to 80 degrees E longitude sector and the F-layer bottom height obtained from the Ionosonde located over the dip equatorial location of Trivandrum (8.5 degrees N, 77 degrees E, dip latitude similar to 0.5 degrees N) in India. It is observed that, for the storms on days 17 and 22, the development of the anomaly was inhibited, probably due to the westward disturbance dynamo electric fields. Subsequently, the post sunset enhancement in the vertical drift of the equatorial F region was also inhibited significantly compared to the quiet day pattern and, as anticipated, no ESF was observed on these days. A large vertical drift of the equatorial F region followed by nearly simultaneous onset of weak ESF was observed on day 18. The late development of the EIA on this day could be due to the eastward prompt penetration electric field associated with the southward turning of the interplanetary magnetic field. Also, strong and distinct F3 layer appeared for a short time in the morning, reappeared later in the noon time, and then quickly ascended to the topside ionosphere during the main phase of the storm on day 18.