## 1 Introduction  
Geomagnetic storms are associated with space weather phenomena and strong storms can impact the whole earth's ionosphere [1, 2, 3, 4]. The intense geomagnetic storms can cause major problems in the satellite communication and navigation systems and ground-based High Frequency communication systems (e.g., scintillations, radio blackouts) [5]. The ionospheric effect of geomagnetic storms known as an ionospheric storm varies with latitude, time of day of storm occurrence, season, and phase of the solar activity. The ionospheric effect involves complicated mechanisms including neutral winds, storm-time thermospheric composition changes and storm-generated high-latitude electric fields propagating toward equatorial latitudes and coupling with normal ionospheric electric field [1; 6; 7; 8]. The ionospheric storm is associated with changes in the ionospheric electron density (\(N_{e}\)) or the critical frequency of the \(F_{2}\)-region, _f_/6F2 [9] based on which it is classified as a positive ionospheric storm (an increase in the _f_/6F2 or \(N_{e}\)) and negative ionospheric storm (a decrease in the _f_/6F2 or \(N_{e}\)). The ionosphere-thermosphere coupling between different latitude regions is associated with the dynamic and electrodynamic coupling from high to low latitudes and the chemical changes during the geomagnetic storms [6; 8]. [PERSON] et al. [6] studied the effects of a major geomagnetic storm of 29-30 October 2003 at the stations, Ho Chi Minh City (10.5\({}^{\circ}\) N, 106.3\({}^{\circ}\) E), Vietnam and Okinawa (26.3\({}^{\circ}\) N, 127.8\({}^{\circ}\) E), Japan, in the East Asian sector. Authors reported both positive and negative ionospheric storms in _f_/6F2 at these stations. Storm-time perturbed high-latitude electric fields play a major contribution in the occurrence of positive and negative ionospheric storms. The perturbed electric fields during geomagnetic storms are categorized into three groups: (1) prompt penetration electric fields (PPEFs), (2) overshielding electric field, and (3) disturbance dynamo electric fields (DDEFs) [6; 7; 9; 10; 11; 12]. The PPEFs are associated with the southward \(Z\)-component of the interplanetary magnetic field (IMF \(B_{Z}\)) and are caused by other solar wind and magnetospheric processes [8]. PPEFs are usually short-lived, around an hour [9; 11] during the main phase of the storms but can exist for 6-8 h during intense geomagnetic storms [13; 14]. Upon the dayside ionosphere, the southward turning of the north-south (\(Z\)) component of IMB \(B_{z}\) results in the eastward PPEFs and northward turning leads to the westward PPEFs. The PPEFs can cause a large enhancement in _f_/6F2 or \(N_{e}\) in the dayside and decrease at the night side _f_/6F2 or \(N_{e}\)[9; 10; 11] due to changes in the normal \(\mathbf{E}\times\mathbf{B}\) drifts which are upward in the daytime and downward at the night. Overshielding electric field is associated with the northward turning of IMF \(B_{Z}\) when PPEFs are withdrawn towards the end of the main phase of the storm and is in the opposite direction in the dayside and nightside [12]. DDEFs are caused by the ionospheric distance dynamo effect that results from thermospheric wind circulation due to Joule heating at the high latitudes associated with geomagnetic storms. DDEFs are in opposite direction to quiet time ionospheric electric field during both day and night sides, that is, DDEFs are westward (eastward) during the dayside (nightside). The westward polarity of DDEFs reduces the _f_/6F2 or \(N_{e}\) in the dayside and results in the contraction of equatorial ionization anomaly (EIA). On the other hand, eastward polarity reduces the F-layer downward drift at night and enhances the _f_/6F2. DDEFs are slowly varying and usually long-lived. Since their arrival at the low latitudes takes about one to two days after the onset of the geomagnetic storm, their effect is dominant during the recovery phase of the storms.