Equatorial ionospheric plasma drifts and O ⁺ concentration enhancements associated with disturbance dynamo during the 2015 St. Patrick\textquoterights Day magnetic storm

Disturbance dynamo is an important dynamic process during magnetic storms. However, very few direct observations of dynamo-induced plasma drifts and ion composition changes in the equatorial ionosphere are available. In this study, we use measurements of the Defense Meteorological Satellite Program (DMSP) satellites to identify the characteristics of the disturbance dynamo process in the topside equatorial ionosphere near dawn during the magnetic storm with a minimum Dst of -223 nT on 17 March 2015. Data from four DMSP satellites with equatorial crossings at 0245, 0430, 0630, and 0730 LT are available for this case. The dynamo process was first observed in the postmidnight sector 3\textendash4.7 h after the beginning of the storm main phase and lasted for 31 h, covering the second storm intensification and the initial 20 h of the recovery phase. The dynamo vertical ion drift was upward (up to 150\textendash200 m s-1) in the postmidnight sector and downward (up to ~80 m s-1) in the early morning sector. The dynamo zonal ion drift was westward at these locations and reached ~100 m s-1. The dynamo process caused large enhancements of the O+ concentration (the ratio of the oxygen ion density to the total ion density) at the altitude of 840 km near dawn. The O+ concentration increased from below 60\% during the prestorm period to 80\textendash90\% during the storm time. More specifically, the O+ density was increased, and the H+ density was decreased. The variations of the O+ concentration were well correlated with the vertical ion drift.