Effects of Geomagnetic Storms on the Postsunset Vertical Plasma Drift in the Equatorial Ionosphere

It has been observed that geomagnetic storms cause suppression of the occurrence of equatorial spread F or plasma bubbles in the evening sector. In this study, we use ion drift data measured by the Communication/Navigation Outage Forecasting System satellite over 6years (2008-2014) to derive the dependence of the vertical ion drift at the prereversal enhancement peak on the strength of magnetic storms (the Dst index). It is found that the average vertical ion drift does not change much for Dst in the range between 0 and -60nT but decreases approximately linearly with the increasing magnitude of Dst for Dst < -60nT. The net decrease in the average vertical ion drift is similar to 30m/s when Dst changes from -60 to -90nT. This result is derived when the ion drift data during the storm main phase are excluded, so the decrease of the vertical ion drift is caused by storm time disturbance dynamo. A possible interpretation of this phenomenon is that geomagnetic activity must be strong enough (e.g., Dst < -60nT) so disturbance winds can reach the equatorial region and change plasma drifts there. The storm time disturbance dynamo becomes dominant in the equatorial ionospheric dynamics near the end of the storm main phase, similar to 4.7hr after the storm onset. The postsunset vertical ion drift is significantly decreased during the early stage of the storm recovery phase but becomes almost fully recovered when Dst increases close to -60 nT.