Westward plasma drifts in the equatorial ionosphere during severe magnetic storms: A new type of penetration electric fields caused by subauroral polarization stream

Abstract:

Penetration electric fields during geomagnetic storms have important effects on the equatorial ionospheric electrodynamics. Previous studies focus on the vertical ion drifts, corresponding to electric fields in the zonal direction, in the equatorial ionosphere. In this study, we analyze the characteristics of the zonal ion drifts in the equatorial region measured by the Defense Meteorological Satellite Program (DMSP) satellites during ten severe magnetic storms with a minimum Dst < -200 nT in the solar maximum years of 2000-2003. It is found that the net change of the equatorial zonal ion drifts caused by magnetic storms is westward and can reach 200-300 m/s in the evening sector and that the westward ion drifts remain in the same direction during both the storm main and recovery phases. The storm-induced zonal ion drifts are highly correlated with the Dst index, the correlation coefficient between the average zonal drift and the average Dst value during the ten storms is 0.89. The magnitude of the storm-induced westward ion drifts is approximately linearly proportional to the Dst index in the evening sector, and on average, a change of -100 nT in Dst causes a change of -52 m/s in the zonal drift. A new mechanism is proposed to explain the generation of the westward plasma drifts in the nighttime equatorial ionosphere. This new mechanism is that the poleward electric fields associated with the subauroral polarization stream (SAPS) penetrate or extend to low latitudes, producing the westward plasma drifts in the equatorial region. The results of this study provide new insight into equatorial ionospheric electrodynamics during geomagnetic storms.