亚极光区极化流发生期间电离层离子下行的统计^*

During geomagnetically disturbed times, enhanced westward plasma flows appear in the duskside subauroral ionosphere and are termed as Subauroral Polarization Streams (SAPS). SAPS are usually believed as one of the most important features of the magnetosphere-ionosphere coupling and play an important role in regulating the ionospheric dynamics. Generally, SAPS can give rise to local ion upward flows, but there are few studies on ion downward flows. Based on the observations of DMSP satellites during 16 magnetic storm from 2001 to 2015, we find 102 cases with ion downward flows in all the 483 SAPS events. The occurrence rate of ion downward flows, the distribution of ion downward flows in the Northern and Southern Hemispheres, and the favored conditions for the formation of ion downward flows are further analyzed in these SAPS events. The results show that the occurrence rate of ion downward flow is relatively low in SAPS events, and the number and distribution of ion downward flow events are different on the two hemispheres. A linear relationship is found between the SAPS and ion downward flow velocities. Furthermore, the formation of ion downward flow is most probably caused by the combined effects of ambipolar diffusion and gravity. The intensity of ion thermal movement has significant influence on the velocity of ion downward flow. These results will help us to further understand the regulating effects of SAPS on the ionosphere.