Solar Activity Dependence of Ionosphere Ion Upflow in the Polar Topside

The influence of solar activity on ionosphere ion upflow was studied, over the northern and southern hemispheric polar topside. The observations were obtained from the Defense Meteorological Satellite between 1995 and 2005. Results show that the response characteristics of upflow events on solar activity are roughly consistent for the two hemispheres, that is, ion upflow fluxes and ion densities are generally larger during high solar activity than those during low solar activity, but ion vertical velocities and upflow occurrence rates show an opposite variation trends, i.e., they are higher and more frequently seen under the conditions of low solar activity, respectively. Besides, we further calculated the mean value of upflow parameters in the southern hemisphere. The ratios of them under high and low solar activity conditions are about 2.26, 3.35, 0.71 and 0.51. Furthermore, the results also show that it is the ion densities instead of ion vertical velocities that have profound effect on the upflow fluxes. The causes of such a dependence is briefly analyzed. Under different levels of solar activity, ionosphere ion densities are different because of the variations of ionization rates and ion-neutral collision frequencies. The collision frequency between ion and neutral atmosphere is closely related to the ion density. Therefore, the ion upflow occurrence rate in the polar topside ionosphere depends on the level of solar activity.