高纬向阳侧磁层顶通量传输事件的特性研究--通量管轴线方位及运动分析

本文分析了2001年2月和3月期间Cluster Ⅱ穿越磁层顶前后的观测资料，检测到13个通量传输事件（FTEs）.用多颗卫星磁场测量资料的最小方差分析（MVAB）方法确定FTE的管轴方向（其中6个方向较可靠）.FTE管轴方向的分布和低纬处不同，在磁顶法线坐标系LMN中对M轴有较大偏离，比较靠近L轴.deHoffmann Teller(HT)分析指出，13个FTEs都存在一个很好的HT参考系，表明它们以一个准稳的MHD结构运动.对垂直于管轴方向的运动分析表明FTEs并不一定和背景等离子体一起对流，它们可快于或慢于背景流，但FTEs的运动和背景流基本沿相同方向，其间可有一不大的夹角.在HT坐标系中，10个FTEs的等离子体速度接近零, 其他3个FTEs的等离子体速度约为局地Alfven波速的14%，都不符合Walen关系.其中北半球事件的Walen曲线为正斜率，南半球事件为负斜率，这说明等离子体沿磁力线（北半球顺着磁场，南半球逆着磁场）流向磁层.

A study of orientation and motion of flux transfer events observed at the high-latitude dayside magnetopause

We investigate 13 FTEs,which were detected by the Cluster II in the magnetosheath near the cusp region during February to March 2001.The orientation of the FTE tube axis is derived by virtue of minimum variance analysis(MVA) using magnetic field data from the four Cluster spacecraft.Of 13 events,6 axis directions are determined reliably,most of them are closer to the L axis than to the M axis in the boundary normal coordinate system LMN.This implies that there are some differences between them and those events observed at the low-latitude magnetopause,whose axes tend to lie in east-west directions.The deHoffmann-Teller(HT) frame for 13 events are also obtained,and there are good HT frames for all FTEs.The correlation coefficients between the convection electric field and the HT electric field are larger than 0.96 except two of them are about 0.9. This means that the event passes the spacecraft as a moving quasi-static structure at V-(HT)(velocity of the HT frame).The FTE moving in the direction perpendicular to its axis is not convected with the ambient magnetosheath plasma,and the velocity of FTE tube may be slower or faster than the ambient plasma.The motion direction of FTE tube is about the same as the ambient plasma,but there may be a small angle between them.The plasma velocity in the HT frame approximates to zero for ten FTEs,and is only about 14% of the local Alfven speed for the others,which are all not satisfied with the Walen relation.The slope of the Walen plot is positive in the northern hemisphere and negative in the southern hemisphere.This implies that the plasma flows along the magnetic field into the magnetosphere(parallel magnetic field in the northern hemisphere and anti-parallel magnetic field in the southern hemisphere).