A Study of Orientation and Motion of Flux Transfer Events Observed at High-Latitude Dayside Magnetopause

Abstract

This paper investigates 13 FTEs, which were detected by Cluster II in 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 the 13 events, 6 axis directions are determined reliably, most of them are closer to L axis than to M axis in the boundary normal coordinate system LMN. This implies that there are some differences between them and those events observed at low-latitude, 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 these events pass spacecraft as a moving quasi-static structure at V HT (velocity of the HT frame). The motion of FTE flux tubes in the direction perpendicular to its axis is not convected with the ambient magnetosheath plasma, the velocity of FTE tube may be slower or faster than the ambient plasma. The motion direction of FTE tube is about the same with the ambient plasma, but there may be a small angle between them. The plasma velocity in HT frame approximates to zero for ten FTEs, and is only about 14% of the local Alfven speed for the others, they are all not satisfied with theWalen relation. The slope of Walen plot is positive in northern hemisphere and negative in 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).