The structure of flux transfer events recovered from Cluster data

摘要

The structure and formation mechanism of a total of five Flux Transfer Events(FTEs), encountered on the equatorward side of the northern cusp by theCluster spacecraft, with separation of ~5000 km, are studied byapplying the Grad-Shafranov (GS) reconstruction technique to the events. Thetechnique generates a magnetic field/plasma map of the FTE cross section,using combined magnetic field and plasma data from all four spacecraft, underthe assumption that the structure is two-dimensional (2-D) andtime-independent. The reconstructed FTEs consist of one or more magnetic fluxropes embedded in the magnetopause, suggesting that multiple X-linereconnection was involved in generating the observed FTEs. The dimension ofthe flux ropes in the direction normal to the magnetopause ranges from about2000 km to more than 1 R_E. The orientation of the flux ropeaxis can be determined through optimization of the GS map, the result beingconsistent with those from various single-spacecraft methods. Thanks to this,the unambiguous presence of a strong core field is confirmed, providingevidence for component merging. The amount of magnetic flux contained withineach flux rope is calculated from the map and, by dividing it by the timeinterval between the preceding FTE and the one reconstructed, a lower limitof the reconnection electric field during the creation of the flux rope canbe estimated; the estimated value ranges from ~0.11 to ~0.26mV m^{-1, with an average of 0.19 mV m-1. This can betranslated to the reconnection rate of 0.038 to 0.074, with an average of0.056. Based on the success of the 2-D model in recovering the observed FTEs,the length of the X-lines is estimated to be at least a few R_E.}