中文摘要：

等离子体片高速流在磁层活动中起着重要作用，其形成机制以及与背景等离子体的相互作用日益引起关注．本文利用搭载于Cluster四颗卫星上的磁场和等离子体观测仪器，对2001和2002两年发生在磁尾等离子体片中高速流事件期间的磁场变化进行了统计研究．结果表明，在高速流前端，伴随着等离子体整体速度的增加，绝大多数高速流前端磁场的BZ分量经常出现先短暂减小然后剧烈增大的现象，符合以往利用Geotail卫星观测数据获得的统计特性．然而个例研究发现BZ的下降与上升常常是不对称的，且BZ分量下降的程度并不是总能达到反向的程度，说明这种变化特征并不一定是存在磁结构的表现．我们认为更多时候这种磁场的变化特征是高速流挤压背景等离子体磁场造成的，是相互作用的结果．当偶极化锋面形成后，由类似间断面的磁场界面反射的热离子产生抗磁效应，可能对BZ下降形成部分贡献，而BZ增加则是高速流携带磁通量堆积的效果．

英文摘要：

Abstract Bursty Bulk Flows （BBFs） play an important role in the magnetotail activities. The formation mechanism of BBFs and the interaction between BBFs and the ambient plasma have become important research subjects recently. Based on the observations made by Cluster during the periods from July to October 2001 and 2002, the variations of magnetic field during the passage of BBFs are statistically analyzed. Superposed epoch analyses show that there is a sharp change in the Bz component as the bulk speed of the flow begins to increase at the front of BBFs： a sharp increase preceded by a transient decrease. This signature in Bz component was found in most of the BBFs events observed by Cluster, which is generally agreed with the result reported based on Geotail observations. However, the change of Bz component is not always symmetry, and the decease of Bz is usually not strong enough to lead to a negative value. This implied that magnetic structure does not always occur together with BBFs. The interaction between BBFs and ambient plasma isresponsible for the change in Bz. After the dipolarization front is formed, the diamagnetic effect due to the hot ions bouncing back from the boundary might contribute to the decrease in Bz. The pile-up of the magnetic flux carried by BBFs mac be the reason for the incre.ase