2001年3月2日磁通量传输事件特性的研究

A STUDY OF THE CHARACTERISTICS FOR THE MAGNETIC FLUX TRANSFER EVENTS OCCURRED ON MARCH 2, 2001

20 0 1年 3月 2日 1 1 :0 0至 1 1 :1 5UT期间 ,ClusterⅡ在南半球极尖区晨侧附近磁鞘内探测到 3个通量传输事件 (简称FTEs) .本文利用ClusterⅡ星簇 4颗卫星观测到的磁场和等离子体资料研究了这些通量传输事件的磁场形态和粒子特征 .并利用它们探测到的空间磁场梯度资料由安培定律直接求出星簇所在区域的电流分布 .结果指出 :(1 )BY 占优势的行星际磁场结构在磁层顶的重联可以在极尖区附近发生 ;(2 )FTEs通量管形成初期内外总压差和磁箍缩应力不一定平衡 ,达到平衡有一发展过程 ;(3)FTEs通量管截面在L M平面内的线度约为 1 .89RE;(4 )FTEs通量管中等离子体主要沿轴向场方向流动 ,整个通量管以慢于背景等离子体的速度沿磁层顶向南向尾运动 ;(5 )FTEs通量管中不仅有轴向电流 ,也存在环向电流 .轴向电流基本沿轴向磁场方向流动 .轴向和环向电流在管内均呈体分布 ,因而轴向电流产生的环向磁场接近管心时不断减小到零 ,而环向电流生成的轴向场则不断增大到极值 ;(6 )在通量管的磁鞘部分观测到磁层能量粒子流量的增强 ,这表明通量管通过磁层顶将磁鞘和磁层内部连通起来了 .

Three flux transfer events (FTEs) were detected in magnetosheath near the south cusp of the dawn side by Cluster Ⅱ during the interval from 11:00 to 11:15 UT on March 2, 2001. In this paper, we analyze the magnetic morphology and the plasma characteristics of these events using the magnetic field and plasma data observed by the four spacecrafts of Cluster Ⅱ. And the electric current density in the region Cluster Ⅱ passed is estimated from the Ampere's law in terms of measured spatial gradients of the magnetic field observed simultaneously by four spacecrafts of Cluster Ⅱ at different places. The results are as follows: (1) The magnetic reconnection at the magnetopause can occur near the cusp when the IMF By component is dominant; (2) The difference of p+ B 2/8π (the sum of magnetic and plasma pressure, measured inside and outside the events) may not be balanced with the tension of the helical magnetic filed at the initial stage when a flux tube is forming, it needs a growing time to reach an equilibrium; (3) The size of the section in the L M plane of the FTEs flux tubes is about 1.89 R E; (4)The plasma inside the flux tubes flows dominantly along the axis aligned field, at the same time the flux tubes move southward to the tail and its velocity is slower than the background plasma; (5) There is not only the axis aligned current in the FTEs flux tubes, but also the annular current. The axis aligned current flows basically along the axis aligned magnetic field. Because both the axis aligned and annular currents distribute in the whole flux tubes, thus the annular magnetic field produced by the axis aligned current will decrease to zero gradually near the center of the flux tube, and the axis aligned magnetic field produced by the annular current will increase to it's maximum by degrees; (6) An enhancement of magnetospheric energetic particle flux was observed inside the magnetosheath portion of the flux tubes. This implies that the flux tubes connect the magnetosheath with the magnetosphere through the magnetopause.