利用Cluster卫星磁尾单事例研究具有复杂核心磁场结构的磁通量绳

2002年8月28日09:50UT-10:50UT，Cluster卫星在地球磁尾观测到一次导向场磁场重联事件.卫星观测到磁场重联扩散区附近清晰的霍尔（Hall）四极型磁结构.由于导向场的存在，该四极型结构被扭曲变形.在该磁场重联事件中，卫星观测到多个磁通量绳，大部分磁通量绳的核心场极性与导向场极性一致.但是，其中一例磁通量绳的核心场结构极性较复杂.该例磁通量绳中心区域核心场强度出现峰值，核心场极性和导向场极性一致；中心以外区域的核心场极性和导向场极性相反.这种复杂核心场结构以前未见报道.通过最小方向导数法，发现该磁通量绳的轴向是弯曲的.C1和C3卫星穿越了磁通量绳弯曲部分，探测到核心场极性变化；C2和C4卫星位于C1和C3卫星的北侧，仅穿越了磁通量绳弯曲处的一部分，故核心场具有单极性. 

Magnetic Flux Rope with a Complicated Core Field by a Cluster Case

The Cluster spacecraft observed a magnetic reconnection event at the Earth magnetotail during 09:50 UT-10:50 UT on 28 August 2002. Cluster observed an evident quadrupole structure of the Hall magnetic field near the magnetic reconnection diffusion region. Due to the existence of the guide field, this quadrupole structure was distorted. During this reconnection event, Cluster observed a few magnetic flux ropes, and the polarity of the core field of most magnetic flux ropes is consistent with the polarity of the guiding field. However, one particular magnetic flux rope has a more complex structure of the core field than the others. In the core region of the flux ropes, the intensity peaks of the core field appeared there and its polarity is agreement with the guide field. Outside the core region, the polarity of the core field is opposite to that of the guide field. This complex structure of the core field within a flux rope has not been reported until now. Using the method of Minimum Directional Derivative (MDD), it is found that the flux rope was curved at the place where Cluster passed through. C1 and C3 satellites crossed the curved flux rope and detected the core field contrary to the guide field. C2 and C4 satellites situated on northward of C1 and C3 satellites, only crossed one part of the curved flux rope and thus detected the core field with a polarity consistent with the ambient magnetic field.