利用Grad-Shafranov重构方法研究磁尾通量绳的特征参量和内部结构

Characteristics and structure of magnetotail flux rope recovered from Grad-Shafranov method

2009年2月21日THEMIS-C卫星在磁尾X=-15.7RE(RE为地球半径,1RE=6371km)观测到典型的磁通量绳事件.采用Grad-Shafranov重构技术研究该磁通量绳的特性、内部磁场和电流结构.研究表明,磁通量绳不变轴位于GSM(geocentric solar magnetospheric coordinates)坐标为(-0.3975,0.8905,0.2213)的方向,基本位于晨昏方向;通量绳的横截面尺度约为2RE,内部轴向磁通量为1.3×106Wb.与经验模型相比,在对磁通量绳几何形状不做约束的情况下,重构出磁尾X=-15.7RE处磁通量绳横截面上的磁场、电流强度分布图像.通量绳核心部位具有无力场位形结构,而随着径向距离的增加,磁场在偏离轴对称分布的区域逐渐表现为非无力位形.

On 21 February 2009,THEMIS-C satellite observed the classical signal of magnetic flux rope at magnetotail of X=-15.7RE.We take the method of Grad-Shafranov reconstruction to investigate the characteristics and structure of magnetotail flux rope.The results give the characteristic physics parameters such as the invariant axis direction,the cross scale length,the magnetic flux in flux rope,etc.With no constraint from model on shape of the flux rope,we reconstruct the distribution of magnetic field and electric current density in the cross section of magnetic flux rope at magnetotail.Our results show that the magnetic field in the central region of magnetic flux rope can be described as being force-free,while with the increase of radial distance,the fields display no force-free feature in the region where the magnetic fields deviate from the axial symmetric distribution.