用非线性磁场梯度方法对空间磁结构模型的重构研究  

Reconstruction of magnetic structure models in space plasmas using nonlinear magnetic gradient method

作　　者：崔博 傅绥燕[1] 濮祖荫[1] 谢伦[1] 张驰[2] CUI Bo;FU SuiYan;PU ZuYin;XIE Lun;ZHANG Chi(School of Earth and Space Sciences,Peking University,Beijing 100871,China;Institute of Geology and Geophysics,Chinese Academy of Sciences,Beijing 100029,China)

机构地区：[1]北京大学地球与空间科学学院,北京100871 [2]中国科学院地质与地球物理研究所,北京100029

出　　处：《地球物理学报》2024年第10期3641-3654,共14页Chinese Journal of Geophysics

基　　金：国家自然科学基金(41731068)资助。

摘　　要：空间等离子体中的各种磁结构的形成是空间物理过程发展的产物,理解磁结构的拓扑形态对于认识空间物理过程有着重要作用.由于磁结构的三维(Three-dimensional, 3D)图像无法直接被观测到,因此通过卫星的局地观测来重建磁结构就成为一种很重要的研究途径.迄今已发展出多种空间磁结构的重构方法,其中“非线性磁场梯度”(Nonlinear Magnetic Gradients, NMG)方法(Shen et al., 2021)能够完备地利用四颗卫星星座的磁场与电流的局地观测和物理原理来计算星座质心处磁场的一阶梯度和二阶梯度以重构出星座附近磁场的近似形态.本文采用NMG方法重构了四种与磁层磁重联相关的动理学尺度磁结构模型:(i)一维(One-dimensional, 1D)Harris电流片与地球偶极磁场的叠加,星座质心位于赤道面X=-20R_E(地球半径)和Y=0;(ii) X线重联模型,星座质心位于X点(r=0);(iii)离子尺度轴对称的磁通量绳模型,星座质心位于通量绳轴线上;(iv) 3D分形线(separator)重联模型,星座质心位于正、负零点连线的中心.本工作重点对重构结果进行细致的误差分析及检验.结果表明,在距离星座质心显著大于10倍卫星平均间距空间范围内,用NMG方法获得的这四种磁场拓扑形态均与模型磁场相符.进而,对于前三个不存在磁零点的磁结构,NMG方法可以近似重现星座周边~10倍卫星平均间距空间范围内磁场模型的数值和方向,重构结果与模型磁场基本一致.对于存在两个磁零点的分形线重联模型,在接近与两个磁零点扇形(fan)面平行的方向上,NMG方法可以近似重现星座周边~5倍卫星平均间距空间范围内的磁场,在其他方向上NMG方法仍可以近似重现星座周边~10倍卫星平均间距范围内的磁场.由此可见,NMG方法为重构空间磁结构提供了一种有效的途径.但是必须注意,对于有磁零点的磁结构,扇形面的存在,会导致重构精度降低.There are various kinds of magnetic structure developed in space plasmas, and revealing of their magnetic topological characteristics is of critical importance for understanding the physical processes involved. Since the three-dimensional (3D) picture of the magnetic structures can hardly be directly obtained by satellite observations, magnetic field reconstruction based on in situ measurements is supposed to be an effective approach. Up to now a few reconstruction techniques based on spacecraft constellation measurements have been developed for this purpose. By completely using the magnetic field and current density measured by the satellites and employing the well-known physical laws (magnetic Gauss theorem, Ampere's law and rotational symmetry of the magnetic field) simultaneously, the "Nonlinear Magnetic Gradient" (NMG) method (Shen et al., 2021) can obtain the first order gradient vector and second order gradient tensor of the magnetic field at the barycenter location of the spacecraft constellation, and hence may best reproduce the magnetic field configuration near the spacecraft orbit. In this paper, we exam the validation of the NMG method in reconstructing four models of kinetic-scale magnetic structures which are in close relation with magnetospheric reconnection processes: (i) the superposition of the 1D Harris current sheet field and the Earth's dipolar field with the barycenter of spacecraft constellation at X=-20 RE (Earth radius) and Y=0 in the equatorial plane;(ii) the X-line reconnection mode,the barycenter of spacecraft constellation is at the X-point (r=0);(iii) the axial symmetric ion-scale flux rope model with the constellation barycenter located at the axis of the flux rope;and (iv) the 3D separator reconnection model with the barycenter located at the center of the ligature connecting the positive and negative magnetic nulls. We carefully analyze the error for each model to verify the construction results. Our results demonstrate that the NMG technique can approximately reproduce the magnet

关 键 词：磁结构 磁场重构 磁重联 磁场拓扑形态 

分 类 号：P353[天文地球—空间物理学]