磁尾等离子体团型结构的数值模拟

考虑横越磁尾不同区域的数密度与离子温度的分布特点,取宁静磁尾温度、密度呈同样形态的非均匀分布,作为模拟计算的初态,对初始By为不同分布的3个算例作模拟计算．数值结果展示了3类磁结构的演化特征．考察磁尾中性片一个给定点的磁场变化,做出3个算例典型事件的磁场矢端图．它们从另一个侧面展示了磁尾通量绳型等离子体团、具有复杂闭合磁力线位形类似于“闭合环”的等离子体团,以及二维“磁岛”型等离子体团的特点．观测表明,多数磁尾等离子体团为具有强核心场的通量绳结构．通过跟踪一个通量绳型等离子体团的发展,作出等离子体团各方向速度与磁场强度随x演化的曲线．其中,vx与磁场强度|B|在近尾至中尾的计算值与Geotail卫星资料统计分析结果大致相符．此外,与Jin等取初始温度为均匀分布的计算结果相比,本文给出的通量绳型等离子体团,其内温度较高、密度较低,与资料分析结果的偏离也随之减小．

NUMERICAL STUDY ON PLASMOID-LIKE STRUCTURES IN THE MAGNETOTAIL

In view of the distribution features of the temperature and density across the different regions of the quiet magnetotail, the same non-uniform distributions of the temperature and density are used as the initial state of the simulation study. In this report, three cases with the different distributions of initial B_y component are investigated. The numerical results illustrate the characteristic evolution of three kinds of magnetic structures. For the varying magnetic field at a given point in the neutral sheet, the hodograms of the typical events are performed. From another point of view, the hodograms of three cases show the features of flux rope structure, plasmoids with Complex closed magnetic field lines and two-dimension "magnetic island", respectively. It was demonstrated from the observations that most plasmoids in the magnetotail are the flux rope structures with a large core field. Monitoring the development of a flux rope structure, the profiles of the directional velocity v_x, v_y, V_z and magnetic field strength B in it as a function of the distance x are given. The computing values of v_x and B are approximately consistent with the observed values in the near and middle tail by Geotail. Moreover, in comparison with Jin et al.'s simulation with a constant initial temperature, in the present work the temperature of plasmoids increases and the density of plasmoids decreases notably, and so the difference between the numerical results and observation is reduced.