地球磁层顶通量传输事件的轴向研究

发布时间：2019-05-16 13:18

【摘要】：磁层顶通量传输事件(Flux Transfer Event,FTE)被认为是与磁场重联相关的现象,其标志为卫星测量磁场在磁层顶法向分量的双极变化。迄今为止,关于FTE的本质、形成机制和产生位置这些基本问题还存在着一些争论。确定FTE的轴向不仅对于研究FTE的几何结构以及理解其中等离子体动力学是必要的,也可以帮助我们区分FTE的不同形成机制和产生位置。本文以FTE轴向研究为主线,主要内容及结论总结如下:(1)发展了一种新的多点分析方法去确定FTE的轴向。这种方法建立在结构为左右对称的纯几何假设上,对于嵌在磁层顶电流片中的FTE结构,不管其物理性质如何,都基本上满足这一对称条件,所以新方法就有着较广泛的应用性。模型测试和实际应用表明,新的方法在确定轴向时比传统方法如Grad-Shafranov(GS)方法和磁场最小方差分析法等以及更近的基于圆柱对称的方法更有效和可靠。(2)在确定了FTE轴向以及在合适的多卫星位形下,设计了一种新方法去重构FTE磁场的空间分布。该方法对THEMIS和Cluster各自观测到的两个FTE的应用表明它能快速而有效地重构出结构的磁场空间分布,从而帮助我们推测结构磁场位形、理解卫星测量数据的时间变化以及获得其它物理量相对于结构磁场的空间分布特征。(3)利用新的FTE轴向确定方法进行了首个关于FTE轴向的统计分析。其结果揭示出:尽管在昏侧低纬观测到的FTE大多有着接近南北方向的轴向,其来源仍然是日下点分量重联线;与传统认识中日下点分量重联线为一条方向几乎不变的分隔线不同,实际的日下点分量重联线在磁层顶有着相当大的曲率。(4)首次发现多X线重联形成FTE的磁场信号。这种磁场信号的表征为的多极变化(比如双或者三-双极变化)。GS重构结果表明在有着如此磁信号的事件中包括多个子结构,这与多X线重联形成多个通量管的图景相符;但各子结构在磁层顶上依序从大到小排列,这一特征是传统的多X线重联模型所未预测到的。为解释这一现象,我们提出了一个新的模型去描述FTE的形成过程:重联持续在磁层顶发生,在主X线附近磁场形成楔形位形;重联率周期性地变化,在其增长时楔形磁场线内由撕裂模不稳定性形成了连续的多个小尺寸的磁岛结构;这些小的磁岛结构互相合并,由于楔形磁场的限制,形成按尺寸大小依序排列的多个FTE,最后在磁鞘流压力和磁张力作用下离开重联位置。(5)分析了在昏侧低纬磁层顶内遥测到的一系列“反常”日向运动的连续FTE。将事件的运动速度与FTE运动模型相比较可发现,为对抗此时在昏侧磁层顶附近较大的尾向磁鞘流速度,实际重联磁场的磁张力要比模型给出的磁张力大很多,这说明在磁流体动力学效应可能起重要作用的时候,现有模型并不能准确地描述磁鞘磁场和等离子体环境,因此需要修正。
[Abstract]:The magnetopause flux transmission event (Flux Transfer Event,FTE) is considered to be a phenomenon related to the reconnection of the magnetic field, which is marked by the bipolar variation of the normal component of the magnetic field measured by the satellite at the top of the magnetopause. So far, there are still some debates about the essence, formation mechanism and location of FTE. It is necessary to determine the axial direction of FTE not only to study the geometric structure of FTE and to understand the plasma dynamics, but also to help us distinguish the different formation mechanism and generation position of FTE. In this paper, the axial research of FTE is taken as the main line, and the main contents and conclusions are summarized as follows: (1) A new multi-point analysis method is developed to determine the axial direction of FTE. This method is based on the pure geometric assumption that the structure is left and right symmetry. For the FTE structure embedded in the magnetopause current sheet, regardless of its physical properties, it basically satisfies this symmetry condition, so the new method has a wide range of applications. The model test and practical application show that, The new method is more effective and reliable in determining the axial direction than the traditional methods such as Grad-Shafranov (GS) method and magnetic field minimum variance analysis method, as well as the closer method based on cylindrical symmetry. (2) the axial and coincidence of FTE are determined. Under the suitable multi-satellite configuration, A new method is designed to reconstruct the spatial distribution of FTE magnetic field. The application of this method to the two FTE observed by THEMIS and Cluster respectively shows that it can reconstruct the magnetic field spatial distribution of the structure quickly and effectively, thus helping us to speculate the magnetic field configuration of the structure. Understand the time variation of satellite measurement data and obtain the spatial distribution characteristics of other physical quantities relative to the structural magnetic field. (3) the first statistical analysis of FTE axis is carried out by using the new FTE axial determination method. The results show that although most of the FTE observed in the low latitudes of the fainting side have an axial direction close to the north and south, the source is still the subsolar component reconnection line. Different from the traditional understanding that the Japanese and lower point component reconnection line is a separation line with almost unchanged direction, the actual diurnal point component reconnection line has considerable curvature at the top of the magnetosphere. (4) for the first time, it is found that the magnetic field signal of FTE is formed by multi-X-ray reconnection. GS reconstruction results show that there are multiple substructures in events with such magnetic signals, which is consistent with the pattern of multiple flux tubes formed by reconnection of multiple X-rays. However, the substructures are arranged sequentially from large to small on the top of the magnetosphere, which is not predicted by the traditional multi-X-ray reconnection model. In order to explain this phenomenon, we propose a new model to describe the formation process of FTE: reconnection continues to occur at the top of the magnetosphere and forms a cuneiform configuration in the magnetic field near the main X-ray; The reconnection rate changes periodically, and a continuous number of small island structures are formed by tearing mode instability in the cuneiform magnetic field line as it grows. These small magnetic island structures merge with each other, and due to the limitation of the cuneiform magnetic field, they form a number of FTE, arranged in order by size. Finally, under the action of magnetic sheath flow pressure and magnetic tension, the reconnection position is left. (5) A series of continuous FTE. of "abnormal" diurnal motion measured in the low latitude magnetopause of the fainting side are analyzed. By comparing the velocity of the event with the FTE motion model, it can be found that the magnetic tension of the actual reconnected magnetic field is much larger than that given by the model in order to counter the larger wake magnetic sheath velocity near the dizzy side magnetopause at this time. This shows that when the magnetohydrodynamic effect may play an important role, the existing model can not accurately describe the magnetic field and plasma environment, so it needs to be modified.
【学位授予单位】：中国科学院国家空间科学中心
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P353

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