地球磁層頂通量傳輸事件的軸向研究
發(fā)布時間：2019-05-16 13:18
【摘要】：磁層頂通量傳輸事件(Flux Transfer Event,FTE)被認為是與磁場重聯(lián)相關的現(xiàn)象,其標志為衛(wèi)星測量磁場在磁層頂法向分量的雙極變化。迄今為止,關于FTE的本質、形成機制和產生位置這些基本問題還存在著一些爭論。確定FTE的軸向不僅對于研究FTE的幾何結構以及理解其中等離子體動力學是必要的,也可以幫助我們區(qū)分FTE的不同形成機制和產生位置。本文以FTE軸向研究為主線,主要內容及結論總結如下:(1)發(fā)展了一種新的多點分析方法去確定FTE的軸向。這種方法建立在結構為左右對稱的純幾何假設上,對于嵌在磁層頂電流片中的FTE結構,不管其物理性質如何,都基本上滿足這一對稱條件,所以新方法就有著較廣泛的應用性。模型測試和實際應用表明,新的方法在確定軸向時比傳統(tǒng)方法如Grad-Shafranov(GS)方法和磁場最小方差分析法等以及更近的基于圓柱對稱的方法更有效和可靠。(2)在確定了FTE軸向以及在合適的多衛(wèi)星位形下,設計了一種新方法去重構FTE磁場的空間分布。該方法對THEMIS和Cluster各自觀測到的兩個FTE的應用表明它能快速而有效地重構出結構的磁場空間分布,從而幫助我們推測結構磁場位形、理解衛(wèi)星測量數(shù)據的時間變化以及獲得其它物理量相對于結構磁場的空間分布特征。(3)利用新的FTE軸向確定方法進行了首個關于FTE軸向的統(tǒng)計分析。其結果揭示出:盡管在昏側低緯觀測到的FTE大多有著接近南北方向的軸向,其來源仍然是日下點分量重聯(lián)線;與傳統(tǒng)認識中日下點分量重聯(lián)線為一條方向幾乎不變的分隔線不同,實際的日下點分量重聯(lián)線在磁層頂有著相當大的曲率。(4)首次發(fā)現(xiàn)多X線重聯(lián)形成FTE的磁場信號。這種磁場信號的表征為的多極變化(比如雙或者三-雙極變化)。GS重構結果表明在有著如此磁信號的事件中包括多個子結構,這與多X線重聯(lián)形成多個通量管的圖景相符;但各子結構在磁層頂上依序從大到小排列,這一特征是傳統(tǒng)的多X線重聯(lián)模型所未預測到的。為解釋這一現(xiàn)象,我們提出了一個新的模型去描述FTE的形成過程:重聯(lián)持續(xù)在磁層頂發(fā)生,在主X線附近磁場形成楔形位形;重聯(lián)率周期性地變化,在其增長時楔形磁場線內由撕裂模不穩(wěn)定性形成了連續(xù)的多個小尺寸的磁島結構;這些小的磁島結構互相合并,由于楔形磁場的限制,形成按尺寸大小依序排列的多個FTE,最后在磁鞘流壓力和磁張力作用下離開重聯(lián)位置。(5)分析了在昏側低緯磁層頂內遙測到的一系列“反�！比障蜻\動的連續(xù)FTE。將事件的運動速度與FTE運動模型相比較可發(fā)現(xiàn),為對抗此時在昏側磁層頂附近較大的尾向磁鞘流速度,實際重聯(lián)磁場的磁張力要比模型給出的磁張力大很多,這說明在磁流體動力學效應可能起重要作用的時候,現(xiàn)有模型并不能準確地描述磁鞘磁場和等離子體環(huán)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.
【學位授予單位】：中國科學院國家空間科學中心
【學位級別】：博士
【學位授予年份】：2017
【分類號】：P353
[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.
【學位授予單位】：中國科學院國家空間科學中心
【學位級別】：博士
【學位授予年份】：2017
【分類號】：P353
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