摘要
等离子体团是行星际和地球磁层中一种常见并十分重要的物理现象。等离子体团的形成与宇宙等离子体的爆发性过程有紧密联系。在行星际空间中等离子体团表现为行星际磁云,在磁层中表现为磁层顶和磁尾的磁通量绳和磁岛。磁层物理中一个重要的方面是研究磁尾重联,亚暴动力学以及与此相联系而产生的各种等离子体结构——磁岛,磁通量绳等等。卫星在磁尾观测的磁岛和磁通量绳信号近似,即先南后北或者先北后南的磁场信号,磁岛和磁通量绳的区别在于在磁通量绳内部有明显的强核心场的存在,而在磁岛中心磁场强度几乎为零。
过去的研究表明,等离子体团存在于由近磁尾到远磁尾的广阔范围内,但研究重点集中于中远磁尾等离子体团。近来随着近地空间探测卫星如ClusterⅡ,双星计划的发射,近磁尾的等离子体团研究逐渐引起人们的重视,本文就是利用双星计划和ClusterⅡ的数据对近地磁尾等离子体团的观测特征,内部磁场和粒子结构,产生机制,及其对应的行星际磁场条件,与磁层亚暴的关系进行了研究,主要研究工作和结果在以下三方面:
1分析了2004年7月至9月双星计划TC-1卫星在磁尾的磁场数据,发现近地磁尾存在等离子体团(plasmoid),首次给出了TC-1对近地( X >-13Re处)等离子体团的观测结果。根据等离子体团内磁场结构的不同,我们分析两个事件:9月14日磁环(magnetic loop )型的等离子体团具有闭合磁力线结构,8月6日磁通量绳(magnetic flux rope)型的等离子体团具有开放磁力线结构。两个事件与背景流场相比都具有高速地向速度。粒子可以沿着开放的磁力线从磁通量绳逃逸出来,而磁环由于其闭合磁力线结构可以束缚住粒子。TC-1对磁尾地向等离子体团的观测为多X线重联在磁尾的发生提供了证据并表明重联地点应该位于X <-10Re的磁层尾部区域。
2我们利用Grad-Shafranov方法研究了近地磁尾X=-14.75Re处的地向运动磁通量绳的内部磁场结构。磁通量绳的不变轴位于晨昏方向,垂直于不变轴的横截面上磁场分布被重构出来,沿着不变轴方向有强核心场存在。对应于这一事件的AE指数表明通量绳被观测到前后没有磁层亚暴发生。最近的一些研究表明等离子体片中的磁通量绳形成可以用多X线重联来解释。我们的计算结果显示横截面上磁场分布为非轴对称圆环结构,这一分布结构确实需要由多X线重联来产生。所以我们的结果为磁尾多X线重联的发生提供了进一步的证据。
3收集了Cluster卫星2001年—2005年间观测到的磁尾磁通量绳事件,并对磁通量绳(magnetic flux rope)形成及其内部磁场结构与行星际磁场(IMF)的关系作了统计研究。考虑磁通量绳被观测到时行星际磁场条件,在所有73个通量绳事件中,IMF By分量在IMF中占有主导地位的事件有80%,78%的事件具有与IMF By相同方向的核心场。IMF通过在磁层顶与地球磁场相互作用改变南北等离子体片内磁场相对方向,形成有利于磁通量绳形成的磁场位形,并且IMF By的方向对通量绳内部核心场的方向具有决定性影响。从统计结果来看,磁通量绳的形成并不会依赖于IMF Bz分量的方向。
论文具体安排如下:第一章对地球磁层及其结构做了简要介绍,由于本文研究内容与磁层亚暴和多X线重联有密切关系,在第二章,第三章对亚暴和多X线重联进行简要介绍,第四章对磁尾等离子团的研究做了回顾和总结,第五、六、七章详细介绍作者自己的工作,第八章是对所做工作的总结和对未来工作的展望。
The plasmoid is a common and very important physical phenomenon which occurs throughout the earth’s magnetosphere and interplanetary space. It is the important feature in the various kinds of eruptive process in cosmic plasmas. In the interplanetary space the plasmoid appears as the magnetic cloud, and it appears as the magnetic island or magnetic flux rope at the magnetopause and the magnetotail. One important subject of the magnetosphere physics is the magnetic reconnection related to the substorm and the resulted plasma structure such as magnetic island and magnetic flux rope at the magnetotail. As the satellite crosses the magnetic island and magnetic flux rope the similar magnetic signals of south-then-north or north-then-south are observed. The different signal observed by the satellite is that there is the strong core field in the center of the flux rope but the field in the center of the magnetic island is nearly 0.
The early studies indicate the plasmoids exist at the wide range from the near-earth magnetotail to the distant magnetotail. The more attention has been paid to plasmoids in the distant magnetotail at X<-60 Re .Now the launch of more and more satellites covering other regions of the magnetotail, such as Geotail, Cluster and Double Star, has allowed observation of plasmoids in the near-Earth magnetotail, at X>-30Re. Using the data from Cluster and Double Star we study the plasmoids at the near tail and our contributions are as follows:
1 We analyze Double Star TC-1 magnetic field data from July to September in 2004 and find plasmoids exist in the very near-Earth magnetotail. It is the first time that TC-1 observes the plasmoids in the magnetotail at X>-13 Re. According to the difference of the magnetic field structure in plasmoids, we choose two typical cases for our study: the magnetic flux rope on 6 August has the open magnetic field and the magnetic loop on 14 September has the closed magnetic field. Both of the cases are associated with the high speed earthward flow and the magnetic loop is related to a strong substorm. The ions can escape from the magnetic flux rope along its open field line, but the case of the closed magnetic loop can trap the ions. The earthward flowing plasmoids observed by TC-1 indicate the multiple X-line magnetic reconnection occurs beyond the distance of X=-10 Re from the earth.
2 We investigate the magnetic structure of a small earthward-moving flux rope observed by Cluster in the near-earth plasma sheet through application of the Grad-Shafranov (GS) technique to reconstruct the transverse magnetic field distribution perpendicular to the flux rope axis at X=-14.75 Re. We find that the principal axis of the flux rope lies approximately along the dawn-dusk direction and that the diameter of the flux rope is about 1.5 Re. There is a strong dusk-ward core magnetic field in the center of the flux rope. According to the AE index, there is no obvious substorm associated with the magnetic flux rope. Recent studies indicate that the formation of the flux rope in the plasma sheet can be understood in terms of simultaneous reconnection at multiple X-line points in the near-tail. The distribution of the transverse magnetic field on the cross section is the asymmetric circles, which requires that the reconnections at multiple X-line points occur. So our results also provide additional evidence for the occurrence of multiple-X line reconnection in the magnetotail.
3 The magnetic flux ropes observed by Cluster from 2001 to 2005 in the magnetotail are surveyed in this work. We have performed a statistic study on the relationship between the formation of the magnetic flux ropes and interplanetary magnetic field (IMF). Considering the IMF condition when the flux ropes are observed, for the 80% of the total 73 flux ropes cases, there are dominant By in IMF accordingly, while in the 78% of all cases there are core fields with the same direction as the IMF By. It is possible that IMF can change the relative direction of the magnetic fields in the northern and southern plasma sheet to form the magnetic topology favoring the formation of the magnetic flux rope. IMF also has a decisive effect on the directions of the core fields in the flux ropes. As indicated in these statistic results, the formation of the flux ropes in the magnetotail does not depend on the direction of the IMF Bz.
We arrange the thesis as follows: A brief introduction of the magnetosphere is presented in Chapter 1. Because our work has the close relation with magnetosphere substorm and multiple-X line reconnection, we introduce them briefly in Chapter 2 and Chapter 3. Chapter 4 gives a review of the early study of the plasmoid and a summary of the nowaday study. From Chapter 5 to Chapter 7 we introduce our works in detail. The summary of our work and prospects are given in the last chapter.
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