Magnetotail Plasmoids Observation And Study

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.