| This dissertation focuses on two magnetic structures in the solar wind at 1 AU, the magnetic holes (MHs) and hot flow anomalies (HFAs). These two mag-netic structures are often observed in the interplanetary space. The formation mechanism of the magnetic holes involves basic plasma physics, such as plasma instabilities. In addition, linear magnetic holes may carry some information of the solar wind heating and acceleration. HFAs are kinetic phenomena observed near planetary bow shocks, and they may lead to disturbances in the magne-tosheath and magnetosphere. Therefore, the study of the characteristics and evolution of HFAs may help to further understand the interaction between the solar wind and the magnetosphere.Magnetic holes, also called magnetic decreases (MDs), are structures in in-terplanetary space with significant decreases in the magnetic field magnitude, and hot flow anomalies (HFAs) are structures with significantly deflected solar wind flow and heated plasma that are observed in the vicinity of planetary bow shocks. The four-spacecraft Cluster mission with tetrahedral configuration makes it possible to carry out the relevant work. In Chapter 1 (introduction), after an introduction to the magnetic holes and hot flow anomalies, we present the open questions and progress of the previous studies on the formation and evolution mechanism of the magnetic holes and hot flow anomalies. In Chapter 2, we show some detail information for the Cluster mission, and the scientific instruments (mainly about FGM and CIS). Then, we present the timing method (considering errors) which is employed to determine the speed and normal direction of spatial structures. Finally, we show the calculation for moments data by use of particle flux data are present.The mechanism for magnetic holes formation is currently unresolved, al-though many observations and theories have been proposed for the generation mechanisms. For example, it is generally thought that linear magnetic holes were generated by mirror-mode instabilities. Besides, the soliton approach was pro- posed for the linear magnetic holes observed in mirror-stable conditions. Then what is the generation mechanism for the linear magnetic holes? Another unre-solved question is whether the formation mechanisms are the same for the linear and nonlinear magnetic holes. In addition, where are the possible sources for the magnetic holes? Further studies are needed for these questions.Analysis of the magnetic holes is presented in Chapters 3 to 5. In Chapter 3, we have investigated the linear magnetic holes (LMHs) in the solar wind at 1AU. It is found that the geometrical structure of the LMHs in the solar wind at 1AU is consistent with rotational ellipsoid, and the occurrence rate is about 3.7 LMHs per day. It is shown that not only the occurrence rate but also the geometrical shape of interplanetary LMHs has no significant change from 0.72 AU to 1AU in comparison with previous studies. It is thus inferred that most of interplanetary LMHs observed at 1AU are formed and fully developed before 0.72AU. In Chapter 4, Plasma and magnetic-field characteristics of magnetic decreases (including LMD trains, NMD trains, linear non-train MDs and nonlinear non-train MDs) in the solar wind at 1 AU are studied by using the magnetic field and plasma data from Cluster-Cl during the period from 2001 to 2009. Compared with the parameters in the average solar wind, the LMD trains occurred in regions with relatively low magnetic-field strengths, high ion-number densities, and large plasmaβs. However, the NMD trains were preferentially observed in regions where the magnetic-field strength was lower, the ion-number density was lower, and the plasmaβ was larger than that of the average solar wind. This suggests that the LMD and NMD trains might be generated by different mechanisms. There are ≈25%(≈25%) LMD trains (NMD trains) in CIRs, indicating that CIRs are important source for the LMD trains (NMD trains). We found that the LMD trains (NMD trains) associated with ICMEs contribute about 20% (20%) to their total number during solar maximum indicating that ICMEs are possible sources of the LMD trains (NMD trains) during solar maximum. However, the occurrence rates of the LMD and NMD trains do not decrease from solar maximum to solar minimum, therefore, other possible mechanisms may present for LMD trains (NMD trains) during solar minimum. In addition, it is found that the magnetic field and plasma characteristics of the trains differ from the non-train events, indicating that the train and non-train events might be generated by different mechanisms. In Chapter 5, we have calculated the propagation velocities of linear magnetic holes in the solar wind frame by using the Timing method (considering errors). Forty-seven stable linear magnetic holes are selected. For these events, the leading and trailing edges propagate at almost the same speed within the error range. It is found that 27 linear magnetic holes are frozen with the solar wind, and these events may be generated by mirror instabilities. The other 20 linear magnetic holes are non-frozen with the solar wind plasmas, indicating that they should not be generated by the mirror instabilities. And these non-frozen events may be formed by solitary waves.The other magnetic structure studied in this dissertation is hot flow anoma-lies (HFAs) which are often observed near the bow shocks. HFAs are generally thought to be formed by the interaction of planetary bow shocks and interplan-etary discontinuities. Previous studies showed that ions are heated due to the interaction between discontinuities and the bow shock,and observations indicate that these structures expand due to the internal hot plasma environment. How-ever, what is the expansion speed of these structures? Is it possible that HFAs can also contract or be stable? In addition, because near the young HFA there exists two diacritical populations of ions:the solar wind flow and the reflected ion bgam, using the temperature calculated from the moments data which is based on the presence of one population will cause great errors in the thermal pressure calculation. However, previous studies did not distinguish the two ion flows.In Chapter 6, based on Cluster observations, the propagation velocities and normal directions of hot flow anomaly boundaries are calculated. Twenty-one young HFAs that have clear leading and trailing boundaries were selected and multi-spacecraft timing method considering errors was employed for the investi-gation. According to the difference in the propagation velocity of the leading and trailing edges, we find that four HFAs are contracting at a speed of a few tens km/s, and five events are expanding at a speed of tens to more than one hundred km/s, and the other twelve cases are stable. Compared with the sum of magnetic and thermal pressure of the nearest point outside of the leading edges, it is found that the variation of the sum of thermal and magnetic pressure is consistent with the evolution of young HFAs. Furthermore, it is found the total pressure (sum of thermal, magnetic and dynamic pressure) variation has a significant effect on the evolution for most (70%) of the HFAs. |
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