The Study Of Turbulence And Waves In The Magnetospheric Boundary Layers

It is widely believed that most of the matters in the Universe are in the plasma state. Earth’s magnetosphere is a natural laboratory for us to study plasma. The wave research has played a core role in plasma physics. Wave is the most basic form of plasma movement. Turbulence and magnetic reconnection are two basic physical processes. Turbulence is a nonlinear physical process, an universal phenomena in the astrophysical physics, and it plays a key role in the plasma transport and energy dissipation. Magnetospheric substorm is one of the most common physical phenomena in Earth’s magnetosphere. Dipolarization is an important composition of the substrom. Dipolarization front is the leading edge structure of the dipolarization, accompanied by the electron acceleration and ion heating. Meanwhile it can act as a carrier, by transporting energy and flux. Magnetic reconnection is a possible mechanism, which can generate subtrom or magnetic storm. It could transfer the magnetic energy to plasma kinetic and internal energy in a short time period.The study of the wave and electron dynamic process in the basic physical phenomenon, can have an important influence for understanding the energy transport and dissipation. Foreshock turbulence is a driven turbulence, how can it develop, how is the wave-particle interaction in it? It affords us a lesson to understand the turbulence evolution in space. There are rich waves around the dipolarization front, and the whistler waves were frequently observed. Some questions are raised up naturally, such as:What is the space distribution of whistlers behind the DFs? What are the essential characteristics? How is the corresponding electron distribution? What is the energy source for whistlers? And we also noted some interesting structure around dipolarization front, such as mirror mode structure. What properties of the structures, and what effects do they have for electron dynamic? Do wave activities associat with the structures, what role do the waves play for energy transport? There are also rich wave activities in the magnetic reconnection. The lower hybrid wave is one of the most important waves, what role does it play in magnetic reconnection. Which relationship is between it and energetic electron? These questions need us to answer. In this thesis, we study these basic physical processes via the Cluster data and numerical simulation.In the first chapter, we introduce the terrestrial magnetosphere, turbulence, substorm and magnetic reconnection. And then we summarize some researches about plasma waves and instabilities.In the second chapter, the spacecraft instruments and data analysis methods are introduced.In the third chapter, in the high (3region the Earth’s foreshock turbulence evolution is reported. First, the quasi-sinusoidal and the irregular wave trains are both detected in high β region of Earth’s foreshock turbulence via Cluster observation. In addition, from the quasi-sinusoidal to the irregular waveform, the corresponding polarizations appear to transit:the right-hand wave of higher frequency is substituted by a left-hand wave with lower frequency in spacecraft frame. The former is believed to be generated by ion-ion right-hand non-resonant instability due to the right-hand polarization and anti-sunward propagation in the plasma frame. The’decay line’ is observed in density spectrum. It indicates that the decay instability leads to the polarization transition. Then the1-D hybrid simulation is applied for two cases with various velocities to study such polarization transition. By comparing observation results with the simulation, such polarization transition and "inversed cascade"(wave energy transferring from large wave number to small wave number) can be understood as the consequence of decay instability. Although decay instability cannot be initiated in high beta (β>1) plasma in MHD theory, such P-dependence can be modified by kinetic effect. Moreover, it is found that in simulation no matter which right-hand instability is dominant in early stage, left-hand wave will be the prime component of magnetic field disturbance in the final stage.In the fourth chapter, we study the waves around the dipolarization fronts. We present a statistical study of whistler waves behind DFs by using the data from Cluster satellites during the years from2001to2007. We obtained the space distribution of the whistler waves, including the global distribution and local distribution; the statistical results of characteristic parameters such as the frequency bandwidth, central frequency, wave amplitude, propagation angle, then the global and local distributions of characteristic parameters and the corresponding electron distribution. We found that the wave amplitude shifted from dusk to dawn with the increasing radial radius in GSM coordinate; in DF local coordinate, the averaged wave amplitudes increased toward the dawn direction. Similar with that, the perpendicular anisotropy for electron at the energy range of5-24keV also shifted from dusk to dawn with the increasing radial radius in GSM coordinate, which may be the consequence of magnetic gradient drift and curvature drift; in DF local coordinate, the averaged perepdicular anisotropy for electron at the energy range of5-24keV increased toward the dawn direction. We deduce that the whistler wave amplitude has a positive correlation with the perpendicular anisotropy which increases towards the dawn direction. All these indicate that whistler waves behind the DFs are provided by locally perpendicular anisotropy of higher energy electrons (5-24keV). An interesting event is found in the statistical events. A magnetic oscillate is observed in a region bounded by two dipolarizations. Features of the structures are that the anti-correlation between the magnetic field strength and plasma density, zero frequency in the plasma rest frame and linear polarization. It suggests that the structure is mirror mode. We noted that the perpendicular ion temperature anisotropy increases after the dipolarization, which might be caused by the betatron heating when the magnetic field strength is enhanced. These observations imply that dipolarization provides favorable plasma conditions for the growth of mirror instability. One interesting feature is the connection of electron dynamics with the mirror mode structures. Thermal electron energy flux has an enhancement at0°and180°pitch angles inside the magnetic dips of the first three mirror structures and an enhancement at90°pitch angle inside the magnetic dip of the last structure. The different electron distributions inside the mirror structures might be results of different evolution stages of the mirror wave. The last structure may be in the nonlinear stage of the mirror instability, whereas the three others with quasi-sinusoidal waveforms may be in the linear stage. In addition, we found that intense whistler waves were confined within the magnetic dips. Based on the electron temperature anisotropy, we conjecture that whistler waves observed in the first three dips were generated in a remote region, then they were trapped in the mirror mode troughs and transported toward the spacecraft; while the whistler wave detected in the last dip was excited locally by the electron anisotropy instability. Mirror mode structure can be viewed as a wave carrier, thus confining and transporting the whistler energy across magnetic field lines. It is worth noting that this mechanism may be important for energy transport and dissipation processes during substorms.In the fifth chapter, we statistically study the distribution and possible roles of lower hybrid waves based on wave data recorded by the Cluster spacecraft during21magnetotail reconnection events. We found that, as the plasma β increasing, magnetic field fluctuations associated with lower hybrid waves increase while electric field fluctuations decrease. As β exceeds10, both magnetic and electric field fluctuations decrease sharply. Furthermore, a two-dimensional wave distribution is constructed based on the two-dimensional reconnection model. The most intense magnetic field fluctuations occur in the outflow region, while they are weaker in the inflow region and separatrix region. The most intense electric field fluctuations occur around the separatrix region, while they are relatively weaker in the inflow and outflow regions. There are positive correlations between wave strength and energetic electron acceleration, as well as between wave strength and reconnection rate. Our results provide a crucial step toward fully understanding the role of lower hybrid waves in the dissipation process of magnetic reconnection.In the sixth chapter, we conclude the thesis, and make an outlook for the future work.