The Study Of Plasma Waves In The Magnetosphere

The instability and plasma wave are one important subject of space physics and laboratory plasma physics. Now it is also a very difficult issue and hot topic. With two-dimensional (2D) particle-in-cell (PIC) simulations and spacecraft observations, this thesis has studied the structure of electron phase-space holes (electron holes), the instabilities driven by the proton temperature anisotropy associated with busrty bulk flows (BBFs), the electron acceleration mechanism behind the dipolarization fronts (DFs). The results are shown as follows.1. The perpendicular electric field in2D electron holesWe perform2D PIC simulations to study the evolution of electron holes at different plasma conditions. We find that the evolution is determined by combined actions between the transverse instability and the stabilization by the background magnetic field. In very weakly magnetized plasma (Ωe＜＜ωpe, where Ωe and ωpe are the electron gyrofrequency and plasma frequency, respectively), the transverse instability dominates the evolution of the electron holes. The parallel cut of the perpendicular electric field (E(?)) has bipolar structures, accompanied by the kinking of the electron holes. Such structures last for only tens of electron plasma periods. With the increase of the background magnetic field, the evolution of the electron holes becomes slower. The bipolar structures of the parallel cut of E(?) in the electron holes can evolve into unipolar structures, In very strongly magnetized plasma (Ωe＞＞ωpe), the unipolar structures of the parallel cut of E(?) can last for thousands of electron plasma periods. At the same time, the perpendicular electric field (E(?)) in the electron holes can also influence electron trajectories passing through the electron holes, which results in variations of charge density along the direction perpendicular to the background magnetic field outside of the electron holes. When the amplitude of the electron hole is sufficiently strong, streaked structures of E(?) can be formed outside of the electron holes, which then emit electrostatic whistler waves because of the interactions between the streaked structures of E1and vibrations of the kinked electron holes.2. The evolution of the magnetic structures in electron phase-space holes 2D electromagnetic PIC simulations are performed in the (x,y) plane to study magnetic structures associated with electron holes under different plasma conditions. In the simulations, the background magnetic field (B0=B0ex) is along the x direction. The combined actions between the transverse instability and stabilization by the background magnetic lead to the generation of the electric field Ey. Then electrons suffer the electric field drift and produce the current in the z direction, which leads to the fluctuating magnetic field along the x and y directions. Meanwhile, the motion of the electron holes along the x direction and the existence of the electric field Ey generate the fluctuating magnetic field along the z direction. In very weakly magnetized plasma (Ωe＜＜ωpe), the transverse instability is very strong and the magnetic structures associated with electron holes disappear quickly. When Ωe is comparable to ωpe, the parallel cut of the fluctuating magnetic field δBx and δBz have unipolar structures in electron holes, while the parallel cut of fluctuating magnetic field δBy has bipolar structures. In strongly magnetized plasma (Ωe＞ωpe), electrostatic whistler waves with streaked structures of Ey are excited. The fluctuating magnetic field δBx and δBz also have streaked structures.3. The proton temperature anisotropy associated with bursty bulk flowsWe study the development of the proton temperature anisotropy T⊥/T‖in bursty bulk flows, as observed by THEMIS Mission. For a set of10selected events, during which at least3spacecraft are aligned in the same flow, we can sample the plasma parameters along the Earth’s magnetotail. The temperature anisotropy in the quiescent tail is negligible. However, as soon as the bursty bulk flow (BBF) passes over the spacecraft a strong anisotropy is measured. We analyze T⊥/T‖as a function of parallel plasma beta β‖(β‖=nkT‖/(B2/2μ0)) for the different THEMIS satellites and compare the spread of the data points with various instability thresholds over ion-scales that can reduce the temperature anisotropy:for T⊥/T‖＜1the parallel and oblique fire hose; for T⊥/T‖＞1the proton cyclotron and mirror mode. It is shown that the anisotropy reduces whilst the BBF is moving Earthward, and the strongest fluctuations are enhanced along the instability thresholds, indicating that these instabilities reduce the proton temperature anisotropy.4. Electron acceleration behind the dipolarization frontsWe investigate the electron acceleration behind the dipolarization fronts (DFs) in the magnetotail from-25RE to-10RE by the examination of the super-thermal electron energy flux (>30keV) with the observations from THEMIS satellites. Statistical results of131DF events are presented based on the dataset from January to April of the years2008and2009. As the DFs propagate earthward, the acceleration of super-thermal electrons behind the DFs is found to last for several RE. The increase in super-thermal electron energy flux can even reach2～4orders of magnitude. The dominant acceleration mechanisms are different in the mid-tail (XGSM≤-15RE) and the near-Earth region (-15RE＜XGSM≤-10RE). In the mid-tail, the majority of DF events show that the dominant electron acceleration mechanism is betatron acceleration. In the near-Earth region, betatron acceleration is dominated in～46%DF events while Fermi acceleration is dominated in～39%DF events.