STUDY ON FORMATION PROCESSES OF ELECTROSTATIC SOLITARY WAVES IN SPACE PLASMAS

通过对Geotail航天器数据的分析，我们发现静电孤立波（ESW）是由沿环境磁场的高能电子通量携带的，这与模拟中假设的静电孤立波形成模型一致。通过对Geotail航天器在磁尾中多个轨道上的ESW波形进行统计分析，发现ESW的产生区域位于中性点附近。我们对磁重联进行了二维粒子模拟，发现在重联点附近存在电子的加速过程。我们对碰撞不稳定性进行了二维模拟，发现一维孤立势是在不稳定性非线性演化后形成的。二维空间中电子的两流不稳定性。另一方面，在周期系统中，我们发现沿磁场方向快速传播的大振幅势可以通过与斜向缓慢传播的低杂波耦合而稳定存在。我们还进行了开放边界的二维模拟，从其中一个边界注入电子束。我们发现，在周期系统中发现的ESW形成过程也可以发生在电子束稳定地穿过未受扰动的热等离子体的顶部。通过基于BGK理论的ESW形成过程的理论分析，我们明确了两个ESW的聚并是一个不可逆的过程，电子空穴的动能转化为困在势中的电子的热能。

Analyzing the data by the Geotail spacecraft, we found that electrostatic solitary waves (ESW) are carried by high energy electron fluxes along the ambient magnetic field, which agree with the ESW formation model postulated from the simulations. Through statistical analysis of ESW waveforms on many orbits of the Geotail spacecraft in the magnetotail, we found that the generation regions of ESW are located near the neutral points. We performed a 2D particle simulation of magnetic reconnection, and we found that there exist acceleration processes of electrons near the reconnection points. We performed 2D simulations of the bump-on-tail instability to find that 1D solitary potentials are formed after nonlinear evolution of the instability. In the two-stream instability of electrons in a 2D.periodic system, on the other hand, we found that large amplitude potentials traveling fast along the magnetic field can exist stably through coupling with lower hybrid waves propagating slowly in the oblique direction. We also performed 2D simulations with open boundaries, injecting electron beam from one of the boundaries. We found that the ESW formation process found in the periodic system can also take place at the top portion of the electron beam traveling stably through the unperturbed warm plasma. Through theoretical analysis of the ESW formation process based on the BGK theory, we clarified that coalescence of two ESW is an irreversible process where the kinetic energy of electron holes is converted to the thermal energy of electrons trapped in the potential.