STUDY ON FORMATION PROCESSES OF ELECTROSTATIC SOLITARY WAVES IN SPACE PLASMAS

通过对观测数据的分析，我们发现静电孤立波是由高能电子通量沿环境磁场携带的，这与模拟所假设的静电孤立波形成模型相一致。通过对磁尾轨道上的地尾航天器多个轨道上的电磁振荡波形进行统计分析，发现电磁振荡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，发现重联点附近存在电子的加速过程。我们对尾撞不稳定性进行了二维模拟，发现一维孤子势是在不稳定性非线性演化后形成的。另一方面，在二维周期系统中电子的双流不稳定性中，我们发现沿磁场快速传播的大幅值势可以通过与沿斜向缓慢传播的低杂波的耦合而稳定存在。我们还进行了开放边界的2D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电子激波的形成过程也可以发生在电子束稳定地通过未受扰动的热等离子体的顶部。通过对基于BGK理论的电子空穴形成过程的理论分析，阐明了两个电子空穴的结合是一个不可逆的过程，电子空穴的动能转化为势垒中电子的热能。

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.