近地磁尾偶极化过程中的动力学Alfvén波研究

Magnetic dipolarization is one of important phenomena taking place in the near- Earth plasma sheet in the magnetotail during substorms, which marks the beginning time of substorm expansion phase. There are many kinds of intense low frequency waves observed during substorm dipolarizations. The exciting mechanism of kinetic Alfvén waves and the coupling between the kinetic Alfvén waves and charged particles are both unsolved problems of the magnetotail dynamic till now. Our project will focus on the conjunction observations of multiple satellites (e.g., TC-1、Cluster、THEMIS、Van Allan Probe、MMS) to analyze the features of low frequency waves and variations of charged particle number density and velocity in the near-Earth magnetotail during substorm dipolarization. The kinetic theory and multiple distribution function of charged particles will be adopted to investigate the dispersion relation of kinetic Alfvén waves and the coupling between kinetic Alfvén waves and charged particles in the high beta, (beta is the ratio of plasma thermal pressure to the magnetic pressure) plasma. Our project will illustrate the exciting mechanism of kinetic Alfvén waves in the near- Earth plasma sheet in the magnetotail during dipolariztion. This project will reveal the rules of the acceleration and heating of O+ ions and provide the new idea of the path of O+ ions entering the plasma sheet in the magnetotail and inner magnetosphere.

磁偶极化是亚暴时近地磁尾等离子体片中发生的重要现象之一，标志着亚暴进入膨胀相阶段。偶极化过程中伴随着强低频电磁场扰动，其中动力学Alfvén波的激发机制以及动力学Alfvén波与带电粒子间相互作用仍是磁尾动力学过程中的一个未解之谜。本项目利用多卫星（TC-1、Cluster、THEMIS、Van Allan Probe、MMS等） 协同观测数据，分析亚暴偶极化过程中近地磁尾低频电磁场扰动和带电粒子密度和速度变化特征；采用多种分布函数（如Bi-Maxiwalian、Loss-cone、Kappa 分布函数）和动理学理论，研究高beta(beta 是等离子体热压与磁压之比值)等离子体中动力学Alfvén波的色散关系及其波粒相互作用，阐明偶极化时近地磁尾等离子体片中动力学Alfvén波的激发机制，揭示偶极化过程中O+离子加速和加热规律，为强亚暴时O+进入磁尾等离子体片和内磁层的路径提供新思路。

本项目已完成相关研究内容，实现了预期研究目标。采用多卫星(THEMIS、Cluster等)在亚暴期间近地磁尾等离子片中的联合观测数据, 详细研究了亚暴偶极化过程电磁场扰动与等离子体离子速度变化的间相关性分析,研究结果表明亚暴期间近地磁尾等离体片中的距离亚暴触发区不同空间存在多类型偶极化现象，明确了亚暴触发区内外偶极化的异同，为亚暴触发区的确定提供了量化指标，即亚暴触发区内磁偶极化初始仰角较大，大于60度，且在亚暴触发之后伴随强的磁场扰动。亚暴偶极化期间等离子体片边界层中的动力学Alfvén波（KAWs）本证模能快速加速低能氧离子进入近地等离子体片内,为亚暴期间近地磁尾等离子片中能量氧离子的快速增加提供了相应机制及其粒子来源。近地磁尾等离子体片高beta环境下KAWs激发机制及其对离子的快速能化过程能被应用到磁层类似等离子体背景中，如磁层顶、外极尖区内。另外采用最近MMS卫星数据统计研究了强亚暴膨胀相和恢复相期间在向阳面昏侧磁层顶中O+离子的分布特征，该研究结果表明强亚暴偶极化过程中近地磁尾等离子片中的能量氧离子能快速传输到向阳面昏侧磁层顶内，且其密度在受IMF调控明显，与IMFBy、Bz呈现指数相关性。