内磁层超低频波与带电粒子的共振作用

The acceleration and transport of charged particles in the Earth’s radiation belts have attracted a lot of research attention in the study of space physics and space weather. One of the major candidate mechanisms of particle acceleration in the radiation belts is the interaction between ultralow frequency waves and charged particles, and the understanding of this mechanism is largely based on a theory developed in the 1980s, the theory of drift resonance and drift-bounce resonance. This theory has successfully reproduced many of the observational characteristics, and therefore has been widely accepted as the classical theory of wave-particle interactions. However, due to the enhanced quality and quantity of the observational data, there have been more and more observational signatures in recent years that cannot be fully understood in the conventional framework. Therefore, it becomes very important to extend the classical theory based on these observations, and this will be the main goal of the proposed study. ..We propose to take into account the temporal evolution and spatial distribution of ultralow frequency waves in the inner magnetosphere, together with nonlinear effects, to the theoretical framework of drift resonance and drift-bounce resonance. This extension will be achieved by an integrated approach that combines theoretical derivations, spacecraft observations and numerical simulations, so that one can better understand the charged particle acceleration and diffusion in the radiation belts. The extended framework will also be compared with observational data to extract information on ultralow frequency waves in the inner magnetosphere, which will provide a solid ground and an innovative approach for a comprehensive model of radiation belts.

带电粒子在地球辐射带的加速和传输一直是空间物理学与空间天气学研究的核心问题。作为粒子加速的主要机制之一，超低频波对粒子的加速与扩散作用一直备受关注，而人们对该机制的理解则很大程度上建立在上世纪八十年代所提出的漂移共振与漂移-弹跳共振理论基础上。该经典理论很好的预测了卫星的观测特征，并因此被广为接受。然而随着卫星观测质量与数量的提高，已有越来越多的观测现象无法被这一理论所解释，因此，如何对该理论进行拓展已成为本领域发展的主要瓶颈问题之一。本课题将紧紧围绕这一问题，从理论推导、卫星观测和数值模拟等多角度出发，充分考虑超低频波的时间演化和空间分布的影响，并讨论非线性等效应，全面拓展现有的理论框架，从而更好的理解粒子在辐射带的加速与扩散过程。拓展后的理论框架与卫星数据的对比也将有助于进一步理解超低频波在内磁层中的产生、演化和空间分布，从而为最终完善现有的辐射带物理模型提供坚实基础和崭新路径。

在地球内磁层带电粒子动力学中，超低频波扮演着极为重要的角色，其对粒子的加速与传输作用被认为是影响范艾伦辐射带的形成与演化的主要机制之一。本项目基于卫星观测数据、数值模拟及理论推导等方法，澄清了长期以来研究中的部分误区，并从多角度深入拓展了人们对超低频波与带电粒子相互作用的理解。.这些研究工作主要包括：对传统超低频波与带电粒子的漂移共振理论进行非线性拓展，推导出了粒子在波场中的运动方程（形式为单摆方程），并利用范艾伦卫星的观测数据，验证了这一非线性理论的准确性；在以上非线性理论的框架内，探讨了磁层对流对共振过程的影响，将粒子运动方程改写为带有准周期性驱动项的单摆方程，并提出了受单摆本征频率和驱动频率共同控制的束缚、穿行和混沌三种运动过程；充分考虑超低频驻波形成的时间尺度，揭示了在超低频波产生初期以行波形式存在时，带电粒子与波场之间的surfratron-type共振图像，并获得了观测验证；从联合观测和理论等角度出发，研究了超低频波的极光响应，即准周期性出现且极向运动的极光弧，从而建立起了一个相对完整的全球图像；从理论上分析了超低频波与带电粒子之间漂移-弹跳共振的物理及观测图像，并获得了卫星数据的验证；探讨了超低频环向模对带电粒子的漂移共振作用，其共振条件与极向模类似，观测信号也极为清晰，从而改变了此前人们对环向模仅在磁层顶被强烈压缩时才可加速粒子的传统认知；利用卫星观测发现了超低频波磁场压缩分量的双倍频现象和方位角分量“肩”型振荡之间的联系，并基于气球-磁镜模理论模型，对这一现象进行了解释和重现。