土星磁尾等离子体团形成和演化的数值模拟研究

The plasmoids are important signatures of the magnetotail reconnection process, with significant influences on the global dynamics of the planetary magnetosphere. The research of plasmoid formation and evolution has been one of the hot topics in the physics of Saturn's magnetosphere. However, most of the current researches mainly focus on the influences of the external solar wind conditions to the plasmoid formation, while the studies of effects of the inner magnetospheric plasma sources on the plasmoid formation are much less. Since currently Cassini is the only spacecraft orbiting in Saturn's magnetosphere, the observations of Saturn's magnetotail is very limited. We need to use the global numerical simulations to further investigate plasmoids in Saturn's magnetotail. This project plans to develop a multi-fluid MHD numerical model of Saturn’s magnetosphere to study the occurrence conditions and the positions of tail reconnection to investigate the physical process of the formation and evolution of the plasmoids in Saturn’s magnetotail. We intend to analyze the changes of the typical characteristics of the plasmoids, such as the three dimensional topological structures, the plasma distributions, and the internal magnetic structures, to study the evolution of the magnetotail plasmoids in Saturn’s magnetosphere. We will further study the influences of the inner magnetospheric plasma sources to the formation of plasmoids in Saturn’s magnetotail. We will make a comparative analysis between the simulation results and Cassini’s observations in Saturn’s magnetotail to validate the simulations and to deepen the understanding of the mechanisms of the formation and evolution of plasmoids in Saturn’s magnetotail. This study will not only deepen the current understanding of the propagation characteristics and physical mechanisms of plasmoids in Saturn’s magnetotail, but also significantly improve the knowledge about the global convection pattern in Saturn's magnetosphere, which is of great significance for the research on the physics of Saturn's magnetosphere.

等离子体团是磁尾磁重联过程的重要特征，对行星磁层全球动力学具有重要影响。对土星磁尾等离子体团形成和演化过程的研究已成为行星磁层物理学的热点课题之一。目前大部分研究主要探讨外部太阳风条件对土星磁尾等离子体团形成的影响，关于内磁层等离子体源对其影响机制的研究还较少。目前土星磁层仅有卡西尼号在轨运行，磁尾观测资料有限，对等离子体团的深入研究还需借助全球数值模拟。本项目计划发展多流MHD数值模型，分析磁尾磁重联的发生条件与位置以研究土星磁尾等离子体团形成的物理机制。分析等离子体团的三维结构、等离子体分布和内部磁场结构等特征的变化研究其演化过程。研究内磁层等离子体源对土星磁尾等离子体团形成的影响机制。将模拟结果与观测数据进行对比，加深理解等离子体团的形成和演化机制。该项目不仅能加深对土星磁尾等离子体团传播特性及物理机制的理解，而且能提高对土星磁层全球对流模式的认识，这对土星磁层物理研究具有重要意义。

对土星磁尾等离子体团形成和演化机制的研究是行星磁层物理学中的热点课题之一。目前，土星空间环境中仅有卡西尼号卫星在轨运行，卫星观测资料相对有限。因此，需要借助全球数值模拟研究土星磁尾等离子体团的形成和演化过程。本项目利用CESE算法发展起来土星磁层全球MHD数值模型，主要探究土星磁尾磁重联的触发条件与位置分布，以及土星磁尾等离子体团的形成与演化机制。研究发现内磁层等离子体源及行星高速自转效应共同驱动导致了土星磁尾等离子体团的形成，而外部太阳风条件主要影响磁尾磁重联的位置分布和等离子体团的空间尺度。结合数值模拟与卫星观测，本项目研究加深了对土星磁层中等离子体传播特性的理解。此外，本项目也对地球磁层的物理特征进行了研究，并获得了重要进展和成果。主要包括：1）弓激波和磁层顶的大小主要受控于太阳风动压强度，而它们的位形主要受行星际磁场强度和方向的影响；2）IMF Bx 分量通过磁重联会破坏磁层位形对称性，引起显著的南北不对称性；3）IMF Bx 也能引起极区出流等离子体输运呈现显著的南北不对称性。相关研究加深了对太阳风与行星磁层相互作用机制的认识和理解。