土星磁层磁通量绳内磁场结构及电子特征的观测研究

The magnetic flux rope, a common structure ever frequently observed in many planetary magnetospheres, provides an important way for planetary particles to escape outside or solar wind particles to penetrate inside. Moreover, flux rope also plays critical role in driving magnetospheric dynamic processes at those planets. At Saturn, the magnetic flux rope can be generated by magnetic reconnection either in the way of the ‘Dungey Cycle’ similar to Earth, or the ‘Vasyliunas Cycle’ similar to Jupiter. Hence, the investigation of flux rope is pivotal to understand macro-scale processes of Saturian magnetosphere. At present, the observational features of the inner structure of flux rope are still poorly understood, and the simulation results cannot well explain the observations. Therefore, to understand the characteristics of the flux rope and its effects on mass transport require a systematic study. This project thus aims to promote the study on the magnetic flux rope in the Saturnian magnetosphere. Based on the in-situ measurements in the Saturnian magnetosphere by Cassini spacecraft since 2004, we will study the inner magnetosphere flux rope magnetic structures and the related electron properties. We also plan to statistically investigate the characteristics of the flux ropes at different local times, to reveal how the flux ropes varying with local times, and to achieve innovative progress on the research of the formation, distribution and evolution of flux ropes. Our research would provide scientific and theoretical foundation for the Chinese upcoming exploration plan to giant planets.

磁通量绳在行星磁层中普遍存在，是磁层粒子逃逸和太阳风进入磁层的重要途径，在行星磁层动力学过程中起重要作用。现有理论认为，土星的磁通量绳既可能由与地球类似的Dungey Cycle磁重联过程产生，又可能由巨行星独有的Vasyliunas Cycle磁重联过程形成。因此，对磁通量绳的研究是理解土星磁层大尺度等离子体过程的一个绝佳突破口。当前，磁通量绳内部的基本观测特征并不足够清楚，数值模拟也尚不能准确描述有限的已知观测特征，更深入的观测研究就显得非常必要。本项目将基于Cassini飞船对土星磁层十余年的局地探测数据，对土星磁层内磁通量绳及相关结构进行研究。通过分析磁通量绳内部磁场结构及电子特征，并对不同地方时的磁通量绳进行分类统计，归纳出磁通量绳随地方时的变化规律。预期结果将能更加准确地描述土星磁通量绳形成、分布及演化等方面的主要特点，并为我国正在论证的巨行星探测计划提供科学依据和理论基础。

本项目利用Cassini卫星数据研究土星磁层动力学相关的磁重联过程的产物磁通量绳结构及电子离子特征。我们利用磁场Bθ分量变为负值来筛选磁通量绳及重联事件，建立了磁通量绳事件列表，研究磁通绳及磁重联区的时空尺度及分布特征。我们首次在巨行星土星的日侧磁层内观测到磁盘磁重联，颠覆了此前科学界认为“巨行星磁层内磁重联只能发生在夜侧”的普遍认知；揭示了日侧磁重联能够非常有效地加速电子和重离子，为巨行星日侧X射线辐射提供了新的激发机制。同时发现土星磁重联区尺度较小，可以存在于各个地方时并绕土星旋转，修改了以往流行的大尺度夜侧重联图像。这一成果为研究巨行星磁层粒子逃逸及行星际物质交换提供了新途径，拓展了对巨行星磁层动力学的认知。此外在磁通绳产生机制方面提出了新的磁零点对的连接形成磁通量绳的理论模型。综上，在项目执行期间，我们在土星磁通量绳结构形成机制方面取得了突破性和创新性进展，加深了对巨行星磁层动力学过程的认知。