磁鞘磁通量绳形成与演化的三维全球混合数值模拟研究

The interaction between interplanetary planes and bow shocks and the Earth's magnetosphere plays an important role in the formation and evolution of the magnetosheath magnetic flux ropes. Interplanetary tangential discontinuities go across the bow shock and then interact with the magnetopause, leading to the evolution of the structure of these discontinuities, and magnetosheath reconnection is generated. Magnetic reconnection process is often accompanied by flux ropes, but the three-dimensional spatial distribution and evolution of the magnetosheath flux ropes generated by the structural change of interplanetary discontinuities are still not very clear. Since the observations of magnetosheath is very limited, we need use the global numerical simulations to further investigate the formation and evolution of magnetic flux ropes in the magnetosheath. In this project, we try to use the 3D global hybrid simulation method, focusing on the formation and evolution of magnetosheath flux ropes, which by the interaction between tangential discontinuities and the bow shock and magnetosphere, to investigative (1) effects of the bow shock and magnetosheath plasma on the structural change of the discontimities; (2) the trigger conditions and positions of the magnetosheath reconnection; (3) the three-dimensional spatial distribution and evolution of the magnetosheath flux ropes under various tangential discontinuities. This study will deepen the currently understanding of the interaction between the interplanetary discontinuities and the Earth's magnetosphere, significantly improve the knowledge about the dynamical processes of magnetosheath reconnection and the evolution of the flux ropes, which is of great significance for the research on the physics of Earth's magnetosphere.

行星际间断面与弓激波和地球磁层的相互作用在磁鞘磁通量绳的形成和演化过程中起到了重要作用。切向间断面穿越弓激波与地球磁层顶发生相互作用后，间断面的结构发生演化，激发磁鞘磁重联。磁重联过程常伴随磁通量绳，但对于行星际切向间断面结构变化所引起的磁鞘磁通量绳的三维空间分布和时间演化过程的研究较少。由于地球磁鞘观测资料有限，对磁鞘磁通量绳形成和演化的深入研究还需借助全球数值模拟。本项目拟采用三维全球混合数值模型，聚焦于切向间断面与弓激波和磁层顶的相互作用所产生的磁鞘磁通量绳的结构和演化过程，重点研究不同间断面位形条件下，（1）弓激波和磁鞘等离子体对间断面结构变化的影响；（2）是否可诱发磁鞘重联及发生位置；（3）磁鞘磁通量绳的三维空间分布和时间演化过程。该项目可加深对行星际间断面与弓激波和地球磁层相互作用的理解，增进对磁鞘磁通量绳演化的动力学过程的认识，对太阳风与磁层耦合过程的研究具有重要意义。

行星际间断面与弓激波和地球磁层的相互作用在磁鞘磁通量绳的形成和演化过程中起到至关重要的作用。通过我们优化改进的三维全球混合数值模拟程序和卫星观测资料，对磁鞘磁通量绳的触发机制及其对磁层顶的影响进行了研究，主要成果如下：（1）行星际切向间断面穿过弓激波到达磁鞘中，分别经历了激波压缩和磁鞘对流压缩过程，间断面两侧磁场强度变强，其宽度变窄，形成很薄很强的重联电流片结构，通过三维补缀重联过程形成磁鞘磁通量绳。该结构在晨昏方向上呈现明显的晨昏不对称性，这与其磁鞘背景等离子体流和局地的电磁结构密切相关。（2）研究了不同切向间断面宽度和旋转角度对磁鞘磁通量绳的影响，发现其宽度越宽，磁鞘磁通量绳越难形成。当间断面的旋转角度越小，磁鞘磁通量绳晨昏方向上的长度越短且晨昏不对称性越明显。（3）研究了磁层顶磁重联对磁鞘磁通量绳的影响，发现初始磁层顶磁重联绳会大大减弱激波压缩和磁鞘对流压缩过程，从而使得磁鞘磁通量绳形成时间更晚且产生位置更靠近磁层顶。随后，在磁鞘磁通量绳与磁层顶磁力线之间发生再次磁重联，形成新的磁层顶磁通量绳结构，为太阳风能量和物质注入地球磁层提供了一种新的机制。（4）通过对行星际旋转间断面与弓激波的相互作用的数值模拟研究，发现弓激波处的压缩过程可改变旋转间断面两侧磁力线位形，在局地可形成了重联电流片。随后，在旋转间断面中心可触发磁重联，从而形成磁通量绳。该项目研究可进一步加深对磁鞘磁通量绳演化以及太阳风与磁层的耦合过程的理解，对未来的磁层空间探测具有重要指导意义。