水星磁层顶通量传输事件产生率及空间分布特征研究

Flux transfer events (FTEs) refer to the magnetic flux ropes, formed through magnetic reconnection (MR) of the magnetospheric and magnetosheath magnetic fields at planetary magnetopauses. Such flux ropes offered channels for the transport of the solar wind magnetic flux and plasma into planetary space. The in situ observations of FTEs reoccurrence rate and spatial distribution provide a crucial key to understand the large-scale MR and the magnetic flux transport rate. The characteristic of FTEs reoccurrence rate and spatial distribution at Earth’s magnetopause have been extensively studied, however, so far, this research has not been systematic conducted at Mercury due to lack of sufficient observational data for a long time. Comparing with the Earth, Mercury has weaker intrinsic magnetic field. Nevertheless, the interplanetary magnetic field (IMF) at Mercury orbit is much stronger and mainly in radial direction. Therefore, the signatures of MR and magnetic flux transport are expected to be largely different from each other. Based on the large MESSENGER database, this project will statistically study how the IMF, especially its radial component, controls the FTEs reoccurrence rate and spatial distribution at Mercury’s magnetopause. Further, the obtained results will be compared with the present knowledge of the Earth case. The role of radial IMF in controlling magnetic flux transport rate will be defined. New understandings of the different planetary magnetopause dynamics will be gained from this project.

通量传输事件（FTEs）是行星际磁场与行星磁层磁场重联产生的磁通量管结构，构成太阳风向行星磁层传输磁通量和等离子体的有效通道。卫星实地观测FTEs产生率及空间分布规律是理解大尺度磁重联和磁通量传输效率的关键。地球磁层的大量卫星数据促使FTEs被广泛地研究，但水星磁层探测数据的缺乏导致该类研究始终没有系统化。水星偶极磁层比地球小且磁场较弱，周围行星际磁场更强并且以径向分量为主，因此其磁重联和磁通量传输特征应当与地球不同。本项目拟利用美国新的水星探测计划“信使”号的长期探测数据，通过统计分析行星际磁场，尤其是径向分量，对FTEs产生率及空间分布的影响，来揭示水星磁层顶FTEs产生率及空间分布规律。进一步与地球空间已有结果比较，明确行星际磁场径向分量对FTEs产生及空间分布的控制作用。以期在不同行星空间环境下磁层顶重联动力学特征的普遍规律上获得新的认识。

通量传输事件是行星际磁场与行星磁层磁场重联的重要产物。相比地球，水星磁层顶磁场重联对磁层动力学过程的影响及太阳风与磁层耦合起着更加重要的作用。本项目主要是建立“信使”号卫星穿越水星磁层顶及通量传输事件的数据库，利用该数据库研究太阳风对水星磁层顶重联的产生及空间分布的控制，并与地球磁层顶重联相关特征进行比较研究。主要取得了三方面的研究进展：（1）建立了完整的水星磁层顶及通量传输事件的事件库；（2）基于事件库建立了水星磁层顶三维不对称模型，并揭示了弱偶极磁场磁层的三维特征。（3）区别于地球，发现了水星磁层顶重联对磁层大尺度动力学特征的影响明显受控于太阳风，在接近水星轨道的近日点，除了平均太阳风动压增大外，太阳风低beta值及低阿尔芬马赫数导致磁层顶磁场重联效应加剧，重联效应甚至可以抵消内核感应电流的影响，从而使水星向阳侧磁层顶明显压缩。分析表明弱偶极磁场并不能像地球一样有效屏蔽太阳风入侵，太阳风在南半球中纬弱磁场区域可以直接轰击水星表面。由此推断水星空间环境是由“太阳风-磁层-水星”以及“太阳风-水星”两种作用方式共同决定的。