太阳系四颗类地行星有很多相似之处，但只有水星和地球具有全球性内禀磁场。水星和地球磁层与太阳风的相互作用也具有明显的相似性。但是，地球磁尾中的重离子多来源于电离层上行，水星磁尾中的重离子则主要来源于其表面的溅射和脱附效应。对水星磁尾离子特征开展研究，不仅有助于认识水星磁尾的动力学过程，更可通过对比分析来深入理解地球磁层和电离层系统的变化规律。本项目拟使用信使号的探测数据对水星磁尾离子特征进行较为全面的分析。使用信使号大约16个水星年的离子和磁场数据，在区分水星磁层不同活跃期的前提下，分别研究其等离子体片内不同成分离子（包括质子、二价氦离子和一价钠离子）温度、密度和不同能量段通量的分布，并总结不同活跃期下的离子特征及可能的原因。最后，将与地球磁尾离子的分布进行比较分析，以期进一步理解太阳风和电离层对磁尾动力学过程的影响。

The terrestrial planets in the Solar System, i.e. Mercury, Venus, Earth, and Mars, share many similarities, but only Mercury and Earth possess the global intrinsic magnetic fields. The interactions between the Mercury and Earth's magnetospheres and the solar wind exhibit great resemblance. The heavy ions in the Earth's magnetotail mainly come from the Earth's ionosphere, but are mainly due to surface sputtering and desorption effects at Mercury. Therefore, the study of ion properties in Mercury's magnetotail could not only help in the understanding of Mercury's tail dynamics, but also the dynamics in the coupling process between Earth's magnetosphere and ionosphere through the comparison between Mercury and Earth. This project aims to investigate the ion properties in Mercury's magnetotail. With the help of MESSENGER magnetic field and plasma measurements in about 16 Mercury years, we propose to study the density, temperature, and particle flux distributions for different ion species (including H+, He++, Na+) under different magnetospheric activities. We will summarize the properties of ions and propose the possible mechanisms. In the last, we will comparatively study the magnetotail ion distributions in Mercury and Earth and try to reveal the roles of solar wind and ionosphere in the tail dynamics, respectively.