水星与地球磁层中亚暴演化过程的比较研究

The substorm is a fundamental mode of magnetospheric disturbances and it is also one type of responses for the magnetosphere to the Interplanetary Magnetic Field (IMF) disturbances. Its evolution and triggering mechanism has always been an important research topic in space physics. Mercury is the innermost planet in the solar system. It is also the only planet that has the same polarity as Earth`s field in the inner solar system. Although the surface field strength of Mercury is only 1% that of Earth`s, the orbital observations have verified that the Earth like magnetospheric substorm activity can occur in Mercury`s magnetosphere. This project will investigate the substorm evolution process occurring in Mercury`s magnetosphere by using the observational data of magnetic field and plasma obtained with MESSENGER. We will also conduct a comparative study of the substorm process occurring in Mercury`s and Earth`s magnetosphere by combining the observations of typical substorms in the geomagnetosphere in order to find out the key differences and similarities between the two. The main contents cover the comparison of the temporal and spatial scales of the substorm evolution as well as the energy release pattern during the expansion phase and the influence of different magnetospheric inner boundaries and different external solar wind conditions on the substorm evolution occurring in Mercury`s and Earth`s magnetosphere. This research is expected to provide the universal theoretical model for the substorm evolution occurring in the planetary magnetospheres with different spatial scales, to lay a foundation for better understanding the substorm process which is an explosive energy release process in the planetary magnetosphere, and to provide references for further carrying out the comparative planetology.

亚暴是磁层扰动的一种基本模式，也是磁层对行星际扰动的一种响应，其演化规律和触发机制一直是空间物理中的重要研究课题。水星是离太阳最近的行星，也是内太阳系中唯一具有和地球磁场极性相同的磁层结构的行星。尽管水星表面磁场强度仅为地球的1%，但观测已证实水星磁层中可发生类似地球的磁层亚暴活动过程。本项目拟利用MESSENGER的磁场和等离子体观测数据，并结合地球磁层中典型亚暴的观测，对发生在水星磁层中的亚暴演化过程进行分析和对比研究，以期找出二者之间的关键差异和相似之处。主要内容包括亚暴演化的时间尺度、空间尺度以及亚暴过程中能量释放方式的对比；水星和地球不同的磁层内边界和外部太阳风条件对亚暴演化过程的影响。本项目的研究预期将对拥有不同尺度内禀磁场的行星磁层中的亚暴演化过程给出普适的理论模式，为更好地理解和认识亚暴这一行星磁层中爆发性能量释放过程打下基础，并能够为进一步开展比较行星学研究提供参考。

作为磁层扰动基本模式的磁层亚暴，是理解宇宙中等离子体空间能量释放过程的一个理想原型。水星是除地球外唯一一个拥有内禀磁场的类地行星，因此对水星磁层亚暴演化过程的研究有助于人们理解不同行星磁层中的能量释放过程。本项目利用MESSENGER卫星四年的在轨观测数据对发生在水星磁层中的亚暴相关现象进行了分析研究。主要研究内容包括：（1）水星亚暴增长相和膨胀相的平均持续时间分别为82.68秒和94.36秒，仅为地球磁层亚暴相应相位持续时间的0.01-0.02%；（2）水星亚暴期间所释放的平均磁通量约占水星总磁通量的11%，远远高于地球磁层亚暴期间的2.6%；（3）发生在昏侧的水星亚暴平均持续时间和磁通量释放量均大于发生在晨侧的水星亚暴，但其磁通量释放率是一致的；（4）水星亚暴膨胀相期间磁尾磁场的南北分量呈现了阶梯状的增加，同时伴随着磁能的释放过程，我们认为造成这一现象的原因极有可能水星磁尾发生了多次磁场重联过程，其产生的高速流在近地发生堆积造成的。本项目的研究成果为我们更好的理解和认识亚暴这一行星磁层中爆发性能量释放过程打下一定的基础，也可为进一步开展比较行星学研究提供参考。在本项目的支持下，我们还在地球磁暴和辐射带研究方面取得了一些进展。