Ion-scale Structures in Flux Ropes Observed by MMS at the Magnetopause
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摘要: 利用MMS观测数据，对磁层顶通量绳内离子惯性尺度（di）的结构进行分析研究.结果发现，许多不同尺度（约1di至数十di）的通量绳内都存在具有di尺度的电流 j m，其方向在磁层顶局地坐标系的-M方向，即与磁层顶查普曼-费拉罗电流同向，由电子在+M方向的运动（ v em）携带.这些电流结构具有以下特征：磁鞘与磁层成分混合，磁场为开放形态；离子去磁化，电子与磁场冻结；N方向（即垂直于磁层顶电流片方向）的电场 E n显著增大，幅度达到约20mV·m-1，并伴有明显的尖峰状起伏，该增强和尖峰状起伏的电场对应于霍尔电场.分析表明，电流、电子与离子运动的偏离以及霍尔电场之间遵从广义欧姆定律，三者密切关联.进一步对磁层顶磁重联的探测数据进行分析发现，在很多重联区内也存在与通量绳内相似的结构，其尺度约为di量级，其中霍尔电场 E N、电流 j M和电子速度 v eM均与通量绳内对应物理量的方向相同且幅度相近.基于上述观测事实，采用经典FTE通量绳模型，对通量绳内电流、电子运动和霍尔电场的起源进行了初步探讨，认为其来源于磁层顶无碰撞磁重联区内的相应结构，并且后者在离子尺度通量绳的形成过程中起到重要作用.Abstract: In this paper the structures with scale of ion inertial length (di) in flux ropes at the magnetopause are studied based on MMS measurements. The results show that currents ( j m) of di scale are found to exist in many flux ropes with different scales, which flow in the -M direction in magnetopause local coordinates (i.e., in the same direction of the Chapman-Ferraro current at the magnetopause) and are carried by electrons' motion in the +M direction ( v em). Within the current structures, magnetosheath and magnetospheric plasma populations are mixed; the magnetic field has open topology; ions are non-magnetized, while electrons are frozen-in with the magnetic field lines; the N-component of electric field ( E n), which is Hall electric field in nature, substantially enhances (up to about 20mV·m-1), accompanying with notable fluctuations. Detailed analysis shows that the current, separation of electrons' motion from ions and the Hall electric field are closely related to each other, and obey the general Ohm's Law. In addition, we have also analyzed the MMS measurements of magnetic reconnection events at the magnetopause. It is found that structures similar to those in flux ropes are also present inside the reconnection region in many cases. Their scales are of di length. The directions (magnitudes) of the Hall electric field E N, current filament j M and electron velocity v eM are as same as (close to) those in flux ropes. On the bases of above observations and making use of the classical flux rope models, how the di-scale structures in flux ropes are formed is studied. It is suggested that they are likely to originate from the corresponding structures in the reconnection region at the magnetopause which play an essential role in the formation process of di-scale flux ropes.
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Key words:
- Magnetic flux ropes /
- Ion-scale current /
- Hall electric field /
- Magnetopause /
- Magnetic reconnection
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