arXiv 1709.096442017-09-27physics.plasm-ph

快速太阳风中离子尺度的相干结构：{ \it Cluster}观测

Coherent structures at ion scales in fast solar wind: {\it Cluster} observations

D. Perrone, O. Alexandrova, O. W. Roberts, S. Lion, C. Lacombe, A. Walsh, M. Maksimovic, I. Zouganelis

DOI：: 10.3847/1538-4357/aa9022

摘要：

利用{ \it Cluster}数据研究了快速太阳风流中离子特征尺度附近的磁湍流涨落的性质。与慢太阳风不同，在\cite []{per 16}中观察到Alfvénic（$δb_{\perp} \gg δb_{\parallel}$）和压缩（$δb_{\parallel} \gg δb_{\perp}$）相干结构，快太阳风的湍流级联由Alfvénic结构（即Alfvén涡旋）主导，具有小的和/或有限的压缩部分，还存在与局部磁场对齐的几个电流片。还识别了几个涡链的例子。虽然离子尺度附近的磁压缩性的增加也观察到快速太阳风，没有强烈的压缩结构被发现，这意味着慢风和快风的性质是本质上不同的。应用于该快风区间的多航天器分析表明，相干结构几乎被气流对流并与局部磁场对齐，即它们的法线垂直于{ \bf B}，这与二维湍流图像一致。理解相干性和相干结构的相关产生可以提供一个关键的洞察磁化和无碰撞等离子体中的非线性能量转移和耗散过程。

英文摘要：

We investigate the nature of magnetic turbulent fluctuations, around ion characteristic scales, in a fast solar wind stream, by using {\it Cluster} data. Contrarily to slow solar wind, where both Alfvénic ($δb_{\perp} \gg δb_{\parallel}$) and compressive ($δb_{\parallel} \gg δb_{\perp}$) coherent structures are observed \cite[]{per16}, the turbulent cascade of fast solar wind is dominated by Alfvénic structures, namely Alfvén vortices, with small and/or finite compressive part, with the presence also of several current sheets aligned with the local magnetic field. Several examples of vortex chains are also recognized. Although an increase of magnetic compressibility around ion scales is observed also for fast solar wind, no strongly compressive structures are found, meaning that the nature of the slow and fast winds is intrinsically different. Multi-spacecraft analysis applied to this interval of fast wind indicate that the coherent structures are almost convected by the flow and aligned with the local magnetic field, i.e. their normal is perpendicular to {\bf B}, that is consistent with a two dimensional turbulence picture. Understanding intermittency and the related generation of coherent structures could provide a key insight into the nonlinear energy transfer and dissipation processes in magnetized and collisionless plasmas.

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英文摘要：

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