MAGNETIC NULL POINTS IN KINETIC SIMULATIONS OF SPACE PLASMAS
MAGNETIC NULL POINTS IN KINETIC SIMULATIONS OF SPACE PLASMAS
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DOI：
10.3847/0004-637x/819/1/52
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发表时间：
2015-12
期刊：
影响因子：
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通讯作者：
V. Olshevsky;J. Deca;A. Divin;I. Peng;S. Markidis;M. Innocenti;E. Cazzola;G. Lapenta
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文献类型：
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作者：
V. Olshevsky;J. Deca;A. Divin;I. Peng;S. Markidis;M. Innocenti;E. Cazzola;G. Lapenta
我们提出了一个系统的尝试，研究磁零点和相关的磁能转换动力学粒子在细胞模拟各种等离子体配置。我们解决了三维模拟与半隐式动力学电磁代码iPic 3D在不同的设置：哈里斯电流片，偶极和四极磁层与太阳风相互作用的变化，和一个轻松的湍流配置与多个零点。螺旋空区更有可能在空间等离子体中产生：在我们所有的模拟中，除了月球磁异常（LMA）和四极微型磁层，螺旋空区的数量超过径向空区的数量3-9倍。我们发现，往往磁零并不表示密集的能量耗散的区域。由径向零点处的拓扑分叉引起的能量耗散事件是相当罕见和短暂的。由Harris电流片和LMA模拟中的径向零点形成的所谓X线相当稳定，并且不表现出任何能量耗散。能量耗散在由磁通绳包围的螺旋空区附近更强大，磁通绳在其轴上具有强电流（其横截面类似于2D磁岛）。这些零线让人想起Z箍缩有效地耗散由于二次不稳定性，如双流或扭结不稳定性，伴随着磁拓扑结构的变化的磁能。伴随着螺旋零点的电流增强可能表明观测数据中的磁能转换点。
We present a systematic attempt to study magnetic null points and the associated magnetic energy conversion in kinetic particle-in-cell simulations of various plasma configurations. We address three-dimensional simulations performed with the semi-implicit kinetic electromagnetic code iPic3D in different setups: variations of a Harris current sheet, dipolar and quadrupolar magnetospheres interacting with the solar wind, and a relaxing turbulent configuration with multiple null points. Spiral nulls are more likely created in space plasmas: in all our simulations except lunar magnetic anomaly (LMA) and quadrupolar mini-magnetosphere the number of spiral nulls prevails over the number of radial nulls by a factor of 3–9. We show that often magnetic nulls do not indicate the regions of intensive energy dissipation. Energy dissipation events caused by topological bifurcations at radial nulls are rather rare and short-lived. The so-called X-lines formed by the radial nulls in the Harris current sheet and LMA simulations are rather stable and do not exhibit any energy dissipation. Energy dissipation is more powerful in the vicinity of spiral nulls enclosed by magnetic flux ropes with strong currents at their axes (their cross sections resemble 2D magnetic islands). These null lines reminiscent of Z-pinches efficiently dissipate magnetic energy due to secondary instabilities such as the two-stream or kinking instability, accompanied by changes in magnetic topology. Current enhancements accompanied by spiral nulls may signal magnetic energy conversion sites in the observational data.