Kinetic entropy-based measures of distribution function non-Maxwellianity: theory and simulations
Kinetic entropy-based measures of distribution function non-Maxwellianity: theory and simulations
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DOI：
10.1017/s0022377820001270
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发表时间：
2020-10-01
影响因子：
2.5
通讯作者：
Zank, G. P.
中科院分区：
文献类型：
--
作者：
Liang, Haoming;Barbhuiya, M. Hasan;Zank, G. P.
我们研究了等离子体中分布函数非极大性的基于动态熵的度量，即局部分布函数与具有与局部分布相同的密度、体积流量和温度的关联的麦克斯韦分布函数的偏离的基于熵的度量。首先，我们考虑Kaufmann&Paterson(J.GePhys.研究报告，第114卷，2009年，A00D04)，对其性质进行了评估，并得出了等价形式。为了提供对它的定量理解，我们推导了三个常见的非麦克斯韦等离子体分布函数的解析表达式。我们证明了这种非马克斯韦度量有一些不受欢迎的特征，包括它可以在不同的物理极限下发散，并解释了发散的原因。然后，我们引入了一种新的基于速度-空间动熵密度的基于动熵的非麦克斯韦度量，它有一个有意义的物理解释，并且不发散。我们使用二维反平行磁重联的无碰撞胞内粒子模拟来评估基于动态熵的非麦克斯韦度量。我们发现，非零非麦克斯韦区域与磁重联过程中发生的动力学过程有关。我们还表明，模拟的非马克斯韦利性与类似于解析计算的分布的预测相当好地吻合。这些结果可能对应用很重要，因为非麦克斯韦效应可以用来识别动力学尺度物理的区域或等离子体中耗散增加的区域。
We investigate kinetic entropy-based measures of the non-Maxwellianity of distribution functions in plasmas, i.e. entropy-based measures of the departure of a local distribution function from an associated Maxwellian distribution function with the same density, bulk flow and temperature as the local distribution. First, we consider a form previously employed by Kaufmann & Paterson (J. Geophys. Res., vol. 114, 2009, A00D04), assessing its properties and deriving equivalent forms. To provide a quantitative understanding of it, we derive analytical expressions for three common non-Maxwellian plasma distribution functions. We show that there are undesirable features of this non-Maxwellianity measure including that it can diverge in various physical limits and elucidate the reason for the divergence. We then introduce a new kinetic entropy-based non-Maxwellianity measure based on the velocity-space kinetic entropy density, which has a meaningful physical interpretation and does not diverge. We use collisionless particle-in-cell simulations of two-dimensional anti-parallel magnetic reconnection to assess the kinetic entropy-based non-Maxwellianity measures. We show that regions of non-zero non-Maxwellianity are linked to kinetic processes occurring during magnetic reconnection. We also show the simulated non-Maxwellianity agrees reasonably well with predictions for distributions resembling those calculated analytically. These results can be important for applications, as non-Maxwellianity can be used to identify regions of kinetic-scale physics or increased dissipation in plasmas.