Turbulence in collisionless plasmas: statistical analysis from numerical simulations with pressure anisotropy
Turbulence in collisionless plasmas: statistical analysis from numerical simulations with pressure anisotropy
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无碰撞等离子体中的湍流：压力各向异性数值模拟的统计分析
DOI：
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发表时间：
2010
期刊：
影响因子：
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通讯作者：
A. Lazarian
中科院分区：
文献类型：
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作者：
Grzegorz Kowal;Grzegorz Kowal;Grzegorz Kowal;D. Falceta;D. Falceta;A. Lazarian
近年来，我们对无碰撞等离子体的物理性质的理解越来越感兴趣，主要是因为大量的天体物理环境（例如，团内介质（ICM））包含磁场，这些磁场足够强，可以与电离气体耦合，并且其特征是密度足够低，可以防止相对于磁力线方向的压力各向同性。在这些条件下，出现了一类新的动力学不稳定性，如消防水龙带和镜像不稳定性，这在文献中已经得到了广泛的研究。然而，在压力各向异性的存在下，它们在湍流演化和叶栅过程中的作用仍然不清楚。在这项工作中，我们提出了第一个统计分析的湍流在无碰撞等离子体中使用三维数值模拟和求解双等温磁流体动力学方程与Chew-Goldberger-Low定律封闭（CGL-MHD）。我们研究了具有不同初始条件的模型，以考虑水龙带和镜像不稳定性，并获得不同的湍流状态。我们发现，CGL-MHD亚音速和超音速的湍流度在大多数情况下与MHD模型相比显示出很小的差异。然而，在强动力学不稳定性的区域中，统计，即密度和速度的概率分布函数（PDF），是非常不同的。在亚音速模型中，不稳定性导致密度色散的增加，而速度色散在某些情况下增加了一个大的因素。此外，密度和速度的谱显示在小尺度上增加的功率，解释了高增长率的不稳定性。最后，我们计算了由磁力线方向定义的局域参考系中速度和密度涨落的结构函数。结果表明，在某些情况下，不稳定性显着增加波动的各向异性。这些结果，即使是初步的，并限制在非常具体的条件下，表明在无碰撞等离子体中的湍流的物理性质，如在ICM中发现的，可能与人们普遍认为的有很大的不同。其影响范围从能量交换到宇宙射线加速。
In recent years, we have experienced increasing interest in the understanding of the physical properties of collisionless plasmas, mostly because of the large number of astrophysical environments (e.g. the intracluster medium (ICM)) containing magnetic fields that are strong enough to be coupled with the ionized gas and characterized by densities sufficiently low to prevent the pressure isotropization with respect to the magnetic line direction. Under these conditions, a new class of kinetic instabilities arises, such as firehose and mirror instabilities, which have been studied extensively in the literature. Their role in the turbulence evolution and cascade process in the presence of pressure anisotropy, however, is still unclear. In this work, we present the first statistical analysis of turbulence in collisionless plasmas using three-dimensional numerical simulations and solving double-isothermal magnetohydrodynamic equations with the Chew–Goldberger–Low laws closure (CGL-MHD). We study models with different initial conditions to account for the firehose and mirror instabilities and to obtain different turbulent regimes. We found that the CGL-MHD subsonic and supersonic turbulences show small differences compared to the MHD models in most cases. However, in the regimes of strong kinetic instabilities, the statistics, i.e. the probability distribution functions (PDFs) of density and velocity, are very different. In subsonic models, the instabilities cause an increase in the dispersion of density, while the dispersion of velocity is increased by a large factor in some cases. Moreover, the spectra of density and velocity show increased power at small scales explained by the high growth rate of the instabilities. Finally, we calculated the structure functions of velocity and density fluctuations in the local reference frame defined by the direction of magnetic lines. The results indicate that in some cases the instabilities significantly increase the anisotropy of fluctuations. These results, even though preliminary and restricted to very specific conditions, show that the physical properties of turbulence in collisionless plasmas, as those found in the ICM, may be very different from what has been largely believed. Implications can range from interchange of energies to cosmic ray acceleration.