论文标题

弱碰撞等离子体中的波动迪纳摩

Fluctuation dynamo in a weakly collisional plasma

论文作者

St-Onge, D. A., Kunz, M. W., Squire, J., Schekochihin, A. A.

论文摘要

宇宙磁场的湍流扩增取决于宿主血浆的材料特性。在许多热稀的天体物理系统中,例如星系簇的簇内培养基(ICM),粒子 - 颗粒碰撞的稀有性允许脱离局部热力学平衡。这些偏离对等离子体运动的各向异性粘应力抑制了它们伸展磁场线的能力。我们介绍了使用磁性水力动力学(MHD)方程在弱碰撞血浆中的波动发电机的数值研究,并具有场并行粘性(Braginskii)应力。当应力仅限于与由消防和镜像不稳定性调节的压力各向异性一致的值时,Braginskii-MHD Dyn​​amo很大程度上类似于其MHD对应物。如果取而代之的是,平行的粘性应力没有减弱 - 与最近对磁化ICM中发动机的动力学模拟以及发电机的早期阶段相关的情况 - 迪纳摩改变了其特征,放大了磁场,同时表现出大量的大型prandtl-number的饱和状态($ rm pm pm pm pm pm {$ rm rm pm} $ nim} $ nim}我们在该方案中构建了Braginskii-MHD发电机的分析模型,该模型成功匹配了磁能光谱。通过我们的模拟证实,该模型的一个预测是,如果垂直粘度和平行粘度的比率太小,则不具有压力 - 动型限制器的Braginskii-MHD等离子体将不支持发电机。该比率反映了场线拉伸和混合的相对速率,后者促进了磁场的电阻耗散。在确实表现出发电机的所有情况下,生成的磁场都被组织成褶皱,持续到饱和状态,并偏向混沌流以获取比例依赖的光谱各向异性。

The turbulent amplification of cosmic magnetic fields depends upon the material properties of the host plasma. In many hot, dilute astrophysical systems, such as the intracluster medium (ICM) of galaxy clusters, the rarity of particle--particle collisions allows departures from local thermodynamic equilibrium. These departures exert anisotropic viscous stresses on the plasma motions that inhibit their ability to stretch magnetic-field lines. We present a numerical study of the fluctuation dynamo in a weakly collisional plasma using magnetohydrodynamic (MHD) equations endowed with a field-parallel viscous (Braginskii) stress. When the stress is limited to values consistent with a pressure anisotropy regulated by firehose and mirror instabilities, the Braginskii-MHD dynamo largely resembles its MHD counterpart. If instead the parallel viscous stress is left unabated -- a situation relevant to recent kinetic simulations of the fluctuation dynamo and to the early stages of the dynamo in a magnetized ICM -- the dynamo changes its character, amplifying the magnetic field while exhibiting many characteristics of the saturated state of the large-Prandtl-number (${\rm Pm}\gtrsim{1}$) MHD dynamo. We construct an analytic model for the Braginskii-MHD dynamo in this regime, which successfully matches magnetic-energy spectra. A prediction of this model, confirmed by our simulations, is that a Braginskii-MHD plasma without pressure-anisotropy limiters will not support a dynamo if the ratio of perpendicular and parallel viscosities is too small. This ratio reflects the relative allowed rates of field-line stretching and mixing, the latter of which promotes resistive dissipation of the magnetic field. In all cases that do exhibit a dynamo, the generated magnetic field is organized into folds that persist into the saturated state and bias the chaotic flow to acquire a scale-dependent spectral anisotropy.

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