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 – pressure anisotropies – exert anisotropic viscous stresses on the plasma motions that inhibit their ability to stretch magnetic-field lines. We present an extensive 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, particularly when the magnetic field is dynamically weak. If instead the parallel viscous stress is left unabated – a situation relevant to recent kinetic simulations of the fluctuation dynamo and, we argue, to the early stages of the dynamo in a magnetized ICM – the dynamo changes its character, amplifying the magnetic field while exhibiting many characteristics reminiscent of the saturated state of the large-Prandtl-number ( ${Pm}\gtrsim {1}$ ) MHD dynamo. We construct an analytic model for the Braginskii-MHD dynamo in this regime, which successfully matches simulated dynamo growth rates and magnetic-energy spectra. A prediction of this model, confirmed by our numerical 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 viable 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.

中文翻译：

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弱碰撞等离子体中的波动发电机

宇宙磁场的湍流放大取决于宿主等离子体的材料特性。在许多热的、稀薄的天体物理系统中，例如星系团的团内介质 (ICM)，粒子-粒子碰撞的罕见性导致偏离局部热力学平衡。这些偏离——压力各向异性——对等离子体运动施加各向异性的粘性应力，抑制它们拉伸磁场线的能力。我们使用具有平行场粘性（Braginskii）应力的磁流体动力学（MHD）方程对弱碰撞等离子体中的波动发电机进行了广泛的数值研究。当应力限制在与受消防软管和镜子不稳定性调节的压力各向异性一致的值时，Braginskii-MHD 发电机在很大程度上类似于其 MHD 对应物，特别是当磁场动态弱时。相反，如果平行粘性应力没有减弱——这种情况与最近对波动发电机的动力学模拟有关，我们认为，这与磁化 ICM 中发电机的早期阶段有关——发电机会改变其特性，放大磁场，同时表现出许多特征让人想起大普朗特数的饱和状态（ ${下午}\gtrsim {1}$ ) MHD 发电机。我们为该状态下的 Braginskii-MHD 发电机构建了一个分析模型，该模型成功地匹配了模拟的发电机增长率和磁能谱。我们的数值模拟证实了该模型的预测是，如果垂直和平行粘度的比率太小，没有压力各向异性限制器的 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允许速率，后者促进了磁场的电阻耗散。在所有显示出可行发电机的情况下，产生的磁场被组织成折叠，这些折叠持续到饱和状态并偏置混沌流以获得与尺度相关的光谱各向异性。