宇宙线压强各向异性不稳定性的动力学模拟：饱和状态下的宇宙线散射率
Kinetic simulations of the cosmic ray pressure anisotropy instability: cosmic ray scattering rate in the saturated state
AI总结：
本文利用MHD-PIC框架下的一维动力学模拟结合自适应δf方法，研究了宇宙线压强各向异性不稳定性的饱和状态，测量了有效散射率νeff并建立了与环境参数的标度关系，为宇宙线反馈的宏观模拟提供亚网格标定依据。
摘要：
宇宙线（CR）反馈通过与磁流体力学波的相互作用，在塑造星系的形成与演化中起着至关重要的作用。在宇宙线自约束图像中，这些波由宇宙线回旋共振不稳定性通过宇宙线流或宇宙线压强各向异性产生，并通过平衡波阻尼而达到饱和。由此产生的波对粒子的有效散射率 νeff 关键性地决定了宇宙线与背景气体之间的耦合，但宇宙线反馈的效率目前仍缺乏良好的约束。我们采用磁流体力学-粒子网格（MHD-PIC）框架下的一维动力学模拟，并结合自适应 δf 方法，来量化在离子-中性摩擦存在时宇宙线压强各向异性不稳定性（CRPAI）饱和状态下的 νeff。我们通过膨胀/压缩模拟盒来驱动宇宙线压强各向异性，以模拟磁场强度背景演化，宇宙线压强各向异性最终通过平衡准线性扩散而达到准稳态。在饱和状态下，我们测量了 νeff 和宇宙线压强各向异性水平，建立了与环境参数之间的标定标度关系。该标度关系与准线性理论一致，并可纳入宇宙线流体模型中，无论是单流体处理还是逐动量（p-by-p）处理。我们的结果为在宇宙线反馈与输运的宏观研究中准确标定亚网格物理奠定了基础。
英文摘要：
Cosmic ray (CR) feedback plays a vital role in shaping the formation and evolution of galaxies through their interaction with magnetohydrodynamic waves. In the CR self-confinement scenario, the waves are generated by the CR gyro-resonant instabilities via CR streaming or CR pressure anisotropy, and saturate by balancing wave damping. The resulting effective particle scattering rate by the waves, νeff, critically sets the coupling between the CRs and background gas, but the efficiency of CR feedback is yet poorly constrained. We employ 1D kinetic simulations under the Magnetohydrodynamic-Particle-In-Cell (MHD-PIC) framework with the adaptive δf method to quantify νeff for the saturated state of the CR pressure anisotropy instability (CRPAI) with ion-neutral friction. We drive CR pressure anisotropy by expanding/compressing box, mimicking background evolution of magnetic field strength, and the CR pressure anisotropy eventually reaches a quasi-steady state by balancing quasi-linear diffusion. At the saturated state, we measure νeff and the CR pressure anisotropy level, establishing a calibrated scaling relation with environmental parameters. The scaling relation is consistent with quasi-linear theory and can be incorporated to CR fluid models, in either the single-fluid or p-by-p treatments. Our results serve as a basis towards accurately calibrating the subgrid physics in macroscopic studies of CR feedback and transport.
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