We explore the physics of the gyroresonant cosmic-ray streaming instability (CRSI) including the effects of ion–neutral (IN) damping. This is the main damping mechanism in (partially ionized) atomic and molecular gas, which are the primary components of the interstellar medium (ISM) by mass. Limitation of CRSI by IN damping is important in setting the amplitude of Alfvn waves that scatter cosmic rays (CRs) and control galactic-scale transport. Our study employs the magnetohydrodynamic (MHD)–particle-in-cell hybrid fluid-kinetic numerical technique to follow linear growth as well as post-linear and saturation phases. During the linear phase of the instability—where simulations and analytical theory are in good agreement—IN damping prevents wave growth at small and large wavelengths, with the unstable bandwidth lower for higher IN collision rates ν _in. Purely MHD effects during the post-linear phase extend the wave spectrum toward larger k. In the saturated state, the CR distribution evolves toward greater isotropy (lower streaming velocity) by scattering off of Alfvn waves excited by the instability. In the absence of low-k waves, CRs with sufficiently high momentum are not isotropized. The maximum wave amplitude and rate of isotropization of the distribution function decrease at higher ν _in. When the IN damping rate approaches the maximum growth rate of CRSI, wave growth and isotropization are suppressed. Implications of our results for CR transport in partially ionized ISM phases are discussed.

我们探索了陀螺共振宇宙射线流不稳定性 (CRSI) 的物理学，包括离子中性 (IN) 阻尼的影响。这是（部分电离的）原子和分子气体的主要阻尼机制，它们是星际介质 (ISM) 质量的主要成分。IN阻尼对CRSI的限制对于设置散射宇宙射线（CR）和控制星系尺度传输的Alfvn波的振幅很重要。我们的研究采用磁流体动力学 (MHD)-细胞内粒子混合流体动力学数值技术来跟踪线性生长以及后线性和饱和阶段。在不稳定性的线性阶段——模拟和分析理论非常吻合——IN 阻尼阻止了小波长和大波长的波增长，对于更高的 IN 碰撞率，不稳定带宽较低ν _在。后线性阶段的纯 MHD 效应将波谱扩展到更大的k。在饱和状态下，CR 分布通过不稳定性激发的 Alfvn 波的散射而向更大的各向同性（更低的流动速度）发展。在没有低k波的情况下，具有足够高动量的 CR 不是各向同性的。分布函数的最大波幅和各向同性率在ν _in较高时减小。当 IN 阻尼率接近 CRSI 的最大增长率时，波的增长和各向同性受到抑制。讨论了我们的结果对部分电离 ISM 相中 CR 传输的影响。