标题： 宇宙射线在多相、动态星际介质中的能量依赖输运 显示英文标题

标题： Energy-Dependent Transport of Cosmic Rays in the Multiphase, Dynamic Interstellar Medium

摘要： 我们研究了在动态多相星际介质（ISM）中，具有动能在$1-100$GeV 之间的光谱分辨宇宙射线（CR）质子的输运，使用一个两矩量CR流体求解器应用于与太阳邻域条件相似的TIGRESS MHD模拟。我们的CR输运方案包含了从由漂移驱动的阿尔芬波增长和耗散过程之间的局部平衡计算得到的空间和动量相关的CR散射系数$\sigma=\kappa^{-1}$。我们发现对流与动量相关的扩散相结合，产生了与观测结果一致的CR分布函数$f(p)\propto~p^{-\gamma}$，其斜率$\gamma\approx4.6$从注入的幂律斜率$\gamma_\mathrm{inj}=4.3$增加了。在高度扩散且主要中性的中平面区域，CR压力是均匀的，但在电离的外平面区域，由于散射效率高，CR压力呈指数下降。为了解释这些数值结果，我们开发了一个两区解析模型，该模型捕捉并联系了多相、动态ISM中CR输运的两个（物理上和空间上）不同的区域。在低动量下，CR输运由气体对流主导，而在高动量下，对流和扩散都起作用。在高动量下，解析预测的谱斜率接近$\gamma=(4/3)\gamma_\mathrm{inj}-1$，并且与动量相关的grammage预测标度为$X\propto p^{1-\gamma_\mathrm{inj}/3}$，与模拟结果一致。 这些结果支持一种物理图景，其中宇宙射线被周围的电离气体限制在中性中平面内，它们的逃逸由电离星系外气体中的宇宙射线散射率以及该气体的速度和阿尔芬速度共同调控，在有效速度$v_\mathrm{c,eff}\approx(1/2)[\kappa_\parallel~d(v+v_\mathrm{A,i})/dz]^{1/2}$处。 显示英文摘要

摘要： We investigate the transport of spectrally resolved cosmic ray (CR) protons with kinetic energies between $1-100$ GeV within the dynamic, multiphase interstellar medium (ISM), using a two-moment CR fluid solver applied to a TIGRESS MHD simulation with conditions similar to the solar neighborhood. Our CR transport prescription incorporates space- and momentum-dependent CR scattering coefficients $\sigma=\kappa^{-1}$, computed from the local balance between streaming-driven Alfv\`{e}n wave growth and damping processes. We find that advection combines with momentum-dependent diffusion to produce a CR distribution function $f(p)\propto~p^{-\gamma}$ with $\gamma\approx4.6$ that agrees with observations, steepened from an injected power law slope $\gamma_\mathrm{inj}=4.3$. The CR pressure is uniform in the highly diffusive, mostly neutral midplane region, but decreases exponentially in the ionized extraplanar region where scattering is efficient. To interpret these numerical results, we develop a two-zone analytic model that captures and links the two (physically and spatially) distinct regimes of CR transport in the multiphase, dynamic ISM. At low momenta, CR transport is dominated by gas advection, while at high momenta, both advection and diffusion contribute. At high momentum, the analytic prediction for the spectral slope approaches $\gamma=(4/3)\gamma_\mathrm{inj}-1$, and the predicted scaling of grammage with momentum is $X\propto p^{1-\gamma_\mathrm{inj}/3}$, consistent with the simulations. These results support a physical picture in which CRs are confined within the neutral midplane by the surrounding ionized gas, with their escape regulated by both the CR scattering rate in the ionized extraplanar gas and the velocity and Alfv\'{e}n speed of that gas, at effective speed $v_\mathrm{c,eff}\approx(1/2)[\kappa_\parallel~d(v+v_\mathrm{A,i})/dz]^{1/2}$.