Cosmic-Ray Transport in Simulations of Star-forming Galactic Disks
  Cosmic ray transport on galactic scales depends on the detailed properties of
the magnetized, multiphase interstellar medium (ISM). In this work, we
post-process a high-resolution TIGRESS magnetohydrodynamic simulation modeling
a local galactic disk patch with a two-moment fluid algorithm for cosmic ray
transport. We consider a variety of prescriptions for the cosmic rays, from a
simple purely diffusive formalism with constant scattering coefficient, to a
physically-motivated model in which the scattering coefficient is set by
critical balance between streaming-driven Alfv\'en wave excitation and damping
mediated by local gas properties. We separately focus on cosmic rays with
kinetic energies of $\sim 1$ GeV (high-energy) and $\sim 30$~MeV (low-energy),
respectively important for ISM dynamics and chemistry. We find that
simultaneously accounting for advection, streaming, and diffusion of cosmic
rays is crucial for properly modeling their transport. Advection dominates in
the high-velocity, low-density, hot phase, while diffusion and streaming are
more important in higher density, cooler phases. Our physically-motivated model
shows that there is no single diffusivity for cosmic-ray transport: the
scattering coefficient varies by four or more orders of magnitude, maximal at
density $n_\mathrm{H} \sim 0.01\, \mathrm{cm}^{-3}$. Ion-neutral damping of
Alfv\'en waves results in strong diffusion and nearly uniform cosmic ray
pressure within most of the mass of the ISM. However, cosmic rays are trapped
near the disk midplane by the higher scattering rate in the surrounding
lower-density, higher-ionization gas. The transport of high-energy cosmic rays
differs from that of low-energy cosmic rays, with less effective diffusion and
greater energy losses for the latter.
