MHD-PIC Simulations of Cosmic-Ray Scattering and Transport in Inhomogeneously Ionized Plasma

Abstract

Cosmic rays (CRs) have critical impacts in the multiphase interstellar medium (ISM), driving dynamical motions in low-density plasma and modifying the ionization state, temperature, and chemical composition of higher-density atomic and molecular gas. We present a study of CR propagation in inhomogeneously ionized plasma, addressing CR transport issues that arise in the cloudy ISM. Using one-dimensional magnetohydrodynamic (MHD) particle-in-cell simulations that include ion–neutral drag to damp Alfvén waves in a portion of the simulation domain, we self-consistently evolve the kinetic physics of CRs and background gas MHD. By introducing a damping region in our periodic domain, our simulations break translational symmetry and allow the emergence of spatial gradients in the CR distribution function. A spatial gradient opposite to the CR flux forms across the fully ionized region as a result of pitch angle scattering. We connect our results with CR hydrodynamics formulations by computing the wave–particle scattering rates as predicted by quasilinear, fluid, and Fokker–Planck theory. For momenta where the mean free path is short relative to the box size, we find excellent agreement among all scattering rates. However, we also find evidence of a reduced scattering rate for less energetic particles that are subject to the μ = 0 barrier in our simulations. Our work provides a first-principles verification of CR hydrodynamics when particles stream down their pressure gradient and opens a pathway toward comprehensive calibrations of transport coefficients from self-generated Alfvén wave scattering with CRs.