Cosmic Ray Perpendicular Superdiffusion and Parallel Mirror Diffusion in a Partially Ionized and Turbulent Medium

Abstract

Understanding cosmic ray (CR) diffusion in a partially ionized medium is both crucial and challenging. In this study, we investigate CR perpendicular superdiffusion and parallel transport in turbulent, partially ionized media using high-resolution 3D two-fluid simulations that treat ions and neutrals separately. We examine the influence of neutral-ion decoupling and the associated damping of turbulence on CR propagation in both transonic and supersonic conditions. Our simulations demonstrate that neutral-ion decoupling significantly damps velocity and magnetic field fluctuations at small scales, producing spectral slopes steeper than those of Kolmogorov and Burgers scaling. In supersonic turbulence, large-scale shock motion is not subject to damping and generates small-scale density enhancements. Moreover, the damping of magnetic field fluctuations substantially decreases pitch-angle scattering, which, however, only slightly affects the CR parallel mean free path $λ_\|$, due to the nonresonant mirror interactions of CRs. In the direction perpendicular to the mean magnetic field, we identify two regimes of the perpendicular superdiffusion of CRs: a diffusive regime ($λ_\|<L_{\rm inj}$, where $L_{\rm inj}$ is turbulence injection scale) with perpendicular separation of CR proportional to $t^{3/4}$, and a ballistic-like regime ($λ_\|>L_{\rm inj}$), with perpendicular separation scaling as $t^{3/2}$. At initially large pitch angles, the effects of magnetic mirroring-naturally arising in magnetohydrodynamic turbulence-become significant, enhancing the confinement of CRs and resulting in $λ_\|<L_{\rm inj}$, despite the damping effect. These results imply that large-pitch-angle CRs can be well confined in the cold ISM, such as molecular clouds.