Slow Cosmic-Ray Diffusion in Supersonic and Super-Alfvénic Turbulence

Abstract

Abstract Extended TeV–PeV gamma-ray halo observations imply cosmic-ray (CR) diffusion that is both substantially slower than the Galactic mean and only weakly anisotropic despite the presence of a large-scale Galactic magnetic field background. We investigate whether such transport can arise in partially ionized source environments, where ion-neutral damping removes the small-scale fluctuations responsible for gyroresonant scattering. Using two-fluid magnetohydrodynamic turbulence simulations with relativistic test-particle tracking, we find that damping increases the parallel diffusion coefficient by approximately 2 orders of magnitude in trans-sonic turbulence, thereby strongly enhancing field-aligned transport. In supersonic turbulence, shock-associated magnetic fluctuations survive the damping and sustain nonresonant pitch-angle scattering, limiting the increase in parallel diffusion to a factor of a few to 10. At high Alfvénic Mach numbers, M A ≳ 5, the magnetic-field direction decorrelates over the Alfvén scale, driving parallel diffusion coefficient D ∥ and perpendicular diffusion coefficient D ⊥ toward equality. For M s ≃ 10, M A ≃ 5–10, and representative parameters for hundred-TeV gamma-ray source environments, we obtain comparable D ∥ and D ⊥ , within the observationally inferred range of ∼10 27 –10 28 cm 2 s −1 . These results show that strongly supersonic, highly super-Alfvénic turbulence in source environments can sustain slow, nearly isotropic CR transport even when the ion-neutral damping effect is important.