Pitch-Angle Scattering of Cosmic Rays: Confronting Theory with Observations

Cosmic ray (CR) propagation is controlled by scattering in turbulent magnetic fields in space. In general, diffusive propagation is governed by pitch-angle diffusion in phase space. In this study, pitch-angle diffusion in the local interstellar medium (LISM) deduced from the analysis of {the CR small scale anisotropy data} from the Tibet AS$\gamma$ experiment is compared with theoretical predictions. While it is difficult to reconcile the inferred LISM pitch angle diffusion coefficient with conventional theoretical results of particle scattering by Alfv\'{e}nic turbulence, we find {very good} agreement with the prediction from particle scattering in quasi-slab fast modes shaped by the damping in the warm ionized medium. These findings offer direct evidence that CR scattering is predominantly governed by fast-mode turbulence. Furthermore, the comparison between experimental and theoretical results imposes strong constraints on plasma and magnetic field parameters within the local bubble, indicating that the LISM is in a low $\beta\simeq 0.1$ condition. The turbulence in the LISM should be compressible with a fast mode component of amplitude approximately $\delta B/B_0 \approx 0.5$.