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

Huirong Yan; Siqi Zhao; Ming Zhang

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arXiv · 2605.24143

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

Huirong Yan·Siqi Zhao·Ming Zhang

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Abstract

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 remarkable agreement with the predictions of particle scattering by quasi-slab fast modes whose properties are shaped by 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 $\beta$ condition. The turbulence in the LISM should be compressible with an amplitude in the range of $0.3\lesssim \delta B/B_0 <1$.

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Huirong Yan, Siqi Zhao, Ming Zhang. 2026-05-22. Pitch-Angle Scattering of Cosmic Rays: Confronting Theory with Observations. https://arxiv.org/abs/2605.24143

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