Cosmic Ray Diffusion in the Turbulent Interstellar Medium: Effects of Mirror Diffusion and Pitch Angle Scattering

TL;DR

This paper investigates cosmic ray diffusion in the turbulent interstellar medium, emphasizing the combined effects of mirror diffusion and gyroresonant scattering, supported by 3D MHD and test particle simulations, with implications for gamma-ray observations.

Contribution

It introduces the concept of mirror diffusion's role in cosmic ray propagation and demonstrates its significance alongside traditional gyroresonant scattering in a turbulent ISM.

Findings

Mirror diffusion significantly affects parallel CR diffusion when mirroring conditions are met.

The combined diffusion mechanism resembles Levy-flight propagation, altering traditional models.

Simulated diffusion coefficients align with gamma-ray observational data.

Abstract

Cosmic rays (CRs) interact with turbulent magnetic fields in the intestellar medium, generating nonthermal emission. After many decades of studies, the theoretical understanding of their diffusion in the ISM continues to pose a challenge. This study numerically explores a recent prediction termed "mirror diffusion" and its synergy with traditional diffusion mechanism based on gyroresonant scattering. Our study combines 3D MHD simulations of star-forming regions with test particle simulations to analyze CR diffusion. We demonstrate the significance of mirror diffusion in CR diffusion parallel to the magnetic field, when the mirroring condition is satisfied. Our results support the theoretical expectation that the resulting particle propagation arising from mirror diffusion in combination with much faster diffusion induced by gyroresonant scattering resembles a Levy-flight-like propagation. Our study highlights the necessity to reevaluate the diffusion coefficients traditionally adopeted in the ISM based on gyroresonant scattering alone. For instance, our simulations imply a diffusion coefficient $\sim10^{27}cm^2/s$ for particles with a few hundred TeV within regions spanning a few parsecs around the source. This estimate is in agreement with gamma-ray observations, which shows the relevance of our results for understanding of diffuse gamma-ray emission in star-forming regions.