Turbulence-level dependence of cosmic ray parallel diffusion

TitleTurbulence-level dependence of cosmic ray parallel diffusion
Publication TypeJournal Article
Year of Publication2020
AuthorsP. Reichherzer, B J. Tjus, E.G. Zweibel, L. Merten, M. Pueschel
JournalMonthly Notices of the Royal Astronomical Society
Date Published11/2020
ISBN Number0035-8711

Understanding the transport of energetic cosmic rays belongs to the most challenging topics in astrophysics. Diffusion due to scattering by electromagnetic fluctuations is a key process in cosmic ray transport. The transition from a ballistic to a diffusive-propagation regime is presented in direct numerical calculations of diffusion coefficients for homogeneous magnetic field lines subject to turbulent perturbations. Simulation results are compared with theoretical derivations of the parallel diffusion coefficient’s dependences on the energy and the fluctuation amplitudes in the limit of weak turbulence. The present study shows that the widely used extrapolation of the energy scaling for the parallel diffusion coefficient to high turbulence levels predicted by quasi-linear theory does not provide a universally accurate description in the resonant-scattering regime. It is highlighted here that the numerically calculated diffusion coefficients can be polluted for low energies due to missing resonant interaction possibilities of the particles with the turbulence. Five reduced-rigidity regimes are established, which are separated by analytical boundaries derived in this work. Consequently, a proper description of cosmic ray propagation can only be achieved by using a turbulence-level-dependent diffusion coefficient and can contribute to solving the Galactic cosmic ray gradient problem.





Alternate TitleMon. Not. Roy. Astron. Soc.

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