The theory of cosmic-ray scattering on pre-existing MHD modes meets data

TL;DR

This study combines theoretical modeling and numerical simulations to understand cosmic-ray scattering by MHD turbulence, revealing the dominant role of magnetosonic modes in cosmic-ray confinement up to PeV energies.

Contribution

It provides first-principles diffusion coefficients considering damping mechanisms and implements them in the DRAGON code to match observed cosmic-ray spectra.

Findings

Magnetosonic modes dominate cosmic-ray scattering up to PeV energies.

Alfvénic turbulence scattering is suppressed when cascade anisotropy is considered.

Simulated cosmic-ray spectra agree with observations above 200 GV rigidity.

Abstract

We present a comprehensive study about the phenomenological implications of the theory describing Galactic cosmic-ray scattering onto magnetosonic and Alfv\'enic fluctuations in the $\mathrm{GeV} - \mathrm{PeV}$ domain. We compute a set of diffusion coefficients from first principles, for different values of the Alfv\'enic Mach number and other relevant parameters associated to both the Galactic halo and the extended disk, taking into account the different damping mechanisms of turbulent fluctuations acting in these environments. We confirm that the scattering rate associated to Alfv\'enic turbulence is highly suppressed if the anisotropy of the cascade is taken into account. On the other hand, we highlight that magnetosonic modes play a dominant role in Galactic confinement of cosmic rays up to $\mathrm{PeV}$ energies. We implement the diffusion coefficients in the numerical framework of the {\tt DRAGON} code, and simulate the equilibrium spectrum of different primary and secondary cosmic-ray species. We show that, for reasonable choices of the parameters under consideration, all primary and secondary fluxes at high energy (above a rigidity of $\simeq 200 \, \mathrm{GV}$) are correctly reproduced within our framework, in both normalization and slope.