A Hierarchical Framework for explaining the Cosmic Ray Spectrum using Diffusive Shock Acceleration

TL;DR

This paper proposes a hierarchical framework where cosmic rays are accelerated across multiple scales, from supernova remnants to intergalactic shocks, explaining the entire spectrum from GeV to EeV energies.

Contribution

It introduces a novel hierarchical model of cosmic ray acceleration involving local and extragalactic shocks, with a bootstrap coupling of magnetic turbulence and cosmic rays.

Findings

Cosmic rays up to 3 PeV originate from supernova remnants.

Higher energy cosmic rays are re-accelerated at galactic and intergalactic shocks.

The model predicts observable signatures in cosmic ray spectrum, composition, and sky distribution.

Abstract

The hypothesis that the entire cosmic ray spectrum, from $\lesssim1\,{\rm GeV}$ to $\gtrsim100\,{\rm EeV}$ energy, can be accounted for by diffusive shock acceleration on increasingly large scales is critically examined. Specifically, it is conjectured that Galactic cosmic rays, up to $\sim3\,{\rm PeV}$, are mostly produced by local supernova remnants, from which they escape upstream. These cosmic rays initiate a powerful magnetocentrifugal wind, removing disk mass and angular momentum before passing through the Galactic Wind Termination Shock at a radius $\sim200\,{\rm kpc}$, where they can be re-accelerated to account for observed cosmic rays up to $\sim30\,{\rm PeV}$. The cosmic rays transmitted downstream from more powerful termination shocks associated with other galaxies can be further accelerated at Intergalactic Accretion Shocks to the highest energies observed. In this interpretation, the highest rigidity observed particles are protons; the highest energy particles are heavy nuclei, such as iron. A universal "bootstrap" prescription, coupling the energy density of the magnetic turbulence to that of the resonant cosmic rays, is proposed, initially for the highest energy particles escaping far ahead of the shock front and then scattering, successively, lower energy particles downstream. Observable implications of this general scheme relate to the spectrum, composition and sky distribution of Ultra-High-Energy Cosmic Rays, the extragalactic radio background, the Galactic halo magnetic field and Pevatrons.