Turbulence-induced magnetic fields and the structure of Cosmic Ray modified shocks

TL;DR

This paper introduces a fluid-based small-scale dynamo mechanism for magnetic field amplification in cosmic ray shocks, offering an alternative to cosmic ray streaming instabilities, and suggests it can accelerate cosmic rays up to 10^16 eV.

Contribution

It presents a novel fluid dynamo model for magnetic field growth in shock precursors, differing from traditional cosmic ray streaming instability models.

Findings

The dynamo mechanism produces rapid magnetic field amplification.

It can generate sufficient magnetic fields for cosmic ray acceleration up to 10^16 eV.

The mechanism is generic and does not require additional processes.

Abstract

We propose a model for Diffusive Shock Acceleration (DSA) in which stochastic magnetic fields in the shock precursor are generated through purely fluid mechanisms of a so-called small-scale dynamo. This contrasts with previous DSA models that considered magnetic fields amplified through cosmic ray streaming instabilities; i.e., either by way of individual particles resonant scattering in the magnetic fields, or by macroscopic electric currents associated with large-scale cosmic ray streaming. Instead, in our picture, the solenoidal velocity perturbations that are required for the dynamo to work are produced through the interactions of the pressure gradient of the cosmic ray precursor and density perturbations in the inflowing fluid. Our estimates show that this mechanism provides fast growth of magnetic field and is very generic. We argue that for supernovae shocks the mechanism is capable of generating upstream magnetic fields that are sufficiently strong for accelerating cosmic rays up to around 10^16 eV. No action of any other mechanism is necessary.