Turbulence-induced magnetic fields in shock precursors

TL;DR

This paper investigates how turbulence in the interstellar medium can amplify magnetic fields in shock precursors, potentially enhancing cosmic ray acceleration at supernova shocks.

Contribution

It introduces a novel mechanism where turbulence-driven magnetic field amplification occurs via small-scale dynamo action in shock precursors, supported by 3D MHD simulations.

Findings

Turbulence in the ISM can significantly amplify magnetic fields in shock precursors.

Magnetic field amplification depends on the level of inhomogeneity and turbulence.

The mechanism provides an alternative to Bell's instability for cosmic ray acceleration enhancement.

Abstract

Galactic cosmic rays are believed to be mostly accelerated at supernova shocks. However, the interstellar magnetic field is too weak to efficiently accelerate galactic cosmic rays up to the highest energies, i.e. $10^{15}$ eV. A stronger magnetic field in the pre-shock region could provide the efficiency required. Bell's cosmic-ray nonresonant streaming instability has been claimed to be responsible for the amplification of precursor magnetic fields. However, an alternative mechanism has been proposed in which the cosmic-ray pressure gradient forms the shock precursor and drives turbulence, amplifying the magnetic field via the small-scale dynamo. A key ingredient for the mechanism to operate are the inhomogeneities present in the interstellar medium (ISM). These inhomogeneities are the consequence of turbulence. In this work we explore the magnetic field amplification in different ISM conditions through 3D MHD numerical simulations.