Magnetic field amplification in supernova remnants

TL;DR

This paper investigates magnetic field amplification in supernova remnants, revealing how turbulence-driven dynamo processes in different regions lead to linear growth of magnetic fields and explain observed X-ray features.

Contribution

It introduces a model of magnetic field amplification considering ion-neutral damping effects, explaining magnetic growth and saturation in supernova remnants.

Findings

Linear-in-time growth of magnetic field strength in preshock turbulence.

Linear-in-time growth of magnetic energy in postshock turbulence.

Magnetic field saturation explains distant X-ray spots behind shocks.

Abstract

Based on the new findings on the turbulent dynamo in \citet{XL16}, we examine the magnetic field amplification in the context of supernova remnants. Due to the strong ion-neutral collisional damping in the weakly-ionized interstellar medium, the dynamo in the preshock turbulence remains in the damping kinematic regime, which leads to {\it a linear-in-time growth of the magnetic field strength}. The resultant magnetic field structure enables effective diffusion upstream and shock acceleration of cosmic rays to energies above the "knee". Differently, the nonlinear dynamo in the postshock turbulence leads to {\it a linear-in-time growth of the magnetic energy} due to the turbulent magnetic diffusion. Given a weak initial field strength in the postshock region, the magnetic field saturates at a significant distance from the shock front as a result of the inefficiency of the nonlinear dynamo. This result is in a good agreement with existing numerical simulations and well explains the X-ray spots detected far behind the shock front.