Evolution of Magnetic Fields in Supernova Remnants

TL;DR

This study uses MHD simulations to investigate how magnetic fields evolve in supernova remnants, aiming to explain the observed radial magnetic field component in early stages, which ideal MHD alone cannot fully account for.

Contribution

The paper demonstrates through simulations that ideal MHD cannot fully reproduce the observed magnetic field configurations in SNRs, suggesting the need for additional physics like higher compression ratios or turbulence.

Findings

Radial magnetic fields arise at contact discontinuity due to Rayleigh-Taylor instability.

No significant radial magnetic component is observed at the forward shock in simulations.

Ideal MHD alone is insufficient to explain observed magnetic field structures in SNRs.

Abstract

Supernova remnants (SNR) are now widely believed to be a source of cosmic rays (CRs) up to an energy of 1 PeV. The magnetic fields required to accelerate CRs to sufficiently high energies need to be much higher than can result from compression of the circumstellar medium (CSM) by a factor 4, as is the case in strong shocks. Non-thermal synchrotron maps of these regions indicate that indeed the magnetic field is much stronger, and for young SNRs has a dominant radial component while for old SNRs it is mainly toroidal. How these magnetic fields get enhanced, or why the field orientation is mainly radial for young remnants, is not yet fully understood. We use an adaptive mesh refinement MHD code, AMRVAC, to simulate the evolution of supernova remnants and to see if we can reproduce a mainly radial magnetic field in early stages of evolution. We follow the evolution of the SNR with three different configurations of the initial magnetic field in the CSM: an initially mainly toroidal field, a turbulent magnetic field, and a field parallel to the symmetry axis. Although for the latter two topologies a significant radial field component arises at the contact discontinuity due to the Rayleigh-Taylor instability, no radial component can be seen out to the forward shock. Ideal MHD appears not sufficient to explain observations. Possibly a higher compression ratio and additional turbulence due to dominant presence of CRs can help us to better reproduce the observations in future studies.