On the maximum magnetic field amplification by the magnetorotational instability in core-collapse supernovae

TL;DR

This study uses 3D simulations to assess the maximum magnetic field amplification by MRI in proto-neutron stars, finding a limit of about 100-fold, which challenges the idea that MRI alone can generate strong magnetic fields in these objects.

Contribution

The paper provides the first detailed 3D simulation-based estimate of the MRI amplification factor in proto-neutron stars, showing it is limited to about 10-100 times, depending on initial conditions.

Findings

MRI channel modes amplify magnetic fields by at most a factor of 100.

Realistic amplification in proto-neutron stars is around a factor of 10.

MRI alone is insufficient for strong magnetic field generation, suggesting other processes are needed.

Abstract

Whether the magnetorotational instability (MRI) can amplify initially weak magnetic fields to dynamically relevant strengths in core collapse supernovae is still a matter of active scientific debate. Recent numerical studies have shown that the first phase of MRI growth dominated by channel flows is terminated by parasitic instabilities of the Kelvin-Helmholtz type that disrupt MRI channel flows and quench further magnetic field growth. However, it remains to be prop- erly assessed by what factor the initial magnetic field can be amplified and how it depends on the initial field strength and the amplitude of the perturbations. Different termination criteria leading to different estimates of the amplification factor were proposed within the parasitic model. To determine the amplification factor and test which criterion is a better predictor of the MRI termination, we perform three-dimensional shearing-disc and shearing-box simula- tions of a region close to the surface of a differentially rotating proto-neutron star in non-ideal MHD with two different numerical codes. We find that independently of the initial magnetic field strength, the MRI channel modes can amplify the magnetic field by, at most, a factor of 100. Under the conditions found in proto-neutron stars a more realistic value for the mag- netic field amplification is of the order of 10. This severely limits the role of the MRI channel modes as an agent amplifying the magnetic field in proto-neutron stars starting from small seed fields. A further amplification should therefore rely on other physical processes, such as for example an MRI-driven turbulent dynamo.