MRI-driven dynamo at very high magnetic Prandtl numbers

TL;DR

This study explores MRI-driven dynamos at extremely high magnetic Prandtl numbers using simulations, revealing a transition to a Pm-independent energy regime and enhanced large-scale dynamo efficiency.

Contribution

It provides the first detailed analysis of MRI-driven dynamos at Pm up to 256, uncovering a new high-Pm regime and its implications for astrophysical magnetic field generation.

Findings

Stress and turbulent energies scale with Pm up to ~50.

At Pm > 100, energies plateau, indicating a Pm-independent regime.

Large Pm enhances small-scale magnetic fields and large-scale dynamo efficiency.

Abstract

The dynamo driven by the magnetorotational instability (MRI) is believed to play an important role in the dynamics of accretion discs and may also explain the origin of the extreme magnetic fields present in magnetars. Its saturation level is an important open question known to be particularly sensitive to the diffusive processes through the magnetic Prandtl number Pm (the ratio of viscosity to resistivity). Despite its relevance to proto-neutron stars and neutron star merger remnants, the numerically challenging regime of high Pm is still largely unknown. Using zero-net flux shearing box simulations in the incompressible approximation, we studied MRI-driven dynamos at unprecedentedly high values of Pm reaching 256. The simulations show that the stress and turbulent energies are proportional to Pm up to moderately high values ($\mathrm{Pm} \sim 50$). At higher Pm, they transition to a new regime consistent with a plateau independent of Pm for $\rm Pm \gtrsim 100$. This trend is independent of the Reynolds number, which may suggest an asymptotic regime where the energy injection and dissipation are independent of the diffusive processes. Interestingly, large values of Pm not only lead to intense small-scale magnetic fields but also to a more efficient dynamo at the largest scales of the box.