Universal Nonlinear Small-Scale Dynamo

TL;DR

This paper investigates the universal behavior of nonlinear small-scale dynamos in astrophysical turbulence, demonstrating that magnetic energy growth rate is a constant fraction of turbulent dissipation, with a measured efficiency around 0.05.

Contribution

It introduces the concept of a universal efficiency of small-scale dynamo, C_E, that remains constant at high Reynolds numbers and is determined by nonlinear dynamics at the equipartition scale.

Findings

Measured C_E around 0.05 in high-resolution simulations.

C_E is a true constant for large Re, independent of specific conditions.

Addresses why C_E is small compared to Kolmogorov constant.

Abstract

We consider astrophysically relevant nonlinear MHD dynamo at large Reynolds numbers (Re). We argue that it is universal in a sense that magnetic energy grows at a rate which is a constant fraction C_E of the total turbulent dissipation rate. On the basis of locality bounds we claim that this "efficiency of small-scale dynamo", C_E, is a true constant for large Re and is determined only by strongly nonlinear dynamics at the equipartition scale. We measured C_E in numerical simulations and observed a value around 0.05 in highest resolution simulations. We address the issue of C_E being small, unlike Kolmogorov constant which is of order unity.