Dissipative magnetic structures and scales in small-scale dynamos

TL;DR

This paper investigates the scales of dissipative magnetic structures in small-scale dynamos, revealing how the cutoff wavenumber depends on magnetic Prandtl number during different dynamo regimes.

Contribution

It provides new insights into the scaling behavior of magnetic energy spectra and cutoff scales across various magnetic Prandtl numbers in small-scale dynamos.

Findings

Cutoff wavenumber scales as expected for large magnetic Prandtl numbers during the kinematic phase.

Contrary to expectations, the same scaling continues for moderately small magnetic Prandtl numbers.

In the saturated regime, the critical magnetic Prandtl number is about one, affecting the cutoff scale scaling.

Abstract

Small-scale dynamos play important roles in modern astrophysics, especially on Galactic and extragalactic scales. Owing to dynamo action, purely hydrodynamic Kolmogorov turbulence hardly exists and is often replaced by hydromagnetic turbulence. Understanding the size of dissipative magnetic structures is important in estimating the time scale of Galactic scintillation and other observational and theoretical aspects of interstellar and intergalactic small-scale dynamos. Here we show that, during the kinematic phase of the small-scale dynamo, the cutoff wavenumber of the magnetic energy spectra scales as expected for large magnetic Prandtl numbers, but continues in the same way also for moderately small values - contrary to what is expected. For a critical magnetic Prandtl number of about 0.3, the dissipative and resistive cutoffs are found to occur at the same wavenumber. In the nonlinearly saturated regime, the critical magnetic Prandtl number becomes unity. The cutoff scale now has a shallower scaling with magnetic Prandtl number below a value of about three, and a steeper one otherwise compared to the kinematic regime.