Small-Scale Dynamo for Full Spectrum of Hydrodynamic Turbulence in Kazantsev Model

TL;DR

This paper introduces a numerical method to compute the Kazantsev dynamo coefficients across the full hydrodynamic turbulence spectrum, analyzing magnetic field growth and energy spectra for various Reynolds numbers.

Contribution

It presents a novel numerical approach to solve the Kazantsev equation for the full turbulence spectrum, including inertial and viscous ranges, and explores dynamo thresholds and growth rates.

Findings

Dynamo threshold Rm_c saturates at ~300 for Re ≥ 10^5.

Magnetic energy peaks at the Ohmic dissipation scale, decreasing with Pm.

Growth rate depends logarithmically on Rm in low Pm regime.

Abstract

A method is proposed for computing coefficients in the Kazantsev equation of small-scale dynamo for the full spectrum of hydromagnetic turbulence comprising the inertial range together with the range of viscous dissipation. The dynamo equation with so-defined coefficients is solved numerically for magnetic (Rm) and hydrodynamic (Re) Reynolds numbers from $10^2$ to $10^8$. The threshold value ${\rm Rm}_c$ for onset of dynamo increases initially with Re but then saturates at a constant value of ${\rm Rm}_c \simeq 300$ for ${\rm Re}\geq 10^5$. In the case of small Prandtl number Pm = Rm/Re << 1, the field growth rate is also small and depends logarithmically on Rm. In this case, the magnetic energy spectrum peaks around the scale of Ohmic dissipation, which decreases with increasing Pm. The decrease stops at the scale of viscous dissipation while the growth rate increases sharply when Pm approaches the value of one. The increase in the growth rate proceeds to ${\rm Pm} > 1$ but slows down and then saturates at a value somewhat below the inverse lifetime of most short-living eddies. An explanation of the results is proposed.