Intermittency assessed through a model of kurtosis-skewness relation in MHD in fast dynamo regimes

TL;DR

This paper investigates intermittency in fast MHD dynamos by analyzing skewness and kurtosis relations through direct numerical simulations, revealing deviations from classical intermittency models.

Contribution

It introduces a model linking kurtosis-skewness relations to intermittency in MHD dynamos, supported by extensive simulations and analytical comparisons.

Findings

Kurtosis-skewness relations follow approximate parabolic laws.

Numerical results show alpha values between 0.2 and 3.0, varying with conditions.

Analytical alpha from She & Leveque model is 2.5, differing from strict parabolic scaling.

Abstract

Intermittency as it occurs in fast dynamos in the MHD framework is evaluated through the examination of relations between normalized moments at third order (skewness S) and fourth order (kurtosis K) for both the velocity and magnetic field, and for their local dissipations. As investigated by several authors in various physical contexts such as fusion plasmas (Krommes (2008)), climate evolution (Sura & Sardeshmukh (2008)), fluid turbulence or rotating stratified flows (Pouquet et al. (2023)), approximate parabolic K(S) ~ alpha laws emerge whose origin may be related to the applicability of intermittency models to their dynamics. The results analyzed herein are obtained through direct numerical simulations of MHD flows for both Taylor-Green and Beltrami ABC forcing at moderate Reynolds numbers, and for up to 314000 turn-over times. We observe for the dissipation 0.2 < alpha <3.0, an evaluation that varies with the field, the forcing, and when filtering for high-skewness intermittent structures. When using the She & Leveque (1994) intermittency model, one can compute alpha analytically; we then find alpha = 2.5, clearly differing from a (strict) parabolic scaling, a result consistent with the numerical data.