The statistics of a passive scalar in field-guided magnetohydrodynamic turbulence

TL;DR

This paper investigates the behavior of passive scalars in magnetohydrodynamic turbulence through high-resolution simulations, revealing complex dynamics and establishing key statistical properties relevant to astrophysical plasma studies.

Contribution

It provides the first detailed analysis of passive scalar statistics in MHD turbulence, including anisotropy, scaling laws, and intermittency, advancing understanding beyond hydrodynamic turbulence.

Findings

Passive scalar anisotropy observed in MHD turbulence

Established a Yaglom-like scaling relation for passive scalars

Passive scalar intermittency is significant and complex

Abstract

A variety of studies of magnetised plasma turbulence invoke theories for the advection of a passive scalar by turbulent fluctuations. Examples include modelling the electron density fluctuations in the interstellar medium, understanding the chemical composition of galaxy clusters and the intergalactic medium, and testing the prevailing phenomenological theories of magnetohydrodynamic turbulence. While passive scalar turbulence has been extensively studied in the hydrodynamic case, its counterpart in MHD turbulence is significantly less well understood. Herein we conduct a series of high-resolution direct numerical simulations of incompressible, field-guided, MHD turbulence in order to establish the fundamental properties of passive scalar evolution. We study the scalar anisotropy, establish the scaling relation analogous to Yaglom's law, and measure the intermittency of the passive scalar statistics. We also assess to what extent the pseudo Alfven fluctuations in strong MHD turbulence can be modelled as a passive scalar. The results suggest that the dynamics of a passive scalar in MHD turbulence is considerably more complicated than in the hydrodynamic case.