On the Existence of Fast Modes in Compressible Magnetohydrodynamic Turbulence

TL;DR

This paper investigates the presence of Fast magnetosonic modes in 3D compressible MHD turbulence using simulations and advanced analysis methods, revealing their minimal contribution under various driving conditions.

Contribution

It introduces a combined mode decomposition and spatio-temporal 4D-FFT approach to quantify Fast modes in compressible MHD turbulence, showing their limited prevalence.

Findings

Fast modes constitute less than 2% of fluctuations in highly compressible turbulence.

In nearly incompressible conditions, Fast modes are nearly absent.

Results have implications for modeling astrophysical plasma turbulence.

Abstract

We study the existence and property of Fast magnetosonic modes in 3D compressible MHD turbulence by carrying out a number of simulations with compressible and incompressible driving conditions. We use two approaches to determine the presence of Fast modes: mode decomposition based on spatial variations only and spatio-temporal 4D-FFT analysis of all fluctuations. The latter method enables us to quantify fluctuations that satisfy the dispersion relation of Fast modes with finite frequency. Overall, we find that the fraction of Fast modes identified via spatio-temporal 4D FFT approach in total fluctuation power is either tiny with nearly incompressible driving or ~2% with highly compressible driving. We discuss the implications of our results for understanding the compressible fluctuations in space and astrophysics plasmas.