Magnetized compressible turbulence with a fluctuation dynamo and Reynolds numbers over a million

TL;DR

This study presents high-resolution simulations of supersonic magnetohydrodynamic turbulence at Reynolds numbers over a million, revealing detailed spectral behaviors and plasma self-organization phenomena relevant to astrophysical plasmas.

Contribution

The paper provides unprecedented simulation data of supersonic MHD turbulence at extremely high Reynolds numbers, uncovering new spectral features and plasma relaxation mechanisms.

Findings

Spectral index of kinetic energy spectrum is -2 at larger scales.

Magnetic energy spectrum follows a -9/5 power law at high magnetic Reynolds numbers.

Plasma self-organizes into relaxed regions with aligned magnetic and velocity fields.

Abstract

Supersonic magnetohydrodynamic (MHD) turbulence is a ubiquitous state for many astrophysical plasmas. However, even the basic statistics for this type of turbulence remains uncertain. We present results from supersonic MHD turbulence simulations at unparalleled resolutions, with plasma Reynolds numbers of over a million. In the kinetic energy spectrum we find a break between the scales that are dominated by kinetic energy, with spectral index $-2$, and those that become strongly magnetized, with spectral index $-3/2$. By analyzing the Helmholtz decomposed kinetic energy spectrum, we find that the compressible modes are not passively mixed through the cascade of the incompressible modes. At high magnetic Reynolds number, above $10^5$, we find a power law in the magnetic energy spectrum with spectral index $-9/5$. On the strongly magnetized, subsonic scales the plasma tends to self-organize into locally relaxed regions, where there is strong alignment between the current density, magnetic field, velocity field and vorticity field, depleting both the nonlinearities and magnetic terms in the MHD equations, which we attribute to plasma relaxation on scales where the magnetic fluctuations evolve on shorter timescales than the velocity fluctuations. This process constrains the cascade to inhomogenous, volume-poor, fractal surfaces between relaxed regions, which has significant repercussions for understanding the nature of magnetized turbulence in astrophysical plasmas and the saturation of the fluctuation dynamo.