Role of magnetic pressure forces in fluctuation dynamo saturation

TL;DR

This study uses magnetohydrodynamic simulations to reveal how magnetic pressure forces influence the saturation of fluctuation dynamos across subsonic to supersonic turbulent flows, highlighting the roles of compression and field line stretching.

Contribution

It demonstrates the significant role of magnetic pressure in dynamo saturation, especially in supersonic flows, and clarifies the mechanisms of magnetic energy growth and suppression.

Findings

Magnetic pressure forces are crucial in dynamo saturation in supersonic flows.

Density and magnetic field strength correlations vary with flow Mach number.

Suppression of field line stretching dominates dynamo saturation in supersonic turbulence.

Abstract

Using magnetohydrodynamic simulations of fluctuation dynamos in turbulent flows with rms Mach numbers $\mathcal{M}_{\rm rms} = 0.2, 1.1$ and $3$, we show that magnetic pressure forces play a crucial role in dynamo saturation in supersonic flows. First, as expected when pressure forces oppose compression, an increase in anticorrelation between density and magnetic field strengths obtains even in subsonic flows with the anti-correlation arising from the intense but rarer magnetic structures. In supersonic flows, due to stronger compressive motions density and magnetic field strength continue to maintain a positive correlation. However, the degree of positive correlation decreases as the dynamo saturates. Secondly, we find that the unit vectors of $\nabla\rho$ and $\nabla B^{2}$ are preferentially antiparallel to each other in subsonic flows. This is indicative of magnetic pressure opposing compression. This antiparallel alignment persists in transonic and supersonic flows at dynamo saturation. However, compressive motions also lead to the emergence of a parallel alignment in these flows. Finally, we consider the work done against the components of the Lorentz force and the different sources of magnetic energy growth and dissipation. We show that while in subsonic flows, suppression of field line stretching is dominant in saturating the dynamo, the picture is different in supersonic flows. Both field line stretching and compression initially amplifies the field. However, growing magnetic pressure opposes further compression of magnetic flux which tends to reduce the compressive motions. Simultaneously, field line stretching also reduces. But, suppression of compressive amplification dominates the saturation of the dynamo.