# Role of the sonic scale in the growth of magnetic field in compressible turbulence

## TL;DR

This paper investigates how the sonic scale influences magnetic field amplification in supersonic turbulence, revealing that below this scale, turbulence behaves incompressibly, affecting the dynamo process.

## Contribution

It introduces a model linking the sonic scale to magnetic field growth, highlighting the role of incompressible eddies in supersonic turbulence dynamo action.

## Key findings

- Magnetic field growth is dominated by eddies below the sonic scale.
- Below the sonic scale, turbulence behaves effectively incompressibly.
- The derived growth mode and rate are supported by a white noise turbulence model.

## Abstract

We study the growth of small fluctuations of magnetic field in supersonic turbulence, the small-scale dynamo. The growth is due to the fastest turbulent eddies above the resistive scale. We observe that for supersonic turbulence these eddies are effectively incompressible which creates a robust structure of the growth. The eddies are localised below the sonic scale $l_s$ defined as the scale where the typical velocity of the turbulent eddies equals the speed of sound. Thus the flow below $l_s$ is effectively incompressible and the field growth proceeds as in incompressible flow. At large Mach numbers $l_s$ is much smaller than the integral scale of turbulence so the fastest growing mode of the magnetic field belongs to small-scale turbulence. We derive this mode and the associated growth rate numerically in a white noise in time model of turbulence. The relevance of this model relies on considering evolution time larger than the correlation time of turbulence.