New scaling for the alpha effect in slowly rotating turbulence

TL;DR

This paper demonstrates through simulations and theory that the alpha effect in slowly rotating turbulence depends on the gradient of kinetic energy density rather than momentum, altering previous assumptions about density stratification's role.

Contribution

It introduces a new analytic theory for the alpha effect at high Reynolds numbers and shows that kinetic energy gradients are more influential than density stratification in turbulent magnetic field generation.

Findings

Alpha effect proportional to logarithmic gradient of kinetic energy density.

Density stratification less important than turbulent velocity in alpha effect.

Results consistent across forced, supernova-driven, and stellar convection turbulence.

Abstract

Using simulations of slowly rotating stratified turbulence, we show that the alpha effect responsible for the generation of astrophysical magnetic fields is proportional to the logarithmic gradient of kinetic energy density rather than that of momentum, as was previously thought. This result is in agreement with a new analytic theory developed in this paper for large Reynolds numbers. Thus, the contribution of density stratification is less important than that of turbulent velocity. The alpha effect and other turbulent transport coefficients are determined by means of the test-field method. In addition to forced turbulence, we also investigate supernova-driven turbulence and stellar convection. In some cases (intermediate rotation rate for forced turbulence, convection with intermediate temperature stratification, and supernova-driven turbulence) we find that the contribution of density stratification might be even less important than suggested by the analytic theory.