Compressible test-field method and its application to shear dynamos

TL;DR

This paper introduces a compressible test-field method (CTFM) for analyzing magnetic and flow properties in MHD, compares it with existing methods, and applies it to shear dynamo systems revealing new insights into dynamo mechanisms.

Contribution

The paper develops a new compressible test-field method for full MHD equations and demonstrates its effectiveness and differences compared to existing methods in shear dynamo studies.

Findings

CTFM agrees with imposed-field method at moderate field strengths.

Different nCTFM variants show consistent results up to equipartition.

Large-scale dynamo action is driven by incoherent effects, not shear-current effect.

Abstract

In this study we present a compressible test-field method (CTFM) for computing $\alpha$ effect and turbulent magnetic diffusivity tensors, as well as those relevant for mean ponderomotive force and mass source, applied to the full MHD equations. We describe the theoretical background of the method, and compare it to the quasi-kinematic test-field method, and to the previously studied variant working in simplified MHD (SMHD). We present several test cases using velocity and magnetic fields of the Roberts geometry, and also compare with the imposed-field method. We show that, for moderate imposed field strengths, the nonlinear CTFM (nCTFM) gives results in agreement with the imposed-field method. Comparison of different flavors of the nCTFM in the shear dynamo case also agree up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study non-helically forced shear flows, which exhibit large-scale dynamo action, and present a re-analysis of low Reynolds number, moderate shear systems, where we previously neglected the pressure gradient in the momentum equation, and found no coherent shear-current effect. Another key difference is that in the earlier study we used magnetic forcing to mimic small-scale dynamo action, while here it is self-consistently driven by purely kinetic forcing. The kinematic CTFM with general validity forms the core of our analysis. We still find no coherent shear-current effect, but do recover strong large-scale dynamo action that, according to our analysis, is driven through the incoherent effects.