Compressible Test-field Method and Its Application to Shear Dynamos

| openaire: EC/H2020/818665/EU//UniSDyn Funding Information: We acknowledge fruitful and inspiring discussions with Prof. Nishant Singh, Dr. Hongzhe Zhou, Dr. Jonathan Squire, and Prof. Amitava Bhattacharjee, many of which took place in the inspiring atmosphere of the Max Planck Princeton Center for Plasma Physics framework. M.J.K. and M.R. acknowledge the support of the Academy of Finland ReSoLVE Centre of Excellence (grant number 307411). This project has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program (Project UniSDyn, grant agreement n:o 818665). A.B. acknowledges support through the Swedish Research Council (Vetenskapsrådet), grant 2019-04234. Publisher Copyright: © 2022. The Author(s). Published by the American Astronomical Society.In this study, we present a compressible test-field method (CTFM) for computing α-effect and turbulent magnetic diffusivity tensors, as well as those relevant for the 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. A comparison of different flavors of the nCTFM in the shear dynamo case also yields agreement up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study nonhelically forced shear flows, which exhibit large-scale dynamo action, and present a reanalysis of low-Reynolds-number, moderate shear systems, where we previously ignored 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 by incoherent effects.Peer reviewe