Virtual Nordic Dynamo Seminar

Non-helical Large-Scale Dynamo in Stellar Radiative Zones

by Valentin A. Skoutnev (Princeton University)

Europe/Stockholm
Description

https://stockholmuniversity.zoom.us/j/940229961

Our understanding of large-scale magnetic fields in stellar radiative zones remains fragmented and incomplete. Such fields, which must be produced by some form of dynamo mechanism, are thought to dominate angular-momentum transport, making them crucial to stellar evolution more generally. A major difficulty is the effect of stable stratification, which generally suppresses dynamo action. We propose a non-helical large-scale dynamo (LSD) mechanism that we find can operate in a stably stratified plasma alongside two ingredients: mean shear and non-helical magnetic fluctuations. Both ingredients are easily sourced in the presence of differential rotation. Our idealized direct numerical simulations, supported by mean-field theory, demonstrate the generation of near equipartition large-scale toroidal fields. The mechanism is robust to increasing stable stratification as long as a source of non-helical magnetic fluctuations is present, e.g. from a small-scale dynamo. Additionally, a scan over magnetic Reynolds number shows no change in the growth or saturation of the LSD, providing good numerical evidence of a quenching-free dynamo mechanism, which has been an issue for helical dynamos. These properties -- the lack of quenching and robustness to stable stratification -- make the mechanism a plausible candidate for generating in-situ large-scale magnetic fields in stellar radiative zones.