Magnetic field morphologies in convective zones influenced by a turbulent surface layer

¹ LIRA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, Paris, France
² Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France

^⋆ Corresponding author: anna.guseva@obspm.fr

Received: 6 January 2025
Accepted: 12 May 2025

Abstract

Context. The magnetism of low-mass stars can have a significant impact on their activity and therefore the detection of exoplanets and their properties. Spectropolarimetric observations show that many low-mass stars possess large-scale poloidal magnetic fields with a considerable dipole component, which in some cases exhibit temporal dynamics (cycles or reversals). Although it is widely accepted that magnetic fields of low-mass stars are generated by the dynamo process (i.e., stretching and twisting of magnetic field lines by helical motions in stellar convective envelopes), numerical dynamo simulations show that it is hard to reproduce coherent oscillations of large-scale magnetic fields with a dipolar symmetry as observed for the Sun when turbulent and compressible regimes are explored.

Aims. Modeling stellar dynamos is a real challenge, as it requires taking into account various interacting physical effects that develop on different scales of time and length. Most previous 3D numerical studies partially avoided this problem by considering a numerical domain with low density stratification, which may correspond to neglecting surface effects where density drops considerably. Our work aims to address this question.

Methods. We performed three-dimensional direct numerical simulations of convective dynamos in extreme parameter regimes of both strong turbulence and strong density stratification using the community-tested numerical software MagIC. The dynamics in such systems, particularly the dominance of the Coriolis effects, depend on the depth of the fluid layer. Our strongly stratified dynamo simulations exhibit rotationally influenced large-scale convective motions surrounded by a turbulent compressible surface layer.

Results. We find complex time variations of the magnetic field with flow regimes of a predominantly dipolar configuration with respect to the few large-scale harmonics that would be captured by spectropolarimetry. In such regimes, the turbulent surface layer induces a global magnetic pumping mechanism, transporting the magnetic energy into the deep interiors of our dynamo model. We find that the dipole magnetic fields are in regimes of transition between solar- and anti-solar differential rotation and that they interact dynamically with it.

Conclusions. The spatial distribution and temporal behavior of the large-scale fields is consistent with observations of low-mass stars, which suggests magnetic pumping could promote time-dependent magnetic fields with a similar dipolar symmetry as observed for the Sun and other solar-like stars. Our results suggest a parameter path in which dynamo models with complex multiscale dynamics should be explored.