Transition from multipolar to dipolar dynamos in stratified systems

TL;DR

This study uses 3D numerical simulations of stratified systems to explore how different dynamo regimes produce dipolar or multipolar magnetic fields in stars, revealing new conditions under which dipolar fields can exist.

Contribution

It demonstrates that dipolar magnetic fields can form at higher Rossby numbers in stratified systems, challenging previous assumptions and linking magnetic morphology to force balances and differential rotation.

Findings

Dipolar solutions occur at Rossby numbers up to 0.4 in stratified simulations.

The ratio of inertial to Lorentz forces controls the magnetic field topology.

Anti-solar differential rotation correlates with dipolar magnetic fields.

Abstract

Observations of surface magnetic fields of cool stars reveal a large diversity of configurations. Although there is now a consensus that these fields are generated through dynamo processes occurring within the convective zone, the physical mechanism driving such a variety of field topologies is still debated. This paper discusses the possible origins of dipole and multipole-dominated morphologies using three-dimensional numerical simulations of stratified systems where the magnetic feedback on the fluid motion is significant. Our main result is that dipolar solutions are found at Rossby numbers up to 0.4 in strongly stratified simulations, where previous works suggested that only multipolar fields should exist. We argue that these simulations are reminiscent of the outlier stars observed at Rossby numbers larger than 0.1, whose large-scale magnetic field is dominated by their axisymmetric poloidal component. As suggested in previous Boussinesq calculations, the relative importance of inertial over Lorentz forces is again controlling the dipolar to multipolar transition. Alternatively, we find that the ratio of kinetic to magnetic energies can equally well capture the transition in the field morphology. We test the ability of this new proxy to predict the magnetic morphology of a few M-dwarf stars whose internal structure matches that of our simulations and for which homogeneous magnetic field characterization is available. Finally, the magnitude of the differential rotation obtained in our simulations is compared to actual measurements reported in the literature for M-dwarfs. In our simulations, we find a clear relationship between anti-solar differential rotation and the emergence of dipolar fields.