Dipolar stability in spherical simulations: the impact of an inner stable zone

TL;DR

This study investigates how an inner stable zone affects dipolar magnetic field stability in spherical dynamo simulations, revealing a transition from dipolar to multipolar regimes influenced by the Rossby number.

Contribution

It introduces a novel analysis of dipolar stability in spherical systems with an inner stable zone, extending understanding of magnetic field behavior in such configurations.

Findings

Transition from dipolar to multipolar magnetic fields occurs as Rossby number increases.

The critical Rossby number at the transition is similar to that in fully convective systems.

Varying the stable zone's stiffness affects the magnetic field regime.

Abstract

Magnetic fields vary in complexity for different stars. The stability of dipolar magnetic fields is known to depend on different quantities, e.g., the stellar rotation, the stratification, and the intensity of convective motions. Here, we study the dipolar stability in a system with an inner stable zone. We present preliminary results of dynamo simulations using the Rayleigh number as a control parameter. The stiffness of the stable zone is accordingly varied to keep a constant ratio of the Brunt-Vaisala frequency to the angular velocity. Similarly to the completely convective spherical shell, we find that a transition exists between a regime where the magnetic field is dipolar to a multipolar regime when the Rossby number is increased. The value of the Rossby number at the transition is very close to the one of the fully convective case.