The role of thermal buoyancy in stabilizing the axial dipole field in rotating two-component convective dynamos

TL;DR

This study investigates how thermal buoyancy influences the stability and characteristics of the axial dipole magnetic field in rotating convective dynamos, with implications for Earth's core dynamics.

Contribution

It reveals that thermal buoyancy stabilizes the axial dipole and extends the dipolar regime, providing new insights into planetary magnetic field generation.

Findings

Thermal buoyancy helps establish the dipole through slow magnetostrophic waves.

A higher compositional buoyancy is needed for polarity transitions.

Thermal buoyancy contributes at least 10% of the total power in the dynamo.

Abstract

Two-component convection driven by both compositional and thermal buoyancy within the fluid core of a rapidly rotating planet produces a predominantly axial dipole field. In a dynamo driven by strong compositional buoyancy that by itself destabilizes the axial dipole, the addition of relatively weak thermal buoyancy establishes the dipole field through the spontaneous generation of slow magnetostrophic waves produced by balances between the magnetic, buoyancy and Coriolis (MAC) forces at several locations within the core. A substantially higher compositional buoyancy is then required to trigger polarity transitions, since the dipolar regime is extended in two-component convection, as predicted by a linear magnetoconvection model that analyses the long-time evolution of a density disturbance. The existence of the axial dipole also prescribes a lower bound for the fraction of the total power contributed by thermal buoyancy, $\approx$ 10%, above which the two-component dynamo with homogeneous boundary heat flux lies deep within the dipolar regime. Two-component convection has implications for Earth's core dynamo: dominant compositional buoyancy ensures the observed polar circulation speed, and a large heterogeneity in the lower-mantle heat flux induces magnetic field excursions and occasional polarity reversals.