Thermochemical coupling at Earth’s inner-core boundary can drive a regime transition in rotating convection, offering a new lens on planetary core dynamics, according to a study hosted on ESS Open Archive. Laboratory experiments and numerical simulations have long sought to explain how convective patterns change under rotation and buoyancy, yet the specific role of boundary thermo-compositional interactions remained incompletely understood. By systematically varying both thermal and compositional driving across a range of rotation rates, researchers identified a transition between rotation-dominated and buoyancy-dominated flow states. The findings suggest that boundary thermochemical coupling acts as a switch, altering the emergent large-scale circulation and heat transport efficiency in the fluid core.
Rotating convection is a cornerstone of geophysical fluid dynamics, governing the generation of Earth’s magnetic field and the dynamics of stellar and planetary interiors. In Earth’s outer core, liquid iron-nickel alloy convects as it cools, releasing latent heat and light elements at the inner-core boundary while losing heat to the mantle above. This double-diffusive setting gives rise to complex feedbacks between thermal and compositional buoyancy, rotation, and magnetic fields. Until now, most studies treated thermal and compositional forcing separately or in simplified configurations, leaving open the question of how their coupling modifies the convective regime.
The new work demonstrates that when thermochemical coupling is present, the transition between rotationally constrained and buoyancy-dominated regimes is sharper and occurs at a lower critical forcing than previously predicted. In the rotation-dominated regime, the flow organizes into narrow convective columns aligned with the rotation axis, with heat transport strongly suppressed by the Coriolis force. As thermochemical forcing intensifies, a threshold is crossed where buoyancy overcomes rotational constraint. The flow shifts to a broad, swirling overturning circulation that transports heat far more efficiently. The study quantifies this threshold via a set of revised dimensionless parameters that combine thermal and compositional driving into an effective buoyancy number.
Experimental and numerical data show strong hysteresis across the transition, indicating that the core may persist in either regime across a range of conditions depending on its history. The work also links the convective regime to macroscale observables such as total heat flux and boundary layer thickness, providing testable hypotheses for seismic and geomagnetic observation. Such links could help reconcile differences between paleomagnetic intensity records and thermal evolution models, suggesting the geodynamo may have operated in different convective regimes over geologic time.
These results have broad implications for understanding Earth’s thermal history, the power balance of the geodynamo, and the likelihood of nucleation events at the inner-core boundary. They also inform the dynamics of other rapidly rotating systems, including the deep atmospheres of gas giants and the convective envelopes of stars. Future efforts will aim to incorporate the effects of a self-consistent magnetic field and lateral heterogeneity at the core-mantle boundary for a more complete picture.
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