Computing the flow in regionally stratified cores of the terrestrial planets

Traditionally, numerical models describing the flow in the fluid cores of terrestrial planets have been developed under the assumption of a spherically symmetric background density. In practice, this means that density gradients produced by chemical and thermal effects are taken to vary only in the radial direction and not horizontally.

Seismic observations of Earth's lowermost mantle, along with the crustal dichotomy observed on the Moon and Mars, however, challenge this spherical symmetry assumption as these observations may be indicative of heterogeneous heat flux at the core-mantle boundary. Indeed, dynamo modelling studies have demonstrated that lateral variations in the heat flux across the core-mantle boundary can, assuming a sufficiently large amplitude, produce laterally varying regions of thermal stratification within the outermost part of the fluid core. These regions of local stratification, sometimes referred to as regional inversion lenses, may have important consequences for the hydromagnetic waves and other core flow processes that are used to interpret observations of planetary magnetic fields and rotation. Yet, despite this potential importance, the influence of laterally varying buoyancy on core dynamics remains largely unmodelled due to the absence of computational tools capable of systematically evaluating it.

To address this, we present a spectral method capable of studying fluid flow including both radial and lateral background buoyancy, which represents a step towards more realistic models of wave dynamics in terrestrial planets. We apply this approach to some well-known wave modes thought to be present in the Earth's core, and discuss the implications of regional stratification for their propagation and detectability.