On the onset of thermal convection in a rotating spherical shell with spatially heterogeneous heat source distribution

Thermal convection in rotating spherical shells effectively model the fluid motions occurring in molten cores of planetary interiors. In the Earth's outer core, such convective motions when subjected to spatial varying thermal buoyancy undergo characteristic modifications that fundamentally affect the consequent magnetic field generation. In addition to the extensively explored mechanism of boundary heat flux (BH) variations, the present study majorly focuses on a novel mechanism of heat source heterogeneity (SH)-driven buoyancy forcing, mimicking non-uniform secular cooling in the outer core. The direct effect of heat sources on the temporal evolution of temperature contrasts the heat flux control exerted by BH forcing. The most prominent difference is the capability of SH forcing to modulate the thermo-fluidic state in the entire shell while the BH-driven anomalies are limited to regions close to the outer boundary only. Dynamically, SH forcing is relatively more effective, with weaker heterogeneity causing transformations in the thermo-fluidic patterns analogous to stronger BH cases. Compared to the homogeneous case, SH leads to a reduction in onset threshold, localization of the convective instabilities, concentrated steady thermal winds, the dominance of anti-cyclonic axial helicity, and overall homogenization with smaller scaled spherical harmonic heterogeneity patterns. Moreover, a novel phenomenon of dual onset is observed for the SH configuration only, marking distinctive transitions in the convective instability features with larger variations. Finally, the effect of SH on the thermal state at the boundaries indicate the plausibility of strong core-mantle and outer-inner core interactions with significant geophysical implications.