Influence of Small-Scale Core-Mantle Boundary Structures on the Dynamics of the Earth’s Outer Core

Recent advances in geodynamo modelling have been very successful in explaining many features of the geo-
magnetic field, including the field reversals and excursions. Previous studies have shown that the dynamics 
of these features depend on spatial variation in the core-mantle boundary (CMB) heat flux pattern. Contrary to 
previous studies, an up-to-date mantle reconstruction for the last 200 Myr provides patterns with a higher degree 
of complexity, featuring a network of interconnected regions with subadiabatic heat flow. We use these patterns 
as outer boundary conditions for dynamo simulation in order to explore whether its evolution can explain the 
observed variation in reversal rate. While the impact of large-scale structures at the core-mantle boundary has 
been thoroughly explored by Frasson et al. (2025), the contribution of smaller scales remains poorly constrained, 
which we aim to cover within the scope of these studies.

For our study, we apply the codensity approach which combines the effects of thermal and compositional density 
to represent both thermally driven convection and the enrichment of the outer core with light elements due to 
the inner core solidification. We first investigate the relative impact of thermal and compositional convection 
a for patterns with various degrees of complexity, defined by the spherical harmonics degree truncation l_max. 
Our models indicate that the field dynamics, including the reversal rate, depends on the truncation l_max, with 
solutions for l_max = 8 and l_max = 16 exhibiting more reversals than higher truncation degrees. This effect is 
present in models with mixed convection (a = 0.33 and a = 0.66). However, when compositional convection 
clearly dominates (a = 0.99), the pattern has no impact on the reversal behaviour, and the model evolves 
similarly to the homogeneous case. We also observe the emergence of subsurface low-radial-velocity regions, 
reminiscent of the stably-stratified lenses discussed by Mound et al. (2019). Our models also show strong 
zonal flows comparable to those discussed in Frasson et al. (2025). Our ongoing work focuses on comparing 
simulations for the CMB heat flux pattern at the present-day time and during the CNS.