Dynamical coupling of lower mantle and inner core

Seismic observations of the inner core indicate a well defined East-West asymmetry in P-wave velocity, anisotropy, and attenuation in the outer most 200 km. One possible mechanism for explaining this hemispherical dichotomy is by imposing a long lived heat flux heterogeneity at the inner core boundary (ICB) maintained by the fluid outer core, which would result in laterally heterogeneous growth of the inner core. However, since the outer core has no intrinsic heat flux asymmetries, so that all departures from homogeneity must arise from the core mantle boundary (CMB), this mechanism requires that the core be in a dynamical regime favorable for CMB-ICB coupling. Here we use a thermochemically driven numerical dynamo to investigate whether inner core growth can be coupled to imposed CMB heat flux patterns. We identify the range of outer core buoyancy fluxes and CMB heterogeneity amplitudes that promote this thermal coupling, and describe the influence that this regime has on the magnetic field morphology and dipole stability. We find that the CMB and ICB thermal coupling is sensitive to the amplitude of the CMB heterogeneity, implying that either changes in the CMB heat flux amplitude or heterogeneity, for example driven by lower mantle dynamics, may evolve the dynamo out of this coupled state. Similarly, we find that the coupling is sensitive to the mean core buoyancy flux so that small changes in this parameter may also disrupt the coupling. Therefore, the long time scale evolution of the lower mantle may cause a time dependent CMB-ICB thermal coupling, which may induce both radial and lateral heterogeneity in the inner core as it grows.