Testing the dynamic coupling of the core‐mantle and inner core boundaries

Abstract

AbstractThe proposal that the seismically observed hemispherical asymmetry of Earth's inner core is controlled by the heat flux structure imposed on the outer core by the lower mantle is tested with numerical dynamo models driven by mixed thermochemical convection. We find that models driven by a single core‐mantle boundary (CMB) spherical harmonic of degree and mode 2, the dominant mode in lower mantle seismic shear velocity tomography, produce a similar structure at the inner core boundary (ICB) shifted 30∘ westward. The sensitivity of the ICB to the CMB is further tested by increasing the CMB heterogeneity amplitude. In addition, two seismic tomographic models are tested: first with CMB resolution up to degree and order 4, and second with resolution up to degree and order 8. We find time‐averaged ICB heat flux in these cases to be similar at large scale, with small‐scale differences due to higher CMB harmonics (above degree 4). The tomographic models produce “Earth‐like” magnetic fields, while similar models with twice the CMB heat flow amplitudes produce less Earth‐like fields, implying that increasing CMB heterogeneity forces the model out of an Earth‐like regime. The dynamic ICB heat fluxes are compared to the proposed translation mode of the inner core to test whether the CMB controls inner core growth and structure. This test indicates that, although CMB tomography is unlikely to be driving inner core translation, the ICB heat flux response is weak enough to not interfere with the most unstable translation mode, if it is occurring.