Geodynamo Models With a Thick Stable Layer and Heterogeneous CMB Heat Flow

Abstract:

The upward revision of the thermal conductivity in the Earth's core makes it
plausible that the mean heat flow at the core-mantle boundary (CMB) could be only a
fraction of what can be conducted down the core adiabat (perhaps one half). The upper
part of the fluid core would be stably stratified to substantial depth. This is
inconsistent with evidence for upwelling flow near the CMB from observations of
of magnetic flux expulsion. Heat flow at the CMB is likely very heterogeneous and
would still be superadiabatic in some regions of the CMB. The dynamics of such a
system is unclear. Gubbins et al. (Phys. Earth Planet. Int., in press, 2015)
suggest that the locally unstable gradient would mix up the stable layer as a whole
and replace it by a weakly convecting one. We study dynamo models driven by a codensity
flux from the inner core. On the outer boundary an inverse (on average) gradient is
imposed, leading to stable stratification of the top 40% of the fluid shell. In
addition to control cases with homogeneous CMB flux, we run models with two unstable
regions centered on the equator. In the latter cases a predominantly horizontal
circulation in a thin layer immediately below the outer boundary redistributes
the heat that is conducted radially upward in the stable layer and transports it
towards the high heat-flow spots. Radial flow below these spots does not penetrate
deeply into the stable layer, nor does the layer become mixed up to a significant
degree. A dynamo operates in the convecting deep interior, however, its dipole
moment is low in comparison to the Earth value. Heat flow heterogeneity at the
CMB does not sem to solve the problems that exist for the geodynamo when the
average heat flux is substantially subadiabatic.