Summary: Physics of the Earth and Planetary Interiors 160 (2007) 143156
Detecting thermal boundary control in surface flows
from numerical dynamos
Julien Auberta,, Hagay Amitb, Gauthier Hulotb
a Dynamique des Syst`emes G´eologiques, Institut de Physique du Globe de Paris, 4 place Jussieu, 75231 Paris Cedex 5, France
b G´eomagn´etisme, Institut de Physique du Globe de Paris, France
Received 29 June 2006; received in revised form 30 October 2006; accepted 6 November 2006
The geomagnetic field and secular variation exhibit asymmetrical spatial features which are possibly originating from an hetero-
geneous thermal control of the Earth's lower mantle on the core. The identification of this control in magnetic data is subject to
several difficulties, some of which can be alleviated by the use of core surface flow models. Using numerical dynamos driven by
heterogeneous boundary heat flux, we confirm that within the parameter space accessible to simulations, time average surface flows
obey a simple thermal wind equilibrium between the Coriolis and buoyancy forces, the Lorentz, inertial and viscous forces playing
only a secondary role, even for Elsasser numbers significantly larger than 1. Furthermore, we average the models over the duration
of three vortex turnovers, and correlate them with a longer time average which fully reveals the signature of boundary heterogeneity.
This allows us to quantify the possibility of observing mantle control in core surface flows averaged over a short time period. A scal-
ing analysis is performed in order to apply the results to the Earth's core. We find that three vortex turnovers could represent between
100 and 360 years of Earth time, and that the heat flux heterogeneity at the core-mantle boundary could be large enough to yield an
observable signature of thermal mantle control in a time average core surface flow within reach of the available geomagnetic data.
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