Effects of buoyancy and rotation on the polarity reversal frequency of gravitationally driven numerical dynamos

SUMMARY We present the results of 50 simulations of the geodynamo using a gravitational dynamo model driven by compositional convection in an electrically conducting 3-D fluid shell. By varying the relative strengths of buoyancy and rotation these simulations span a range of dynamo behaviour from strongly dipolar, non-reversing to multipolar and frequently reversing. The polarity reversal frequency is increased with increasing Rayleigh number and Ekman number. Model behaviour also varies in terms of dipolarity, variability of the dipole, and core–mantle boundary field spectra. A transition region is found where the models have dipolar fields and moderate (Earth-like) reversal frequencies of approximately 4 per Myr. Dynamo scaling laws are used to compare model dipole intensities to the geodynamo. If the geodynamo lies in a similar transition region then secular evolution of the buoyancy flux in the outer core due to the growth of the inner core and the decrease in the Earth’s rotation rate due to tidal dissipation can account for some of the secular changes in the reversal frequency and other geomagnetic field properties over the past 100 Myr.

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