Geodynamo reversal frequency and heterogeneous core–mantle boundary heat flow

Abstract

We analyze more than two hundred and forty magnetic polarity reversals and excursions over forty million years of time from numerical dynamos with heterogeneous boundary heat flux. Comparisons between a dynamo with uniform boundary heat flux, dynamos with boundary heat flux patterns consisting of a single spherical harmonic, and dynamos with tomographic heat flux patterns proportional to the large-scale lateral variation of seismic shear wave velocity in the lower mantle reveal that boundary heat flux heterogeneity tends to increase the average frequency of polarity reversals relative to uniform boundary conditions, particularly if the heterogeneity increases the average equatorial heat flux. Non-axisymmetric spherical harmonic degree ℓ = 1 and 2 boundary heterogeneity and the seismically derived tomographic heterogeneity produce comparable reversal and excursion frequencies in our models, suggesting that geomagnetic polarity reversal rates may be relatively insensitive to the non-axisymmetric planform of core–mantle boundary heat flux. In contrast, the average polarity reversal frequency in our models is quite sensitive to the total boundary heat flow and to the total heat flow at the equator, with non-reversing behavior at low heat flow and frequent reversals at high heat flow conditions. Reversal frequency also increases with the amplitude of the boundary heterogeneity, although it is less sensitive to this parameter. Our results suggest that geomagnetic superchrons may correspond to times with reduced core–mantle boundary heat flow, either globally or equatorially, and conversely, periods with high paleomagnetic reversal frequency may correspond to times with increased heat flow at the core–mantle boundary.