The role of the Earth's mantle in controlling the frequency of geomagneticreversals

A series of computer simulations of the Earth's dynamo illustrates how the thermal structure of the lowermost mantle might affect convection and magne tic-field generation in the fluid core. Eight different patterns of heat fl ux from the core to the mantle are imposed over the core-mantle boundary. S pontaneous magnetic dipole reversals and excursions occur in seven of these cases, although sometimes the field only reverses in the outer part of the core, and then quickly reverses back. The results suggest correlations amo ng the frequency of reversals, the duration over which the reversals occur, the magnetic-field intensity and the secular variation. The case with unif orm heat flux at the core-mantle boundary appears most 'Earth-like'. This r esult suggests that variations in heat flux at the care-mantle boundary of the Earth are smaller than previously thought, possibly because seismic vel ocity anomalies in the lowermost mantle might have more of a compositional rather than thermal origin, or because of enhanced heat flux in the mantle' s zones of ultra-low seismic velocity,