An examination of simulated geomagnetic reversals from a palaeomagnetic perspective

Four magnetic polarity reversals that occurred during two numerical simulat ions of the Glatzmaier-Roberts geodynamo display a range of behaviour that resembles records of real reversals of the Earth's magnetic field in some w ays, and suggests additional insights in others. Two reversals happened dur ing the homogeneous simulation, which prescribes spatially uniform heat flu x at the core-mantle boundary (CMB); and two occurred during the tomographi c simulation, which specifies variable CMB heat flux patterned after a low- order seismic velocity model from tomographic investigation of the lower ma ntle. All but one were accomplished within 2000-7000 (model) years, whereas the second tomographic reversal took 22 000 years. The two homogeneous tra nsitions display low intensities typical of real reversals, with, longer-te rm variation resembling what has been called 'sawtooth' behaviour. During t he first tomographic reversal extremely high non-dipole fields occur in som e regions, the result of strong patches of vertical flux that appear in les s than 100 years and grow rapidly for several hundred more. The intensity d uring the second tomographic reversal is unusually low for a long time, and large-amplitude oscillations in direction are common. The fields in the mi ddle of the polarity transitions are dominantly nondipolar for all but the first tomographic reversal. One consists of spherical harmonics that are ma inly antisymmetric about the equator, two by symmetric harmonics, and one b y a mixture of symmetric and antisymmetric harmonics. Despite this wide var iety of characteristics, all reversals occur when the non-dipole energy tre nd is upward. Finally, after running 300 kyr and reversing twice, the densi ty of transitional virtual geomagnetic poles in the tomographic simulations exhibits a crude statistical correlation with areas of higher-than-average CMB heat flux, offering some support for hypotheses of preferred bands and patches.