Millennial scale cyclicity in the geodynamo inferred from a dipole tilt reconstruction

Constraining past changes in the strength and configuration of the geomagnetic field provides a valuable perspective on the processes that govern the geodynamo. Here we update a modelled dipole tilt reconstruction for the last 9000 years by using palaeomagnetic records that originate from globally well-distributed sites. Some features predicted by earlier dipole models become more distinct due to added data sets. We identify a dominant 1350-year cycle in the dipole tilt variations and two preferred states of the dipole axis with north geomagnetic pole longitudes confined to either c. 120 degrees West or c. 30 degrees East. The dipole tilt reconstruction is shown to be consistent with independent geomagnetic field intensity data from western Eurasia, which show generally higher intensities when the dipole is tilted towards this region. Our study implies that VADM reconstructions that are constrained by a biased spatial distribution of data can show variations resulting from dipole tilt instead of true dipole moment. The preferred states of the modelled dipole axis can be related to the four semi-stationary high latitude flux lobes that have been observed at the core-mantle boundary in long-term time averaged palaeomagnetic field models. The tilt episodes towards 30 degrees East in the northern hemisphere are perhaps related to the appearance of a high intensity flux lobe beneath western Eurasia. The dipole tilt reconstruction is highly correlated to millennial scale variations in the length of day that have been reconstructed from ancient records of eclipses, which indicates that the cyclicity may constitute an important component in core flow dynamics. (C) 2011 Elsevier B.V. All rights reserved.