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Leonhardt, Roman; Fabian, Karl; Winklhofer, Michael; Ferk, Annika; Laj, Carlo; Kissel, Catherine (2009): Geomagnetic field evolution during the Laschamp excursion. In: Earth and Planetary Science Letters, Vol. 278, No. 1-2: pp. 87-95
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Since the last geomagnetic reversal, 780,000 years ago, the Earth’smagnetic field repeatedly dropped dramatically in intensity. This hasoften been associated with large variations in local field direction,but without a persistent global polarity flip. The structure anddynamics of geomagnetic excursions, and especially the differencebetween excursions and polarity reversals, have remained elusive so far.For the best documented excursion, the Laschamp event at 41,000 yearsBP, we have reconstructed the evolution of the global field morphologyby using a Bayesian inversion of several high-resolution palaeomagneticrecords. We have obtained an excursion scenario in which inversemagnetic flux patches at the core-mantle boundary emerge near theequator and then move poleward. Contrary to the situation during thelast reversal (Leonhardt, R., Fabian. K., 2007. Paleomagneticreconstruction of the global geomagnetic field evolution during theMatuyama/Brunhes transition: Iterative Bayesian inversion andindependent verification. Earth Planet. Sci. Lett. 253,172-195). theseflux patches do not cross the hydrodynamic boundary of the inner-coretangent cylinder. While the last geomagnetic reversal began with asubstantial increase in the strength of the non-dipolar fieldcomponents, prior to the Laschamp excursion, both dipolar andnon-dipolar field decay at the same rate. This result suggests that thenature of an upcoming geomagnetic field instability can be predictedseveral hundred years in advance. Even though during the Laschampexcursion the dipolar field at the Earth’s surface was dominant, thereconstructed dynamic non-dipolar components lead to considerabledeviations among predicted records at different locations. The inversemodel also explains why at some locations no directional change duringthe Laschamp excursion is observed.