Statistics – Computation
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp33a..04w&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP33A-04
Statistics
Computation
1510 Dynamo: Theories And Simulations, 1513 Geomagnetic Excursions, 1535 Reversals: Process, Timescale, Magnetostratigraphy
Scientific paper
Self-consistent numerical dynamo simulations have been used for more than a decade now to simulate geomagnetic field reversals. The computer models, in theory, allow to explore the underlying dynamics which is largely inaccessible to paleomagnetic exploration. However, the complex spatial and temporal nature of the dynamo mechanism complicates the numerical computation as well as the interpretation. Therefore typically rather simple numerical models have been analyzed that are quite remote from realistic parameters. It is thus essential to test their behavior against paleomagnetic inferences. On the other hand, the models may nevertheless provide valuable help in interpreting and understanding paleomagnetic records. We have analyzed several reversing models to infer some common characteristics. Numerical reversals are a three stage process: First, plume-like outflows in the equatorial region or at higher latitudes produce significant inverse field and step-wise degrade the predominant dipole. The second stage is a period where the field is dominated by higher harmonics and changes rather fast. Finally, the dipole grows again with opposite polarity. The slow degradation is also observed in paleomagnetism, but the second low-dipole period seems not a common feature. Also, in paleomagnetic data the dipole recovers typically faster after the reversal than it had decrease before while both processes have a comparable duration in the simulations. Though the overall reversal process tends to last longer in the computer models than in the geodynamo, the latitudinal dependence of durations agrees with paleomagnetic findings. The simulations also suggest some additional features that can be tested with paleomagnetic data: Reversals as well as excursion are rarely simple dipole swings but typically involve several polarity changes. Also, excursions tend to be more pronounced at low and high latitudes where the upwellings produce more inverse field, but excursions may be hard to discern from the background variation at mid latitudes. The simulations thus predict that fewer excursions are reported from mid-latitude regions. We have also analyzed the statistics of chron durations in our simulations and find that a log-normal or power-law probability distribution function is most likely. This is somewhat surprising, since these distributions require a long-term memory we have no explanation for. However, a log-normal statistics has also been suggested for the paleomagnetic record.
Jonkers Art
Wicht Johannes
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