Geomagnetic Estimates of the Mean Rates of Paleomagnetic Dipole Power Excursions and Reversals

Details Geomagnetic Estimates of the Mean Rates of Paleomagnetic Dipole Power Excursions and Reversals Geomagnetic Estimates of the Mean Rates of Paleomagnetic Dipole Power Excursions and Reversals

Dec 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmgp23a..02v&link_type=abstract

American Geophysical Union, Fall Meeting 2007, abstract #GP23A-02

Mathematics

Logic

1507 Core Processes (1213, 8115), 1513 Geomagnetic Excursions, 1535 Reversals: Process, Timescale, Magnetostratigraphy, 5440 Magnetic Fields And Magnetism, 5734 Magnetic Fields And Magnetism

Scientific paper

To further and better test some kinematic, dynamical, and statistical hypotheses about the core geodynamo, previous predictions for the mean rates and durations of dipole power excursions and axial dipole reversals [Voorhies & Conrad, 1996] are re-examined and refined. The hypotheses yield a theoretical form for the low degree, core-source part of the Lowes-Mauersberger spectrum R(n) - the mean square field, averaged over the sphere of radius a, in spherical harmonics of degree n. This form is \{R(n)\} = K[(n + 1/2)/n(n + 1)](c/a)**(2n+4); core radius c = 3.5 +/- 0.1 Mm is recovered with no significant error by fitting log-theoretical to log-observational spectra for Earth [Voorhies, 2004]. Jupiter may also have a "1/n" spectrum, but Mercury's field seems too weak and for its core dynamo to be very Earth-like. Our statistical hypothesis distributes (2n+1)R(n)/\{R(n)\} as chi-square with 2n+1 degrees of freedom. The implied field is usually mainly dipolar and can be primarily axial. During a small fraction of geologic time, however, dipole power R(1) is expected to be quite small. An unusually weak axial dipole might reverse during such an interval. So we defined "dipole power excursion" as an interval when absolute dipole moment is less than or equal to a threshold value, taken to be the absolute equatorial dipole moment at 1980. Our amplitude estimate implied that such excursions would occupy 2.5% of geologic time. We used the 1945-1980 dipole power time scale to convert this fraction into mean duration, hence mean rate. For dipole power excursions, we predicted a mean duration of 2767 years and a mean rate of 9.04 per million years; for reversals, we predicted a mean duration of 5534 years and a mean rate of 2.26 per million years [Voorhies & Conrad, 1996]. The method and results are critically reviewed and updated, with special attention to definitions, calibration, time scale, the one quarter rule for reversal/excursion rates, and uncertainty estimates. For example, amplitude K has now been calibrated by a self-consistent fit to satellite-era spectra for degrees 1-12 and used to estimate mean intensity [Voorhies, 2006 JGR-SE]. The expected Virtual (Axial) Dipole Moment, 6.5 +/- 1.0 (x 10**22 Am**2), is centered in the range of published mean paleointensities. The calibrated model also predicts large intensity variations: 80% of the time, absolute dipole moment is expected to be between 2.7 and 9.0, but non-dipole fields broaden the 80% range for Virtual Axial Dipole Moment to about 2 - 12 (x 10**22 Am**2).

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