HR: 08:00h
AN: GP41A-01 INVITED    [Abstracts]
TI: Secular Variation of the Geomagnetic Dipole during the past two Thousand Years
AU: * Valet, J
EM: valet@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252, France
AU: Herrero-Bervera, E
EM: herrero@soest.hawaii.edu
AF: Hawaii Institute of Geophysics, 2525 Correa Road, Honolulu, HI 96822, United States
AU: LeMouel, J
EM: lemouel@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252, France
AU: Plenier, G
EM: plenier@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris, 4, Place Jussieu, Paris, 75252, France
AB: We have constructed a very simple model of a time varying geocentric dipole based on the archeomagnetic records obtained at four widely separated sites on the globe for the past 2 ka. The predictions of the model in terms of directional variations have been tested against actual archeomagnetic data from 12 sites distributed over the globe, being aware of the uneven distribution of the sites for this period. The directions compiled at each site are compared with those derived from the time-varying spherical harmonic models produced by Hongre et al (1998) and recently by Korte et al (2005) who developed CALS7K-2 to predict both the field and secular variation with harmonics up to degree 10. We find that the misfits between our simple dipole and the actual records is equivalent to the performance of the spherical harmonic models for the European sites and not strikingly larger for the rest of the world. Many discrepancies can be accounted for by uncertainties inherent to the archeomagnetic records, which, along with the small number and poor geographical distribution of sites, leads us to conclude that the present state of the database does not allow to extract secular variations described by terms going beyond degree 2 and maybe even degree 1. It appears also that dipole tilt could be responsible for the main part of the secular variation associated with time constants exceeding 102 years. As a second step, we used the paleointensity records contained in the same database to construct the curve depicting the variations of the true dipole moment. The present decrease of the dipole did not begin prior to one thousand years ago and the dipole was actually increasing from 0 until AD 500. The dipole moment of CALS7K is moslty and sometimes significantly lower than the present estimate. The tilt and strength of the dipole give the dipole field at any site. The curve can thus be used to predict dipole field intensity at any location and offers potential for studies related to the evolution of the magnetosphere. As a direct application, we calculated the variations of dipole field intensity in Paris and compared them with the archeomagnetic records of total field intensity. The large amplitude of the dipolar changes is reflected by the records of total field and the archeomagnetic jerks coïncide with periods of either strong dipolar or non dipolar fields.
DE: 1560 Time variations: secular and longer
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting