Genomagnetic secular variation during the Brunhes epoch inferred from the Paleomagnetism and the last 200 years geomagnetic field.

Abstract

Paleomagnetic data were compiled from the previous studies on volcanic materials in the southern part of Central Japan to investigate the geomagnetic secular variation during the Brunhes epoch. The distribution of compiled 113 VGPs is examined whether or not the Fisher statistics can be applied. Chi-square test indicates that the hypothesis of uniform azimuthal distribution around a mean VGP, which is assumed in the Fisher statistics, is rejected at a 0.05 significance level. Besides, the VGPs tend to concentrate in a certain direction. Azimuthal distributions of the Brunhes epoch VGPs for other regions, France, the Aleutian Islands, the Canary Islands, Comores Island, Crozet Island, Reunion Island and Amsterdam Island, also show similar elongation and azimuthal uniformity can be rejected for these assemblages. Hence the elongation of VGP distribution appears to be a global characteristic in the Brunhes epoch. These elongations can be explained neither by the westward drift of the nondipole fields, by the wobble of the geocentric dipole, nor by the contortion due to transforming field directions into VGPs. Gauss coefficients of the last 200 years geomagnetic field indicate that several nondipole terms changed linearly with the geocentric axial dipole component of g01. Some of these relationship is possible to have held well not only in the last 200 years but also throughout the Brunhes epoch. In addition, the previous paleointensity studies for the Plio-Pleistocene infer that g01 changed approximately in a normal distribution. On the basis of these results, a new model for the secular variation is proposed, in which the intensity of dipole term g01 changes in a normal distribution and those of the correlating nondipole terms vary linearly with a deviation of g01 from its mean intensity. All combinations of nondipole components are examined and consequently the model with a quadrupole component h12 seems to predict the elongation in azimuthal distribution and the global trend of latitude dependence in angular dispersion. This model appears to have an ability to give a significant and quantitative constraint to the homogeneous dynamo process.