コア表面のトロイダル磁場の強度と形について

*浜野 洋三[1]

東京大学大学院理学系研究科[1]

On the strength and the shape of the toroidal magnetic field at the core surface

*Yozo Hamano[1]
Dept. Earth&Planet. Sci., Univ. of Tokyo[1]

If the electrical conductivity in the mantle is horizontally non- uniform, the heterogeneous structure produces a poloidal field from a toroidal source field by the coupling effect. Based on the result of the numerical calculation of this effect and comparison with the geomagnetic non-dipole field, the strength and the shape of the toroidal field at the core surface were estimated assuming the conductivity heterogeneity in the D" layer. Root-mean-square strength of the inferred toroidal field is 1..2 mT, which is consistent with the previous estimates from the electric potential measurements and the observations of the core-mantle torque. The estimated latitudinal dependence of the toroidal field gives additional information on the origin of the toroidal field and the dynamics of the core.

The Earth's magnetic field is generated by magneto- hydrodynamic dynamo action in the electrically conducting fluid iron core, in which the coupling between the poloidal and toroidal magnetic fields due to the fluid motion play an important role. Although the toroidal field is trapped within the conducting part of the Earth's interior, and vanishes at the Earth's surface, the information on the toroidal field in the core gives an important clue on the dynamics of the core. Strength of the toroidal field at the core surface has been estimated from the measurements of the electric potential over long submarine cables, and the observations of the core-mantle torque inferred from the decadal length of day variation. Here, we report a new estimate on the strength and the shape of the toroidal field at the core surface. If the electrical conductivity in the mantle is horizontally non-uniform, the heterogeneous structure produces a poloidal field from a toroidal source field by the coupling effect between the magnetic fields and the structure. Within the mantle, seismic observations indicates that the D" layer at the lowermost part of the mantle is laterally heterogeneous, and the existence of the electrical conductivity heterogeneity is highly possible. In the present work, the effect of the lateral conductivity heterogeneity in the D" layer was calculated by using a newly developed semi- analytical approach to simulate the coupling effect in a laterally heterogeneous spherical shell. As a source field, zonal toroidal fields with harmonic degrees 1 - 10 are used. Root-mean-square strength of the generated stable poloidal field for each toroidal field ranges from 1/10 to 1/100 of the strength of the source toroidal field. At the Earth's surface, the strength decreases due to the filtering effect through the mantle, and give a negative slope as a function of the harmonic degree. The source toroidal field at the core surface was obtained by fitting the generated field at the Earth's surface to the observed geomagnetic non-dipole field. Root-mean-square strength of the toroidal source field at the core surface is 1.2 mT, which is about the same order of the strength of the poloidal field. As for the latitudinal dependence of the zonal toroidal field, strong peaks in the eastward and westward directions are observed at the northern and the southern high latitudes inside the inner-core tangent cylinder (higher than ~70 degrees in latitude), whereas the toroidal field outside the cylinder is relatively weak. North-south asymmetry is evident both in the fields inside and outside of the tangent cylinder. This information is useful to investigate the dynamical process in the core.