[S-EM13] Geomagnetism, paleomagnetism and rock magnetism

[SEM13-06] An attempt to detect a stratified layer at the core surface from a geomagnetic field model

*Masaki Matsushima¹ (1.Department of Earth and Planetary Sciences, School of Science, Tokyo Institute of Technology)

Keywords:core surface flow, geomagnetic field, stratified layer

A stably stratified layer at the top of the Earth's outer core can be present as suggested by recent seismic studies. If it is the case, upwelling and downwelling flows would not be present just below the core-mantle boundary (CMB); that is, only toroidal flows contribute to temporal changes in the geomagnetic field. Such fluid flows near the CMB can be estimated from a geomagnetic field model which provides information on the spatial distribution of the geomagnetic field and its temporal variation at the Earth's surface. The information at the CMB can be obtained through downward continuation of the geomagnetic potential field on the assumption that the mantle is an electrical insulator. Most of core flow models are estimated on the basis of the frozen-flux hypothesis. If a viscous boundary layer with finite thickness is present below the CMB, not only the motional induction but also the magnetic diffusion inside the boundary layer should cause temporal variations of the geomagnetic field. Hence core surface flows near the CMB can be estimated from a geomagnetic field model as follows; the magnetic diffusion contributing to temporal variations of the geomagnetic field is neglected below the boundary layer as in the frozen-flux approximation, whereas the magnetic diffusion is assumed to cause the temporal variations inside the boundary layer. Core surface flows below the boundary layer are assumed to be in the tangentially magnetostrophic state.

In this method to estimate a core surface flow, depths from the core surface are provided as parameters; one (ξ₁) is taken to be less than the thickness of the boundary layer, and the other (ξ₂) be larger than the thickness of the boundary layer. Hence, it might be possible to investigate the existence of a stably stratified layer at the top of the core in the following way. In case 1, only the toroidal flow is allowed inside the stratified layer, while in case 2, both the toroidal and poloidal flows are permitted there. Correlation coefficients and misfits between a temporal variation of the magnetic field obtained from a geomagnetic field model and that due to core surface flows in case 1 are calculated. Also correlation coefficients and misfits in case 2 are calculated. Then correlation coefficients and misfits thus obtained are compared with each other. The result provides information on the stratified layer at the top of the outer core.