| abstract: |
Recent seismological observations and their analysis suggest the existence of a stable layer near outer boundary of the Earth's outer core(e.g., Lay and Young 1999, Helffrich and Kaneshima 2004, 2010). In an unstable layer beneath the stable layer, intrinsic magnetic fields are generated due to the dynamo action. In the stable layer, magnetic field generated in lower unstable layer is expected to decay since convective motions are suppressed there. Some recent numerical studies show the existence of an upper stable layer has a decaying effect on magnetic disturbances in the surface of the outer core (e.g., Christensen 2006, Nakagawa 2011)．
The extent of penetration of the deep convective motion into the outer stable layer is one of the important key issue for considering magnetic field generation through the dynamo process as well as origin of the magnetic secular variation of the Earth. Takehiro and Lister (2001) theoretically derives the scalling law of penetration thickness of the convection motion into the uppper stable layer in the case of no magnetic field, and shows that penetration depth is proportional to the ratio of Brunt-Vaisala frequency in upper stable layer and to the horizontal wavenumber of the disturbance. However, scaling law of penetration under the existence of magnetic field has not been examined. In this study, the effect of penetration of deep convection of conducting fluid in an upper stable layer are investigated with numerical experiments on MHD convection and the dynamo effect.
With linear theory, it is shown that Alfven wave can propagate in stable layers regardless of stability under the existence of magnetic field. The distance of propagation can be estimated with phase speed of excited Alfven wave divided by arithmetic mean of eddy viscosity coefficients and total wave number. we perform linear analyses of MHD thermal convection in a rapidly rotating spherical shell with an upper stably stratified layer embedded in the axially uniform basic magnetic field. When the strong stratification of the stable layer is given, the neutral modes of columnar fluid motions and magnetic field disturbances trapped below the stable layer gradually penetrate into the stable layer as the basic magnetic field is strengthened. The penetration distances of the obtained neutral modes are in good agreement with those of the theoretical scaling. It is also observed that vortical fluid motions and toroidal magnetic field disturbances deeply penetrate into the stable layer in the case of MHD dynamo calculation.
One of important quantity for Earth planetary science is radial component of magnetic field which can be observed at Earth's surface. According to the above theory, disturbances excited in upper stable layers is Alfven wave. Fluid motion of Alfven wave is restricted in horizontal direction, thus it has no radial component of magnetic field. However, radial component of magnetic field is excited in upper stable layers of rotating spheres by MHD convection, although the strength is weak. If a stable layer exists in upper region of Earth's core, non-dipole component of magnetic field which is observed at Earth's surface may be excited in the stable layer by the above described mechanism.
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