MHD dynamo in a rotating spherical shell: Influence of mechanical boundary conditions

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  • 回転球殻MHDダイナモにおける力学的境界条件の影響(宇宙・惑星(1),一般講演)

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Numerical experiments of magnetohydrodynamic dynamos driven by thermal convection in a rotating spherical shell with a free-slip top boundary and a no-slip bottom boundary are performed. Non-dimensional numbers for the experiments are the modified Rayleigh number Ra=100, the Ekman number E=10^<-3>, the Prandtl number Pr=1, the ratio of inner and outer radii ξ=0.35. The magnetic Prandtl number Pm is varied from 5 to 50. Time integration of non-magnetic thermal convection is carried out until a quasi-steady state is established, and then MHD dynamo calculation is carried out starting from the quasi-steady state with a dipole and toroidal magnetic field. In the non-magnetic thermal convection case, the obtained solution consists of well-organized spiral vortex columns aligned with the rotation axis. A strong prograde zonal flow is produced at the top of the spherical shell in contrast to the simulations with the both no-slip boundaries. In the MHD dynamo cases, calculated magnetic fields fall into decay at all magnetic Prandtl numbers. The strong prograde zonal flow, which is prominent in the case of non-magnetic thermal convection, does not appear, but a slow retrograde zonal flow is produced at the top of spherical shell. As a result, the omega effect expected from the strong zonal flows does not seem to operate effectively.

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