HR: 0800h
AN: GP11D-0850    [Abstracts]
TI: Thermal convection and magnetic field generation in a rotating spherical shell at high Rayleigh number
AU: * Takahashi, F
EM: futoshi@stp.isas.jaxa.jp
AF: Department of Space Plasma Physics, Institute of Space and Astronautical Science, JAXA, 3-1-1, Yoshinodai, Sagamihara, Kanagawa, 229-8510 Japan
AU: Matsushima, M
EM: mmatsush@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AU: Honkura, Y
EM: yhonkura@geo.titech.ac.jp
AF: Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo, 152-8551 Japan
AB: We have carried out 3D numerical simulations of MHD dynamo in a rotating spherical shell to investigate behavior of the convection structure and the generated magnetic field. Here, we focus on Rayleigh number ($Ra$) dependence of numerical dynamos to make extrapolation to the geodynamo regime, and thus, performed parameter survey varying $Ra$ up to about 53 times the critical value, $Ra_c$. To make wider parameter survey, we use moderate value of the Ekman number ($10^{-4}$). Using the other fixed parameters, the Prandtl number $Pr=1$, the magnetic Prandtl number $Pm=2$, and radius ratio $r_i/r_o=0.35$, dynamos undergo a series of bifurcations with increasing $Ra$ from well known dipolar dynamos without convection inside the tangent cylinder (TC), DP1, to the ones with active convection inside TC generating the toroidal magnetic field through the $\omega$-effect, DP2, non-dipolar dynamos, NDP, and then TC-dominating dynamos, TCD. Between DP2 and NDP transition, the magnetic energy shows a dramatic reduction, and then increases again, whereas the kinetic energy monotonically increases. TCD dynamos are also found in Glatzmaier and Roberts dynamo model using hyperdiffusivity. In this regime, advection term in the momentum equation plays a role in momentum balance, while other dynamos at lower $Ra$ are essentially in geostrophic state. The Lorentz force counteracts the advection term to inhibit disturbance in convection structure due to vortex stretching. It seems that viscous effects still play a role in momentum balance. Using the results obtained in this study, it is suggested that the magnetic Reynolds number similar to the Earth's core would be achieved at $Ra\sim 80Rac$.
DE: 1507 Core processes (8115)
DE: 1510 Dynamo theories
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2004 AGU Fall Meeting