HR: 0800h
AN: GP31A-0082    [Abstracts]
TI: Numerical study of dynamo action at low Ekman numbers in rotating spherical shells
AU: * Takahashi, F
EM: futoshi@stp.isas.jaxa.jp
AF: 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, 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, Tokyo, 152-8551 Japan
AB: It is generally believed that the geomagnetic field is generated by convective motion of an electrically conducting fluid in the Earth's outer core, known as the geodynamo. However, it is still unclear whether numerical models represent the dynamics in the Earth's core, properly, since nondimensional parameters in the present numerical models of the geodynamo are still far from those in the Earth's core. The most serious problem arises from the difference in the Ekman number, E. It is required that the Ekman number is low enough to reach nearly inviscid regime, as is the case with the Earth's core of E~ 10^-9. Here, numerical models of the geodynamo at the Ekman number as low as 10^-6 are provided. In our results, many small-scale convection cells attached to the inner portion of the core appear, while dominantly dipolar magnetic field is generated by such convection cells. What is worth noting here is the large difference in characteristic length scale between the convection and magnetic field. Such scale disparity indicates that small-scale convective motion generates large-scale magnetic field more significantly than in a case at higher Ekman number. This fact suggests that backward energy cascade plays a major role in maintaining dipolar magnetic field in low Ekman number dynamos such as the geodynamo. Effects of the small magnetic Prandtl number are also discussed.
DE: 1507 Core processes (1213, 8115)
DE: 1510 Dynamo: theories and simulations
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005