HR: 1340h
AN: GP33A-0917    [Abstracts]
TI: Torsional Oscillations in a Numerical Geodynamo Model
AU: * Sakuraba, A
EM: sakuraba@eps.s.u-tokyo.ac.jp
AF: Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Tokyo, 113- 0033, Japan
AU: Roberts, P
EM: roberts@math.ucla.edu
AF: Institute of Geophysics and Planetary Physics, University of California Los Angeles, 405 Hilgard Avenue, Los Angeles, CA 90095,
AB: In a first approximation, the Earth's core is in a Taylor state, where viscous and inertial forces are negligibly small compared with Coriolis and Lorentz forces. To decrease viscous effects in a numerical geodynamo, the Ekman number, E = ν/Ømega L², should be small, where ν is kinematic viscosity, Ømega is angular velocity of the core and L is the core size. To decrease inertial effects, the ratio of kinetic to magnetic energy densities should be smaller than unity, implying that the magnetic Ekman number, E_m = η/Ømega L², where η is magnetic diffusivity, should be small, assuming that that the magnetic Reynolds number and the Elsasser number are close to those expected for the Earth's core. Deviations from the Taylor state are believed to propagate as torsional waves inside the core with timescales of some decades. Torsional oscillations are of particular interest for the angular momentum balance in the Earth system. They might also be related to abrupt changes in the geomagnetic field such as magnetic jerks. Torsional waves can exist when the viscous diffusion timescale is much longer than the magnetic one, i.e., E \ll E_m. Early dynamo models succeeded in creating dynamos with both E and E_m smaller than unity (typically 10^-5 ~ 10^-6) but with E ≥ E_m. For a better understanding of torsional oscillations, numerical calculations with E < E_m, i.e., small magnetic Prandtl number, are desirable. We attempted to simulate Earth-type dynamos with E < E_m < 10^-5 and to analyze the torques on axial cylinders and the propagation of torsional waves. We paid particular attention to the effect of the freely rotating conductive inner core. Since the computation is demanding even on super-parallel computers, we approximated the magnetic field in the inner and outer cores by using the same number of Chebyshev spectral modes as those used for the velocity in the outer core. As a result, we could integrate the dynamo with the conductive inner core without increasing the computation time. This treatment is reasonable when E < E_m and the magnetic length scale exceeds the velocity length scale.
DE: 1507 Core processes (1213, 8115)
DE: 1510 Dynamo: theories and simulations
DE: 1555 Time variations: diurnal to decadal
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting