HR: 1340h
AN: GP33A-0922    [Abstracts]
TI: Simulation Study Of The Symmetry-Breaking Instability And The Dipole Field Reversal In A Rotating Spherical Shell Dynamo
AU: Nishikawa, N
EM: nnishikawa@jamstec.go.jp
AF: Super Computer System Planning and Operations Department, Japan Agency for Marine- Earth Science and Technology, 3173-25 Showa-machi, Kanazawa-ku, yokohama, 236-0001, Japan
AU: * Kusano, K
EM: kusano@jamstec.go.jp
AF: The Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology, 3173- 25 Showa-Machi, Kanagawa-ku, yokohama, 236-0001, Japan
AB: It is known by the paleomagnetic measurement of the geomagnetic field that, although the magnetic field of the Earth is usually dominated by a dipole component, the polarity is suddenly reversed many times so far at irregular intervals. The understanding of the mechanism for both the sustainment and the reversal of the dipole field still remains the most important problem not only in the geoscience but also in the nonlinear magnetohydrodynamics. In this study, the reversal mechanism of dipole magnetic field generated by the dynamo action in a rotating spherical shell is investigated by the three-dimensional nonlinear magnetohydrodynamic simulations as well as the linear stability analyses based on the decomposition technique with respect to the equatorial symmetry. As a result, first, we found that there is the threshold of the magnetic Prandtl number, below which the dipole field is never reversed, but above which the reversal may occur at irregular intervals like the paleomagnetic evolution of the geodynamo. Second, it is shown that the dynamo process responsible for the generation of dipole field (called " a-dynamo") consists only of the anti-mirror symmetric magnetic field and the mirror symmetric velocity field with respect to the equatorial plane. Third, it is found that the components of the opposite symmetry to the a-dynamo can survive only in the case that the reversal may occur, but they quickly decay in no reversal case. It indicates that the dipole field reversal and the loss of the equatorial symmetry are tightly connected. In fact, it is clearly demonstrated by the numerical analyses that the a-dynamo process is linearly unstable for the perturbation of the opposite symmetry when the magnetic Prandtl number exceeds the threshold for the dipole reversal. The mode coupling between the longitudinal Fourier components plays a crucial role in making the instability. Based on the results above, we propose that the symmetry-breaking instability could work as the primary cause of the dipole field reversal in the geodynamo process, although the trigger mechanism of the reversal events is still a puzzle.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1510 Dynamo: theories and simulations
DE: 7544 Stellar interiors and dynamo theory
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting