TERRAPUB Earth, Planets and Space

Earth Planets Space, Vol. 59 (No. 7), pp. 665-673, 2007

A numerical study on magnetic polarity transition in an MHD dynamo model

Futoshi Takahashi, Masaki Matsushima, and Yoshimori Honkura

Department of Earth and Planetary Sciences, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8551, Japan

(Received September 30, 2006; Revised June 1, 2007; Accepted June 1, 2007; Online published July 20, 2007)

Abstract: Magnetic polarity transitions in a Takahashi-Matsushima-Honkura dynamo model are analyzed. Distinctive differences in behavior of the axisymmetric poloidal magnetic field are found among a polarity reversal and excursions, including short polarity events. At the beginning of magnetic polarity transitions, the magnetic field with the reversed polarity is generated by anti-cyclonic convection columns deep within the outer core. In the case of excursion, it is soon advected by the radial flow toward a shallow interior of the core, and the transition can be detected at the core surface. However, the same process retrieves the original polarity from the deep interior, and the reversed field eventually vanishes. In the case of polarity reversal, on the other hand, the reversed polarity field is persistently generated deep within the core. It is then advected toward a shallow interior of the core, while the generation process of the reversed field occurs successively. The reversed polarity field near the core surface is collected by the downwelling flow associated with convection columns, as is the case for the original polarity field. The polarity reversal is completed by the advection process, the duration of which is consistent with the flow speed in the core.
Key words: Dynamo, magnetic polarity transition, advection, stretching.

Corresponding author E-mail: futoshi@geo.titech.ac.jp

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