Abstract
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.
Highlights
The mechanism of the polarity reversal of the geomagnetic field is one of the unsolved problems in geophysics, and it has been investigated through magnetic measurements of paleomagnetic samples and numerical simulations of magnetohydrodynamic (MHD) processes in rapidly rotating spherical shells
Takahashi et al (2003) pointed out, that the dynamo action inside the tangent cylinder (TC) plays a minor role for the polarity transition, since the magnetic field generated inside the TC is insignificant because of small-scale and highly time-dependent convection there
Using a Takahashi-Matsushima-Honkura dynamo model, we have investigated the internal process of dynamo action during some transitional periods
Summary
The mechanism of the polarity reversal of the geomagnetic field is one of the unsolved problems in geophysics, and it has been investigated through magnetic measurements of paleomagnetic samples and numerical simulations of magnetohydrodynamic (MHD) processes in rapidly rotating spherical shells. As in the E1 event, the weak reversed field appears in the shallow interior of the core, but only in the southern hemisphere at t = 0.960 This phenomenon can be observed as an excursion without polarity change outside the core. In the R1 event, the modest correlation with active inverse field generation lasts for a sufficiently long time, and the negative radial component is collected by the down-welling flow associated with columnar convective motions near the core surface, as usually found in dynamo models with stable polarity Takahashi et al (2003) pointed out, that the dynamo action inside the TC plays a minor role for the polarity transition, since the magnetic field generated inside the TC is insignificant because of small-scale and highly time-dependent convection there. We conclude that polarity reversals are initiated at low-latitudes outside the TC, as found in this paper as well as by Takahashi et al (2005)
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