| Budget Amount *help |
¥3,800,000 (Direct Cost: ¥3,800,000)
Fiscal Year 2004: ¥1,100,000 (Direct Cost: ¥1,100,000)
Fiscal Year 2003: ¥900,000 (Direct Cost: ¥900,000)
Fiscal Year 2002: ¥1,800,000 (Direct Cost: ¥1,800,000)
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| Research Abstract |
The axial and equatorial rotations of both cores relative to the mantle associated with the Quaternary glacial cycles were evaluated based on a realistic earth model in density and elastic structures. The rheological model is composed of compressible Maxwell viscoelastic mantle, inviscid outer core and incompressible Maxwell viscoelastic inner core. In models with no frictional torques at the boundaries of the outer core, the maximum magnitude of the predicted axial rotations of the outer and inner cores amounts to 〜2° yr^<-1> and 〜1° yr^<-1>, respectively, although the predictions are sensitive to the lower mantle viscosity. These values are significantly larger than the flow velocity of the outer core inferred from the observed magnetic westward drift of 〜0.2° yr^<-1>. Pulsation terms with a period of 〜225 years are, however, predicted in the equatorial rotations for both cores. If we consider that the recent geodynamo modelling produces realistic features on the core flow, then the
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results for axial rotations may require some coupling mechanism at the boundaries. I therefore evaluated the differential rotations by taking into account a time-dependent electromagnetic(EM) coupling at the core-mantle boundary(CMB) and inner core boundary(ICB). For a realistic radial magnetic field at the CMB estimated from a downward continuation of surface magnetic field, the axial and equatorial rotations couple through the fricctional torques at the boundaries. The differential rotations were evaluated for conductivity models with the conductance of 10^8 S of the bottom layer of the mantle inferred from the studies of nutation and precession of the Earth and variations of length of day(LOD). The secular parts of equatorial rotations are less sensitive to these parameters describing the EM torque, and similar to those with no frictional torques. The maximum magnitude of the secular axial rotations of both cores is 〜0.02° yr^<-1> at most. These models, however, produce both pulsation terms in the equatorial rotations of both cores and pulse-like terms in the axial rotations of the whole Earth and outer and inner cores. While the coupling through the frictional torque probably causes the excitation of the pulse-like terms, the excitation of pulsation terms may be inherent in the Euler's equations describing the equatorial rotations of both cores. The periods of these terms are sensitive to both the magnitude of the radial magnetic field at the CMB and the conductivity structure. A sharp isolated spectral peak is not predicted for a model with a thin conductive layer (〜200 m) at the bottom of the mantle. For a model with a conductive layer of 100 km thickness, however, sharp spectral peaks are predicted at the periods of 〜225 and 〜25 years for pulsation and pulse-like terms, respectively, although these depend on the strength of the radial magnetic field at the CMB. The results obtained in this study may suggest that the geodynamo may be affected by climate changes through both the secular and chaotic axial and equatorial differential rotations of both cores. Less
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