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Earth's rotations and core-mantle coupling

地球自转和地核-地幔耦合

基本信息

批准号：
14540396
负责人：
NAKADA Masao
金额：
$ 2.43万
依托单位：
KYUSHU UNIVERSITY
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (C)
财政年份：
2002
资助国家：
日本
起止时间：
2002 至 2004
项目状态：
已结题

项目摘要

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 … More 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

在密度和弹性结构的真实地球模型的基础上，评估了与第四纪冰川旋回有关的两个核心相对于地幔的轴向和赤道自转。流变模型由可压缩Maxwell粘弹性地幔、无粘性外核和不可压缩Maxwell粘弹性内核组成。在外核边界没有摩擦力矩的模型中，预测的外核和内核轴向旋转的最大量级分别为~2°yr^&lt；-1和~1°yr^&lt；-1&gt；，尽管预测对下地幔粘性很敏感。这些值明显大于由观测到的~0.2°yr^&lt；-1&gt；的磁西移所推断的外核的流速。然而，在两个核心的赤道自转中，都预测了周期为~225年的脉动项。如果我们认为最近的地球发电机模型在核心流动上产生了真实的特征，那么…轴向旋转的更多结果可能需要在边界处有某种耦合机制。因此，我通过考虑核-地幔边界(CMB)和内核边界(ICB)的依赖于时间的电磁(EM)耦合来评估差异旋转。对于从表面磁场向下延续估计的CMB处的实际径向磁场，轴向旋转和赤道旋转通过边界处的摩擦力矩耦合。根据地球章动和岁差以及日长变化的研究，估算了地幔底层电导率为10^8 S的电导率模型的微分自转。赤道自转的长期部分对这些描述电磁扭矩的参数不那么敏感，与没有摩擦力矩的部分相似。两个岩心的长期轴向旋转的最大震级至多为0.02°yr^&lt；-1。然而，这些模型既产生了地核赤道自转的脉动项，也产生了整个地球以及外核和内核的轴向自转的脉冲项。虽然通过摩擦力矩的耦合可能导致类脉冲项的激发，但脉动项的激发可能是描述两个核心的赤道自转的欧拉方程所固有的。这些项的周期对CMB处的径向磁场大小和导电性结构都很敏感。对于地幔底部有一个薄的导电层(~200m)的模式，没有预测到明显的孤立光谱峰。然而，对于具有100公里厚度的导电层的模型，脉动项和类脉冲项的尖锐光谱峰值分别在~225年和~25年的周期内被预测，尽管这取决于CMB处的径向磁场的强度。这项研究的结果可能表明，地球发电机可能通过两个核心的长期和混沌的轴向和赤道差异旋转而受到气候变化的影响。较少