Earth's rotations and core-mantle coupling

基于密度和弹性结构的真实地球模型，评估了与第四纪冰期旋回相关的两个岩心相对于地幔的轴向和赤道旋转。流变模型由可压缩麦克斯韦粘弹性地幔、不可压缩外核和不可压缩麦克斯韦粘弹性内核组成。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震级分别为~ 2°yr^<-1>和~ 1°yr^<-1>，尽管预测对下地幔粘度很敏感。这些数值明显大于由观测到的磁向西漂移（~ 0.2°yr^<-1>）推断的外核流速。然而，在两个地核的赤道旋转中预测了周期为~ 225年的脉动项。如果我们考虑到最近的地球动力学模拟对岩心流动产生了真实的特征，那么对于轴向旋转的更多结果可能需要在边界处存在某种耦合机制。因此，我通过考虑核幔边界（CMB）和内核边界（ICB）的时变电磁（EM）耦合来评估微分旋转。对于从表面磁场向下延拓估计的CMB实际径向磁场，轴向和赤道旋转通过边界处的摩擦力矩耦合。根据地球的章动、岁差和日长变化，对地幔底层电导率为10^8 S的电导率模型进行了微分旋转评价。赤道旋转的长期部分对这些描述电磁扭矩的参数不太敏感，与没有摩擦扭矩的参数相似。两个岩心的最大长期轴向旋转幅度最大为~ 0.02°yr^<-1>。然而，这些模型在两个地核的赤道旋转中产生脉动项，在整个地球以及内外地核的轴向旋转中产生脉动项。虽然通过摩擦转矩的耦合可能引起脉冲项的激励，但脉动项的激励可能是描述两个核赤道旋转的欧拉方程所固有的。这些项的周期对微波背景的径向磁场大小和电导率结构都很敏感。对于在地幔底部有薄导电层（~ 200 m）的模型，预计不会出现尖锐的孤立谱峰。然而，对于具有100 km厚度导电层的模型，脉冲和类脉冲项分别在~ 225年和~ 25年的周期内预测出尖锐的光谱峰值，尽管这些峰值取决于CMB的径向磁场强度。本研究的结果可能表明，地球动力可能通过两个地核的长期和混沌轴向和赤道的微分旋转受到气候变化的影响。少

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