Fluctuations in the Earth's rotation and the topography of the core-mantle interface
Fluctuations in the Earth's rotation and the topography of the core-mantle interface
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地球自转的波动和核幔界面的地形
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发表时间：
1989
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通讯作者：
R. Hide
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作者：
R. Hide
在过去20年里，支持高粘性地幔中的非常缓慢的对流仅限于上700公里的观点的论据逐渐减弱，因此，地球物理学家越来越愿意接受这样一种观点，即温度和其他结构参数的显著水平变化发生在下地幔的各个层面。伴随的密度变化，包括核幔界面形状的扭曲所引起的密度变化，将大大有助于地球引力的长波长特征，也会影响地震传播时间。深部地幔对流对下面的低粘度液态金属核所施加的热力学和力学边界条件偏离轴对称性，不仅会影响主地磁场长波特征的空间变化，（这是由发电机作用产生的，涉及核心中相对快速的混沌磁流体动力学），而且在所有相关时间尺度上的时间变化，从地磁长期变化的几十年和几个世纪的特征到极性反转频率变化的几千万年的特征。核心运动应该影响“固体”地球的旋转（地幔、地壳和冰冻圈），在没有任何数量上合理的替代攻击线的情况下，地球物理学家一直认为，在大约5 × 10-3秒的一天长度中出现的不规则的“十年”波动，一定是地核和地幔之间的角动量交换的表现，这种角动量交换是由地核处随时间变化的扭矩产生的。地幔界面造成这些力矩的应力包括：（a）切向应力，它是由界面正下方薄的Ekman-Hartmann边界层中的粘性力产生的，也是由与弱导电下地幔中的电流与保持在那里的磁场相互作用有关的洛伦兹力产生的;以及（B）主要由作用在不规则界面形貌（即形状偏离轴对称）上的动态压力产生的正应力。作者在20世纪60年代提出了地形应力可能对扭矩起主要作用的假设，本文详细介绍了他最近提出的利用地球自转和其他地球物理数据对这一假设进行新检验的方法。该方法提供了一种方案，用于研究假设与（a）基于地磁长期变化数据的核外部运动和（B）基于重力和地震数据的核-幔界面地形的“模型”的各种组合的一致性，从而阐明了关于地球深部内部动力学和结构的基本假设的有效性，是基于。该方案目前正应用于与R. W.克莱顿，B。H. Hager，M. A. Spieth和C. V. Voorhies.
As arguments in favour of the notion that very slow convection in the highly viscous mantle is confined to the upper 700 km gradually weakened over the past 20 years, so geophysicists have increased their willingness to entertain the idea that significant horizontal variations in temperature and other structural parameters occur at all levels in the lower mantle. Concomitant density variations, including those caused by distortions in the shape of the core-mantle interface, would contribute substantially to long-wavelength features of the Earth’s gravity held and also affect seismic travel times. The implied departures from axial symmetry in the thermal and mechanical boundary conditions thus imposed by deep mantle convection on the underlying low-viscosity liquid metallic core would affect not only spatial variations in the long-wavelength features of the main geomagnetic held (which is generated by dynamo action involving comparatively rapid chaotic magnetohydrodynamic how in the core) but also temporal variations on all relevant timescales, from decades and centuries characteristic of the geomagnetic secular variation to tens of millions of years characteristic of changes in the frequency of polarity reversals. Core motions should influence the rotation of the ‘solid’ Earth (mantle, crust and cryosphere), and in the absence of any quantitatively reasonable alternative line of attack, geophysicists have long supposed that irregular ‘decade’ fluctuations in the length of the day of about 5 x 10-3 s must be manifestations of angular momentum exchange between the core and mantle produced by time-varying torques at the core-mantle interface. The stresses responsible for these torques comprise (a) tangential stresses produced by viscous forces in the thin Ekman-Hartmann boundary layer just below the interface and also by Lorentz forces associated with the interaction of electric currents in the weakly conducting lower mantle with the magnetic held there, and (b) normal stresses produced largely by dynamical pressure forces acting on irregular interface topography (i.e. departures in shape from axial symmetry). The hypothesis that topographic stresses might provide the main contribution to the torque was introduced by the author in the 1960s and the present paper gives details of his recently proposed method for using Earth rotation and other geophysical data in a new test of the hypothesis. The method provides a scheme for investigating the consistency of the hypothesis with various combinations of ‘models’ of (a) motions in the outer reaches of the core based on geomagnetic secular variation data, and (b) core-mantle interface topography based on gravity and seismic data, thereby elucidating the validity of underlying assumptions about the dynamics and structure of the Earth’s deep interior upon which the various ‘models’ are based. The scheme is now being applied in a complementary study carried out in collaboration with R. W. Clayton, B. H. Hager, M. A. Spieth and C. V. Voorhies.