Topographic core-mantle coupling and polar motion on decadal time-scales
Topographic core-mantle coupling and polar motion on decadal time-scales
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十年时间尺度上的地形核幔耦合和极移
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发表时间：
1996
期刊：
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通讯作者：
A. Jackson
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文献类型：
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作者：
R. Hide;D. Boggs;J. Dickey;D. Dong;R. Gross;A. Jackson
与地球液态金属核中的非稳态磁流体动力学（MHD）流（典型的相对速度为几分之一毫米/秒）相关的是约10 ~ 3 Nm-2的动压波动。作用在非球形核幔边界（CMB）上，这些压力波动在上覆固体地幔上产生波动的净地形力矩Li（t）（i = 1，2，3 t，其中t表示时间）。地球物理学家现在接受了我们中的一个人（RH）的建议，即L1（t）对由地核运动直接或间接产生的地幔总扭矩LT（t）做出了重要的、可能占主导地位的贡献。其他的贡献是“重力”扭矩与波动的密度梯度在核心，“电磁”扭矩与洛伦兹力在弱导电下地幔，和“粘性”扭矩与剪切运动在边界层下面的CMB。轴向分量L; LT（t）的Lf（t）对观测到的日长波动有贡献，CLOD是固体地球（地幔、地壳和冰冻圈）旋转角速度的逆量度]，而赤道分量（Lf（t），Li（t））= L*（t）对观测到的极移有贡献，如通过测量地球自转轴相对于其形状轴的变化所确定的。在一项基于从地球物理数据确定Li（t）的方法（大约10年前由Hide和Le Mouel独立提出）的持续研究方案的早期阶段，研究表明，不高于约0.5 km的不规则CMB地形依赖于重力，可以产生足以解释观测到的年代际时间尺度上LOD波动幅度的L3（t）值。在这里，我们报告了Li（t）的赤道分量（L1（t），L2（t））= L（t）的研究，仅考虑了CMB的一个地形特征--尽管可能是最明显的--即轴对称的赤道隆起，其赤道半径超过极半径9.5 kO.1 km（地核的平均半径为3485.2km，是整个地球的0.547倍）。通过假设在核心的外部区域--“极球层”（Hide 1995 t）的几乎所有地方，压力梯度和科里奥利力之间都达到地转平衡，可以从CMB下方“自由流”中的局部欧拉流速获得CMB附近脉动压力场的局部水平梯度。极地
SUMMARY Associated with non-steady magnetohydrodynamic (MHD) flow in the liquid metallic core of the Earth, with typical relative speeds of a fraction of a millimetre per second, are fluctuations in dynamic pressure of about lo3 N m-2. Acting on the non-spherical core-mantle boundary (CMB), these pressure fluctuations give rise to a fluctuating net topographic torque Li(t) (i = 1,2,3twhere t denotes time--on the overlying solid mantle. Geophysicists now accept the proposal by one of us (RH) that L,(t) makes a significant and possibly dominant contribution to the total torque LT( t) on the mantle produced directly or indirectly by core motions. Other contributions are the ‘gravitational’ torque associated with fluctuating density gradients in the core, the ‘electromagnetic’ torque associated with Lorentz forces in the weakly electrically conducting lower mantle, and the ‘viscous’ torque associated with shearing motions in the boundary layer just below the CMB. The axial component L;(t) of LT(t) contributes to the observed fluctuations in the length of the day CLOD, an inverse measure of the angular speed of rotation of the solid Earth (mantle, crust and cryosphere)], and the equatorial components (Lf(t), L;(t)) = L*(t) contribute to the observed polar motion, as determined from measurements of changes in the Earth’s rotation axis relative to its figure axis. In earlier phases of a continuing programme of research based on a method for determining Li( t) from geophysical data (proposed independently about ten years ago by Hide and Le Mouel), it was shown that longitude-dependent irregular CMB topography no higher than about 0.5 km could give rise to values of L3(t) sufficient to account for the observed magnitude of LOD fluctuations on decadal time-scales. Here, we report an investigation of the equatorial components (L,(t), L2(t)) = L (t) of Li(t) taking into account just one topographic feature of the CMB-albeit possibly the most pronounced-namely the axisymmetric equatorial bulge, with an equatorial radius exceeding the polar radius by 9.5 kO.1 km (the mean radius of the core being 3485 2 km, 0.547 times that of the whole Earth). A measure of the local horizontal gradient of the fluctuating pressure field near the CMB can be obtained from the local Eulerian flow velocity in the ‘free stream’ below the CMB by supposing that nearly everywhere in the outer reaches of the core-the ‘polosphere’ (Hide 1995tgeostrophic balance obtains between the pressure gradient and Coriolis forces. The polospheric