Gravitational torque on the inner core and decadal polar motion
Gravitational torque on the inner core and decadal polar motion
复制标题
内核的重力扭矩和十年极移
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
M. Dumberry
中科院分区：
文献类型：
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作者：
M. Dumberry
总结
在过去的世纪中，观测到一个振幅约为25毫角秒（mas）的十年极移，这种运动被称为马科维茨摆动。这一运动的起源仍然不明。在本文中，我们调查的可能性，随时间变化的轴向失调的密度结构的内核和地幔可以解释这个信号。内核密度结构的纵向位移导致了地球整体转动惯量的变化。此外，由于密度失调，赤道引力力矩导致内核的扁圆几何形状倾斜，导致全球惯性矩的进一步变化。为了保持角动量，旋转矢量必须发生调整，导致极移。我们发展的理论表达式的惯性矩和重力扭矩的变化方面的角度纵向错位和密度结构的地幔。还提出了一个模型来计算响应于随时间变化的轴向内核旋转的极运动。我们表明，由这种机制产生的极移可以极化的纵轴，预计有十年周期，两个一般特征的马科维茨摆动。极移的振幅主要取决于地幔密度的Y12球谐分量、内核与地幔之间的纵向错位以及内核的体积粘度。我们建立约束的前两个这些量的轴向分量的考虑，这重力扭矩和观察到的变化，在一天的长度。这些限制表明，从这个机制的最大极运动小于1质量，太小，无法解释马科维茨摆动。
SUMMARY
A decadal polar motion with an amplitude of approximately 25 milliarcsecs (mas) is observed over the last century, a motion known as the Markowitz wobble. The origin of this motion remains unknown. In this paper, we investigate the possibility that a time-dependent axial misalignment between the density structures of the inner core and mantle can explain this signal. The longitudinal displacement of the inner core density structure leads to a change in the global moment of inertia of the Earth. In addition, as a result of the density misalignment, a gravitational equatorial torque leads to a tilt of the oblate geometric figure of the inner core, causing a further change in the global moment of inertia. To conserve angular momentum, an adjustment of the rotation vector must occur, leading to a polar motion. We develop theoretical expressions for the change in the moment of inertia and the gravitational torque in terms of the angle of longitudinal misalignment and the density structure of the mantle. A model to compute the polar motion in response to time-dependent axial inner core rotations is also presented. We show that the polar motion produced by this mechanism can be polarized about a longitudinal axis and is expected to have decadal periodicities, two general characteristics of the Markowitz wobble. The amplitude of the polar motion depends primarily on the Y12 spherical harmonic component of mantle density, on the longitudinal misalignment between the inner core and mantle, and on the bulk viscosity of the inner core. We establish constraints on the first two of these quantities from considerations of the axial component of this gravitational torque and from observed changes in length of day. These constraints suggest that the maximum polar motion from this mechanism is smaller than 1 mas, and too small to explain the Markowitz wobble.