Evidence for MAC waves at the top of Earth's core and implications for variations in length of day
Evidence for MAC waves at the top of Earth's core and implications for variations in length of day
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DOI：
10.1093/gji/ggv552
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发表时间：
2016-03-01
影响因子：
2.8
通讯作者：
Holme, Richard
中科院分区：
文献类型：
--
作者：
Buffett, Bruce;Knezek, Nicholas;Holme, Richard
地球的液体核心承载着一系列不同的波，周期从几天到几千年不等。已知有一类周期为几十年的波是由磁力、阿基米德力和科里奥利力之间的相互作用引起的。这些所谓的MAC波被认为与解释地磁场的历史波动有关。在这项研究中，我们表明，MAC波提供了一个很好的描述随时间变化的纬向流在顶部的核心。同样的波的集合也提供了一个简单的解释在偶极场中观察到的波动。这两种预测都需要在地核顶部有一个厚度为130-140公里的分层层，浮力频率与地球自转速率相当。我们扩展这些预测，包括在一天的长度（LOD）的变化，并发现MAC波可以占约一半的观测到的波动在十年周期。当电磁应力耦合MAC波以在芯的内部流动时，更大的波动是可能的。事实上，耦合运动的理想化模型高估了LOD波动，可能反映了这种理想化模型的局限性。我们的研究结果提供了支持稳定分层的核心顶部，并建议在地磁场和LOD的十年波动的共同起源。
Earth's liquid core hosts a diverse set of waves with periods ranging from days to thousands of years. One class of waves with periods of several decades is known to arise from an interplay between magnetic, Archimedes and Coriolis forces. These so-called MAC waves are thought to be relevant for interpreting historical fluctuations in the geomagnetic field. In this study, we show that MAC waves provide a good description of time-dependent zonal flow at the top of the core. The same collection of waves also offers a simple explanation for observed fluctuations in the dipole field. Both of these predictions require a stratified layer at the top of the core with a thickness of 130-140 km and a buoyancy frequency comparable to Earth's rotation rate. We extend these predictions to include changes in the length of day (LOD) and find that MAC waves can account for about half of the observed fluctuation at decadal periods. Larger fluctuations are possible when electromagnetic stresses couple MAC waves to flow in the interior of the core. In fact, an idealized model for the coupled motion overestimates the LOD fluctuations, probably reflecting limitations in this idealized model. Our results offer support for stable stratification at the top of the core and suggest a common origin for decadal fluctuations in the geomagnetic field and the LOD.