Excitation of high-latitude MAC waves in Earth’s core
Excitation of high-latitude MAC waves in Earth’s core
复制标题
地核中高纬度 MAC 波的激发
DOI：
10.1093/gji/ggad047
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发表时间：
2023
影响因子：
2.8
通讯作者：
Buffett, Bruce
中科院分区：
文献类型：
--
作者：
Nicolas, Quentin;Buffett, Bruce
最近的地磁观测揭示了高纬度地区地磁场长期加速度的局部振荡，周期约为20年。人们已经提出了几种类型的波在旋转磁化流体解释赤道振荡具有类似的高频率。其中包括非轴对称的阿尔文波、磁科里奥利波，以及在流体分层的情况下的磁阿基米德科里奥利（MAC）波。我们探讨的假设，所观察到的高纬度模式是MAC波的签名，通过模拟他们在地球的核心。我们定量评估几个发电机模拟输出的理论框架，由于莱特希尔的生成机制。虽然从发电机模拟的源的时空结构预计是现实的，其振幅外推，以反映模拟的参数空间和地球一样的条件之间的差异。我们估计全波谱跨越每月到百年的频率为三个合理的激发源：热波动，洛伦兹力和磁感应。当专注于十年的频率，洛伦兹力似乎是最有效的产生高纬度MAC波的幅度估计下降的数量级内观察到的振荡。总的来说，这项研究提出了MAC波作为一个可行的解释，在地球的核心顶部的流体分层的存在下，在高纬度地区观察到的场变化。
Recent geomagnetic observations reveal localized oscillations in the field’s secular acceleration at high latitudes, with periods of about 20 yr. Several types of waves in rotating magnetized fluids have been proposed to explain equatorial oscillations with similar high frequencies. Among these are non-axisymmetric Alfvén waves, magneto-Coriolis waves and, in the presence of fluid stratification, magnetic-Archimedes–Coriolis (MAC) waves. We explore the hypothesis that the observed high latitude patterns are the signature of MAC waves by modelling their generation in Earth’s core. We quantitatively assess several generation mechanisms using output from dynamo simulations in a theoretical framework due to Lighthill. While the spatio-temporal structure of the sources from the dynamo simulations are expected to be realistic, their amplitudes are extrapolated to reflect differences between the simulation’s parameter space and Earth-like conditions. We estimate full wave spectra spanning monthly to centennial frequencies for three plausible excitation sources: thermal fluctuations, Lorentz force and magnetic induction. When focusing on decadal frequencies, the Lorentz force appears to be most effective in generating high-latitude MAC waves with amplitude estimates falling within an order of magnitude of observed oscillations. Overall, this study puts forward MAC waves as a viable explanation, in the presence of fluid stratification at the top of Earth’s core, for observed field variations at high latitudes.