Dynamo constraints on the long-term evolution of Earth's magnetic field strength
Dynamo constraints on the long-term evolution of Earth's magnetic field strength
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DOI：
10.1093/gji/ggab342
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发表时间：
2021-09-12
影响因子：
2.8
通讯作者：
Biggin, Andrew J.
中科院分区：
文献类型：
--
作者：
Davies, Christopher J.;Bono, Richard K.;Biggin, Andrew J.
弄清液核中在过去3.5 Gyr期间产生了观测到的古强度变化的过程，对于了解地球深部的动力学和长期演化至关重要。我们将数值地球发电机模拟与理论标度律相结合来研究地球磁场强度随地质时间的变化。我们的方法遵循Aubert等人的研究，适应于包括在数值模拟、矿物物理和古地磁学方面的最新进展。我们首先比较了一套314个发电机模拟和两个基于功率的理论标度律在发电机区域内和核幔边界(CMB)上的场强。标度律既基于前导阶准地转力平衡(QG)平衡，也基于一阶磁、阿基米德和科里奥利(MAC)平衡，不同之处在于将对流的特征长度尺度视为固定的(QG-MAC固定的)或确定为解的一部分(QG-MAC-自由)。当对数据集进行过滤以仅保留磁动能比至少大于2的模拟时，我们发现内场以及均方根场和偶极子CMB场在不同加热模式和边界条件引起的不确定范围内表现出与这两种标度兼容的幂定律行为。然而，尽管基于QG-MAC自由标度的外推强度与地球现代CMB磁场相匹配，但QG-MAC修正的预测偏高，也严重高估了过去3.5年的古强度。我们结合QG-MAC-Free标度和275次核-地幔热演化实现的结果，构造了跨越最后3.5Gyr的合成真偶极矩(TDM)曲线。最佳拟合的TDM在Bruhnes期间和内核成核(ICN)之前重现了在观测不确定因素内的二品脱数据，但Pint不包含预测的强劲增长和随后的高TDM在内核增长的早期阶段。对于现今CMB热流为11-16TW，在4GA时增加到17-22TW，得到了最佳拟合模型，并预测了ICN的最小TDM。
Elucidating the processes in the liquid core that have produced observed palaeointensity changes over the last 3.5 Gyr is crucial for understanding the dynamics and long-term evolution of Earth's deep interior. We combine numerical geodynamo simulations with theoretical scaling laws to investigate the variation of Earth's magnetic field strength over geological time. Our approach follows the study of Aubert et al., adapted to include recent advances in numerical simulations, mineral physics and palaeomagnetism. We first compare the field strength within the dynamo region and on the core-mantle boundary (CMB) between a suite of 314 dynamo simulations and two power-based theoretical scaling laws. The scaling laws are both based on a Quasi-Geostropic (QG) force balance at leading order and a Magnetic, Archimedian, and Coriolis (MAC) balance at first order and differ in treating the characteristic length scale of the convection as fixed (QG-MAC-fixed) or determined as part of the solution (QG-MAC-free). When the data set is filtered to retain only simulations with magnetic to kinetic energy ratios greater than at least two we find that the internal field together with the root-mean-square and dipole CMB fields exhibit power-law behaviour that is compatible with both scalings within uncertainties arising from different heating modes and boundary conditions. However, while the extrapolated intensity based on the QG-MAC-free scaling matches Earth's modern CMB field, the QG-MAC-fixed prediction shoots too high and also significantly overestimates palaeointensities over the last 3.5 Gyr. We combine the QG-MAC-free scaling with outputs from 275 realizations of core-mantle thermal evolution to construct synthetic true dipole moment (TDM) curves spanning the last 3.5 Gyr. Best-fitting TDMs reproduce binned PINT data during the Bruhnes and before inner core nucleation (ICN) within observational uncertainties, but PINT does not contain the predicted strong increase and subsequent high TDMs during the early stages of inner core growth. The best-fitting models are obtained for a present-day CMB heat flow of 11-16 TW, increasing to 17-22 TW at 4 Ga, and predict a minimum TDM at ICN.