A physical interpretation of stochastic models for fluctuations in the Earth's dipole field
A physical interpretation of stochastic models for fluctuations in the Earth's dipole field
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地球偶极场涨落随机模型的物理解释
DOI：
10.1093/gji/ggu153
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发表时间：
2014
影响因子：
2.8
通讯作者：
H. Matsui
中科院分区：
文献类型：
--
作者：
B. Buffett;E. King;H. Matsui
作者：Buffett，BA; King，EM; Matsui，H|摘要：最近的几项研究已经使用古地磁估计的虚拟轴向偶极矩构建一个定量的随机模型的波动和逆转在地球的偶极场。我们调查的物理意义的条款，在一个标准的随机（朗之万）模型，从一个数值地球发电机模型的输出。第一项，称为漂移项，表征偶极场向时间平均状态的缓慢调整。我们发现，这种缓慢的调整的时间尺度是由磁偶极子波动的衰减时间。这些波动通常由前几个衰减模式表示。第二项通常被称为噪声项，因为它表征了核心中短周期对流波动的影响。我们建立了噪声项和磁感应均方根变化之间的联系。将这些结果应用于古地磁场表明，偶极生成的均方根变化超过了平均生成速率。这种大的波动可能是允许磁场逆转所必需的。古地磁估计的漂移项有利于高电导率的核心。电导率的下限为0.6 × 106 S m-1。类似地，我们建立了湍流磁扩散率的上限（0.8 m2 s-1），尽管现实的估计可能要少得多。由牛津大学出版社代表皇家天文学会出版。
Author(s): Buffett, BA; King, EM; Matsui, H | Abstract: Several recent studies have used palaeomagnetic estimates of the virtual axial dipole moment to construct a quantitative stochastic model for fluctuations and reversals in the Earth's dipole field. We investigate the physical significance of the terms in a standard stochastic (Langevin) model using output from a numerical geodynamo model. The first term, known as the drift term, characterizes the slow adjustment of the dipole field toward a time-averaged state. We find that the timescale for this slow adjustment is set by the magnetic decay time of dipole fluctuations. These fluctuations are typically be represented by the first few decay modes. The second term is often called the noise term because it characterizes the influence of short-period convective fluctuations in the core. We establish a connection between the noise term and the rms variation in magnetic induction. Applying these results to the palaeomagnetic field suggests that the rms variation in dipole generation exceeds the mean rate of generation. Such large fluctuations may be necessary to permit magnetic reversals. Palaeomagnetic estimates of the drift term favour a high electrical conductivity in the core. A lower bound on electrical conductivity is 0.6 × 106 S m-1. Similarly, we establish an upper bound on turbulent magnetic diffusivity (0.8 m2 s-1), although realistic estimates may be much less. © The Authors 2014. Published by Oxford University Press on behalf of The Royal Astronomical Society.
DOI：
10.1073/pnas.1111841109
发表时间：
2012-03-13
影响因子：
11.1
作者：
de Koker, Nico;Steinle-Neumann, Gerd;Vlcek, Vojtech
通讯作者：
Vlcek, Vojtech