Accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation
Accounting for magnetic diffusion in core flow inversions from geomagnetic secular variation
复制标题
考虑地磁长期变化的核心流反演中的磁扩散
DOI：
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
U. Christensen
中科院分区：
文献类型：
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作者：
H. Amit;U. Christensen
总结
我们使用数值发电机研究磁扩散在核心顶部的可能作用。我们发现，在很宽的控制参数范围内，径向磁扩散对长期变化的贡献与切向磁扩散的贡献相关。这两个扩散项之间的相关性被解释在极向流沿着一个柱状流管的强度的变化。两个扩散项的振幅比被用来估计径向磁扩散在类地条件下对长期变化的可能贡献。然后，我们应用一个模型，径向磁扩散是成比例的切向扩散的核心流反演的地磁长期变化数据。我们发现，包括磁扩散并没有显着改变的全球流量，但出现了一些显着的局部变化。在非冻结通量核心流模式（称为“扩散”），半球形的二分法之间的活跃的大西洋和平静的太平洋是较弱的，一个气旋性的涡旋以下的北美出现和亚洲以下的涡旋较强。我们的研究结果有几个重要的地球物理意义。首先，我们的扩散流模型包含一些流动活动在低纬度地区的太平洋，这表明在该地区的磁场平流和扩散之间的局部平衡。其次，在我们的扩散流中，北美下方的气旋调和了地幔驱动的热成风预测和冻结通量核心流模型之间的差异，并且与地磁场模型中北美下方突出的强磁通量补丁一致。最后，我们假设，磁扩散附近的核心表面起着更大的作用，在地磁长期变化比通常假设的。
SUMMARY
We use numerical dynamos to investigate the possible role of magnetic diffusion at the top of the core. We find that the contribution of radial magnetic diffusion to the secular variation is correlated with that of tangential magnetic diffusion for a wide range of control parameters. The correlation between the two diffusive terms is interpreted in terms of the variation in the strength of poloidal flow along a columnar flow tube. The amplitude ratio of the two diffusive terms is used to estimate the probable contribution of radial magnetic diffusion to the secular variation at Earth-like conditions. We then apply a model where radial magnetic diffusion is proportional to tangential diffusion to core flow inversions of geomagnetic secular variation data. We find that including magnetic diffusion does not change dramatically the global flow but some significant local variations appear. In the non frozen-flux core flow models (termed ‘diffusive’), the hemispherical dichotomy between the active Atlantic and quiet Pacific is weaker, a cyclonic vortex below North America emerges and the vortex below Asia is stronger. Our results have several important geophysical implications. First, our diffusive flow models contain some flow activity at low latitudes in the Pacific, suggesting a local balance between magnetic field advection and diffusion in that region. Second, the cyclone below North America in our diffusive flows reconciles the difference between mantle-driven thermal wind predictions and frozen-flux core flow models, and is consistent with the prominent intense magnetic flux patch below North America in geomagnetic field models. Finally, we hypothesize that magnetic diffusion near the core surface plays a larger role in the geomagnetic secular variation than usually assumed.