Core flow inversion tested with numerical dynamo models
Core flow inversion tested with numerical dynamo models
复制标题
使用数值发电机模型测试核心流反演
DOI：
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发表时间：
2000
期刊：
影响因子：
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通讯作者：
Johannes Wicht
中科院分区：
文献类型：
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作者：
Steffen Rau;U. Christensen;A. Jackson;Johannes Wicht
摘要
我们用合成数据检验了地磁长期变化数据对岩心表面附近流体流动模式的反演方法。这些来自旋转球壳中对流驱动的磁流体发电机的自洽三维模型，它产生具有类似地球的形状的偶极主导的磁场。我们发现，作为所有反演方案的基础的冻结通量近似在模式中得到了相当程度的满足。为了减少反演的非唯一性，通常对气流施加先验条件；例如，要求它是纯环向或地转的。我们的模型外表面附近的流动几乎满足这两个条件。然而，地表速度场大部分位于反演问题的零空间内。尽管如此，先验约束减少了零空间，通过将磁数据与它们中的任何一个倒置，我们恢复了流动的很大一部分。在地转条件下，根据阻尼参数的选择，反演速度场与真实速度场之间的相关系数可达0.65。对于L=26以下的大多数球谐函数，这种相关性在95%的水平上是显著的。然而，当我们将磁场数据集截断到L≤14，即核场模型的分辨率时，即使在长波长，它也会显著退化。在后一些倒转中，赤道地区出现了明显的纬向流，类似于根据地磁数据得出的岩心流动模型中所看到的那样。然而，真正的流不包含该流组件。结果表明，可以从长期变化数据中提取一些关于岩心流动模式的有意义的信息，但也表明磁芯场的有限分辨率可能会产生严重的伪影。
Summary
We test inversion methods of geomagnetic secular variation data for the pattern of fluid flow near the surface of the core with synthetic data. These are taken from self-consistent 3-D models of convection-driven magnetohydrodynamic dynamos in rotating spherical shells, which generate dipole-dominated magnetic fields with an Earth-like morphology. We find that the frozen-flux approximation, which is fundamental to all inversion schemes, is satisfied to a fair degree in the models. In order to alleviate the non-uniqueness of the inversion, usually a priori conditions are imposed on the flow; for example, it is required to be purely toroidal or geostrophic. Either condition is nearly satisfied by our model flows near the outer surface. However, most of the surface velocity field lies in the nullspace of the inversion problem. Nonetheless, the a priori constraints reduce the nullspace, and by inverting the magnetic data with either one of them we recover a significant part of the flow. With the geostrophic condition the correlation coefficient between the inverted and the true velocity field can reach values of up to 0.65, depending on the choice of the damping parameter. The correlation is significant at the 95 per cent level for most spherical harmonic degrees up to l=26. However, it degrades substantially, even at long wavelengths, when we truncate the magnetic data sets to l≤ 14, that is, to the resolution of core-field models. In some of the latter inversions prominent zonal currents, similar to those seen in core-flow models derived from geomagnetic data, occur in the equatorial region. However, the true flow does not contain this flow component. The results suggest that some meaningful information on the core-flow pattern can be retrieved from secular variation data, but also that the limited resolution of the magnetic core field could produce serious artefacts.