Abstract It has been proposed that magnetic flux expulsion due to outer core fluid upwellings may affect the geomagnetic secular variation on the core-mantle boundary ( Bloxham, 1986 ). In this process intense horizontal field lines are concentrated below the outer boundary, introducing small radial length scales and consequently strong radial diffusion. Here we explore such magnetic boundary layers in numerical dynamo simulations with heterogeneous outer boundary heat flux inferred from a tomographic model of lower mantle seismic shear waves velocity anomalies. Our scheme associates magnetic boundary layers to peak horizontal magnetic fields at the top of the shell. In our models mean magnetic boundary layer thickness ranges ≈200 − 400 km and decreases with increasing magnetic Reynolds number. Extrapolation or interpolation to Earth's core conditions based on total core flow amplitude or its poloidal part gives mean magnetic boundary layer thickness of ≈220 and ≈260 − 330 km, respectively. We find magnetic boundary layers associated with the azimuthal field at the equatorial region, whereas magnetic boundary layers associated with the meridional field are found at mid latitudes. Negative outer boundary heat flux anomalies yield preferred locations of expulsion of azimuthal field below Africa and the Pacific, while positive outer boundary heat flux anomalies yield preferred locations of expulsion of meridional field below the Americas and East Asia. Furthermore, we find a tendency of the azimuthal field to low latitudes of the Northern Hemisphere. Our results suggest that the local diffusion time is on the order of several kyr and the local magnetic Reynolds number is on the order of ≈10, both much smaller than classical estimates.

中文翻译：

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具有异质外边界热通量的数值发电机中的磁边界层

摘要 外核流体上涌引起的磁通量驱逐可能会影响核-幔边界上的地磁长期变化（Bloxham，1986）。在这个过程中，强烈的水平场线集中在外边界以下，引入了小的径向长度尺度，从而产生了强烈的径向扩散。在这里，我们利用从下地幔地震横波速度异常的层析模型推断出的异质外边界热通量，在数值发电机模拟中探索了这种磁性边界层。我们的方案将磁边界层与壳顶部的峰值水平磁场相关联。在我们的模型中，平均磁边界层厚度范围约为 200 - 400 公里，并且随着磁雷诺数的增加而减小。外推或内插到地球' 基于总核心流动幅度或其极向部分的 s 核心条件分别给出了 ≈220 和 ≈260 - 330 公里的平均磁边界层厚度。我们在赤道地区发现了与方位场相关的磁边界层，而在中纬度地区发现了与经向场相关的磁边界层。负外边界热通量异常产生了非洲和太平洋以下方位角场的优选排出位置，而正外边界热通量异常产生了美洲和东亚以下经向场排出的优选位置。此外，我们发现方位角场趋向于北半球的低纬度地区。