Scale separated low viscosity dynamos and dissipation within the Earth's core.
Scale separated low viscosity dynamos and dissipation within the Earth's core.
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DOI：
10.1038/s41598-018-30864-1
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发表时间：
2018-08-22
影响因子：
4.6
通讯作者：
Jackson A
中科院分区：
文献类型：
--
作者：
Sheyko A;Finlay C;Favre J;Jackson A
地球磁场的产生机制被认为是金属液态铁芯中的热对流。在这里，我们给出了一套自洽的超低粘度球壳计算的结果，这些计算重复了这一机制，但使用了彼此显著不同的动量和磁场扩散系数。这导致磁场和速度场之间存在显著的尺度分离，后者以小尺度为主。我们证明了在大尺度上，当磁场和动量的扩散系数相差很大时，科里奥利力和洛伦兹力的方位平均部分之间存在零级平衡，就像我们的模型一样。在边界层外，粘性力的大小约为洛伦兹力的千分之一。在这种发电机中的耗散几乎完全是欧姆的，就像在地球上一样，所谓的正切圆柱体内的对流起着至关重要的作用；它也处于“强场”状态，具有比动能大得多的磁能(如在地球中)。最后，我们证明了磁耗散和磁能之间的一个稳健的经验标度律。
The mechanism by which the Earth’s magnetic field is generated is thought to be thermal convection in the metallic liquid iron core. Here we present results of a suite of self-consistent spherical shell computations with ultra-low viscosities that replicate this mechanism, but using diffusivities of momentum and magnetic field that are notably dissimilar from one another. This leads to significant scale separation between magnetic and velocity fields, the latter being dominated by small scales. We show a zeroth order balance between the azimuthally-averaged parts of the Coriolis and Lorentz forces at large scales, which occurs when the diffusivities of magnetic field and momentum differ so much, as in our model. Outside boundary layers, viscous forces have a magnitude that is about one thousandth of the Lorentz force. In this dynamo dissipation is almost exclusively Ohmic, as in the Earth, with convection inside the so-called tangent cylinder playing a crucial role; it is also in the “strong field” regime, with significantly more magnetic energy than kinetic energy (as in the Earth). We finally show a robust empirical scaling law between magnetic dissipation and magnetic energy.
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