Sheyko Andrey, Finlay Christopher, Favre Jean, Jackson Andrew
Earth and Planetary Magnetism Group, Institute of Geophysics, ETH Zurich, Zürich, Switzerland.
Division of Geomagnetism, DTU Space, Technical University of Denmark, Lyngby, Denmark.
Sci Rep. 2018 Aug 22;8(1):12566. doi: 10.1038/s41598-018-30864-1.
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.
地球磁场产生的机制被认为是金属液态铁核中的热对流。在此，我们展示了一组具有超低粘度的自洽球壳计算结果，这些计算结果复制了这一机制，但使用的动量扩散率和磁场扩散率彼此显著不同。这导致了磁场和速度场之间的显著尺度分离，后者由小尺度主导。我们展示了在大尺度上科里奥利力和洛伦兹力的方位平均部分之间的零阶平衡，这种平衡在磁场扩散率和动量扩散率差异如此之大时出现，就像我们的模型那样。在边界层之外，粘性力的大小约为洛伦兹力的千分之一。在这种发电机模型中，耗散几乎完全是欧姆耗散，就像在地球中一样，在所谓的切圆柱内部的对流起着关键作用；它也处于“强场”状态，磁能比动能显著更多（就像在地球中一样）。我们最终展示了磁耗散和磁能之间一个稳健的经验标度律。
