Turbulent geodynamo simulations: a leap towards Earth's core
Turbulent geodynamo simulations: a leap towards Earth's core
复制标题
DOI：
10.1093/gji/ggx265
复制
发表时间：
2017-10-01
影响因子：
2.8
通讯作者：
Fournier, A.
中科院分区：
文献类型：
--
作者：
Schaeffer, N.;Jault, D.;Fournier, A.
我们提出了在地球动力学的直接数值模拟中达到真实湍流状态的尝试。我们依靠在一个快速旋转的球壳中连续三次对流驱动的模拟。最极端的情况是降低黏度（磁普朗特数P-m = 0.1），同时保持强劲的对流（磁雷诺数Rm bbb500）和快速旋转（埃克曼数E= 10(-7)），这在今天的超级计算机上是可行的。详细而全面的分析强调了与地磁观测或发电机理论预测相匹配的几个关键特征——所有这些都出现在同一个模拟中——但它也揭示了与地核动力学相关的有趣见解。在这个强磁场、偶极子主导的发电机模拟中，磁能比动能大一个数量级。磁场强度的空间分布是高度不均匀的，切线圆柱体（与环绕内核的旋转轴平行的圆柱体）的内部和外部之间存在着明显的动态对比。在内部，磁场是最强的，并与一个强烈的扭曲的极涡有关，其动力学可能偶尔导致在壳表面形成一个反向的极通量补丁。此外，强磁场还允许轻物质在切线圆柱体内积累，导致那里的稳定分层。扭转阿尔芬波经常在切线柱附近被触发并向赤道传播。切线柱体外磁场抑制纬向风生长，动能多为非纬向风。时空分析表明，低频、非纬向流是相当地转的（柱状），并且主要是大规模的：在我们最极端的模拟中，一个m = 1的涡流自发出现，没有任何非均质边界强迫。我们的时空分析进一步揭示了：(1)低频、大规模的流动是由科里奥利力和浮力之间的平衡所控制的——磁场和流动趋于对齐，使洛伦兹力最小化；（ii）高频流遵循磁力和科里奥利力之间的平衡；（iii）对流羽流主要生活在一个中等尺度，其动力学是由一个涉及科里奥利力、洛伦兹力和浮力的三期MAC平衡驱动的。然而，在低磁场强度区域仍然存在小尺度（类似或等于E-1/3）准地转对流。
We present an attempt to reach realistic turbulent regime in direct numerical simulations of the geodynamo. We rely on a sequence of three convection-driven simulations in a rapidly rotating spherical shell. The most extreme case reaches towards the Earth's core regime by lowering viscosity (magnetic Prandtl number P-m = 0.1) while maintaining vigorous convection (magnetic Reynolds number Rm > 500) and rapid rotation (Ekman number E= 10(-7)) at the limit of what is feasible on today's supercomputers. A detailed and comprehensive analysis highlights several key features matching geomagnetic observations or dynamo theory predictions-all present together in the same simulation-but it also unveils interesting insights relevant for Earth's core dynamics. In this strong-field, dipole-dominated dynamo simulation, themagnetic energy is one order of magnitude larger than the kinetic energy. The spatial distribution of magnetic intensity is highly heterogeneous, and a stark dynamical contrast exists between the interior and the exterior of the tangent cylinder (the cylinder parallel to the axis of rotation that circumscribes the inner core). In the interior, the magnetic field is strongest, and is associated with a vigorous twisted polar vortex, whose dynamics may occasionally lead to the formation of a reverse polar flux patch at the surface of the shell. Furthermore, the strong magnetic field also allows accumulation of light material within the tangent cylinder, leading to stable stratification there. Torsional Alfven waves are frequently triggered in the vicinity of the tangent cylinder and propagate towards the equator. Outside the tangent cylinder, the magnetic field inhibits the growth of zonal winds and the kinetic energy is mostly non-zonal. Spatio-temporal analysis indicates that the low-frequency, non-zonal flow is quite geostrophic (columnar) and predominantly large-scale: an m = 1 eddy spontaneously emerges in our most extreme simulations, without any heterogeneous boundary forcing. Our spatio-temporal analysis further reveals that (i) the low-frequency, large-scale flow is governed by a balance between Coriolis and buoyancy forces-magnetic field and flow tend to align, minimizing the Lorentz force; (ii) the high-frequency flow obeys a balance between magnetic and Coriolis forces; (iii) the convective plumes mostly live at an intermediate scale, whose dynamics is driven by a threeterm MAC balance-involving Coriolis, Lorentz and buoyancy forces. However, small-scale (similar or equal to E-1/3) quasi-geostrophic convection is still observed in the regions of low magnetic intensity.