Turbulent convective length scale in planetary cores
Turbulent convective length scale in planetary cores
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DOI：
10.1038/s41586-019-1301-5
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发表时间：
2019-06-20
期刊：
影响因子：
64.8
通讯作者：
Schaeffer, Nathanael
中科院分区：
文献类型：
--
作者：
Guervilly, Celine;Cardin, Philippe;Schaeffer, Nathanael
对流是行星流体核心的基本物理过程。它是热量和化学物质的主要传输机制，也是行星磁场的主要能源。对流的关键特性，如特征流速和长度尺度，在行星核心中很难量化，因为这些特性强烈依赖于行星旋转，浮力驱动和磁场，所有这些都很难使用现实条件进行建模。在没有强磁场的情况下，核心的对流预计将处于快速旋转的湍流状态（1），这在很大程度上尚未探索。在这里，我们使用的非磁性的数值模型，旨在探索这一制度的组合表明，对流长度尺度变得独立的粘度时，接近现实的参数值，是完全由流速和行星旋转。速度在较小的尺度上下降得非常快，所以这个湍流对流长度尺度是流中能量携带长度尺度的下限。使用这种方法，我们可以逼真地模拟月球等小型非磁性核心的动力学。虽然模拟较大行星核心的条件仍然遥不可及，但湍流对流长度尺度与粘度无关的事实允许对这些物体进行可靠的外推。对于地球的核心条件，我们发现，在没有磁场的湍流对流长度尺度将是约30公里，这是数量级大于10米粘性长度尺度。因此，在未来更现实的地球发电机模拟中，至少在弱磁化区域，可以放松对解决数值上无法达到的粘性尺度的需要。
Convection is a fundamental physical process in the fluid cores of planets. It is the primary transport mechanism for heat and chemical species and the primary energy source for planetary magnetic fields. Key properties of convection-such as the characteristic flow velocity and length scale-are poorly quantified in planetary cores owing to the strong dependence of these properties on planetary rotation, buoyancy driving and magnetic fields, all of which are difficult to model using realistic conditions. In the absence of strong magnetic fields, the convective flows of the core are expected to be in a regime of rapidly rotating turbulence(1), which remains largely unexplored. Here we use a combination of non-magnetic numerical models designed to explore this regime to show that the convective length scale becomes independent of the viscosity when realistic parameter values are approached and is entirely determined by the flow velocity and the planetary rotation. The velocity decreases very rapidly at smaller scales, so this turbulent convective length scale is a lower limit for the energy-carrying length scales in the flow. Using this approach, we can model realistically the dynamics of small non-magnetic cores such as the Moon. Although modelling the conditions of larger planetary cores remains out of reach, the fact that the turbulent convective length scale is independent of the viscosity allows a reliable extrapolation to these objects. For the Earth's core conditions, we find that the turbulent convective length scale in the absence of magnetic fields would be about 30 kilometres, which is orders of magnitude larger than the ten-metre viscous length scale. The need to resolve the numerically inaccessible viscous scale could therefore be relaxed in future more realistic geodynamo simulations, at least in weakly magnetized regions.