Double diffusive convection in stable layers within Earth’s core

地磁场是由地球流体外核中的热成分对流产生的。然而，并不是整个地核都是不稳定的。目前地核的最外层通常被认为是稳定分层的，而且有强有力的论据表明，在地球历史上，特别是在固体内核形成之前的时代，稳定分层的存在。虽然稳定分层有抑制垂直运动的趋势，但稳定层仍可能支持双扩散型流体动力学不稳定性。这些不稳定性发生在小的空间尺度上（几厘米到几米），可以显著增加这些核心区域的垂直混合强度。在有利的条件下，它们甚至可以将稳定区域的内部结构转变为一堆强有力的对流层，由薄的高梯度界面分开。本项目采用直接数值模拟的方法，研究了行星核心条件下双扩散对流的特性。特别地，将考虑到地球自转和地磁场所施加的科里奥利力和洛伦兹力的联合效应。研究明确地集中在小的，局部区域的核心，这样的小尺度双扩散运动可以解决在现实的方式。模拟结果将有助于约束通过地核稳定层的径向输运，并且还将约束在何种条件下小规模双扩散运动将地核内的稳定区域转变为一堆对流层。这项研究有助于更好地理解行星核心的小尺度双扩散对流，并将有助于最终将其参数化到大尺度地球动力学模型中。

The geomagnetic field is generated by thermo-compositional convection in the Earth’s fluid outer core. However, not the entire core is expected to be convectively unstable. The outermost part of the current core is generally believed to be stably stratified, and there are strong arguments for the presence of stable stratification in the Earth's history, especially during the epoch before the formation of the solid inner core. While stable stratification has a tendency to damp vertical motions, stable layers may still support fluid dynamical instabilities of double-diffusive type. These instabilities, which occur on small spatial scales (on the order of several centimetres to meters), can significantly increase the strength of vertical mixing across these core regions. Under favourable conditions, they can even transform the internal structure of the stable region into a stack of vigorously convecting layers, separated by thin, high gradient interfaces. In this project, the properties of double-diffusive convection under conditions characteristic for planetary cores are studied using direct numerical simulations. In particular, the combined effects of Coriolis and Lorentz forces, exerted by the Earth's rotation and by the geomagnetic field, will be taken into consideration. The study explicitly focusses on small, local regions in the core such that the small-scale double diffusive motions can be resolved in a realistic fashion. The simulation results will help to constrain the radial transport through stable layers in the Earth's core, and they will also constrain under which conditions small-scale double diffusive motions would transform a stable region inside Earth's core into a stack of convecting layers. This research contributes to a better understanding of small-scale double diffusive convection in planetary cores and will help to eventually parameterize it in large-scale models of the geodynamo.