The Earths's Core: Dynamics and Reversals

地核：动力学与逆转

• 批准号: NE/J007080/1
• 负责人: David Hughes
• 金额: $ 42.09万
• 依托单位: University of Leeds
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2013
• 资助国家: 英国
• 起止时间: 2013 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FJ007080%2F1
• 关键词: Earths; Core; Dynamics; Reversals

Obtaining an understanding of the physical mechanisms responsible for the generation of the Earth's magnetic field is one of today's outstanding scientific challenges. Paleomagnetic records provide a long history of the Earth's field, revealing long epochs in which the magnetic field is of a certain polarity, interspersed with relatively short periods during which the field reverses.Understanding why the Earth's magnetic field exhibits this characteristic behaviour can only come from a full understanding of the processes that maintain the magnetic field against its tendency otherwise to decay -- the geodynamo mechanism. The interior of the Earth, beneath the crust, has three distinct regions: a solid, predominantly iron, inner core; a liquid metal outer core; and an electrically conducting mantle, in which motions can occur only over extremely long time scales. The dynamo is thus located in the outer core, and results from the motions in this region maintaining the magnetic field via induction. The most widely accepted theory for the motions of the outer core is that they result from a combination of thermal and compositional convection.Although the equations governing the dynamics of the Earth's core are known, they cannot be readily solved, owing to the extreme values of the dimensionless parameters involved. However, with today's extremely powerful, parallel processor computers, it is possible to go some way towards the true parameter regime and, crucially, then to obtain new insights into the physics involved, and, subsequently, to lead to new physical explanations.We therefore propose to investigate, via numerical simulations on massively parallel computers, dynamo action driven by rotating thermal convection. Previous studies of this problem have revealed that in certain parameter regimes the magnetic field is small-scale, and hence not reminiscent of the Earth's dipolar field, whereas if the rotation rate is sufficiently rapid then the convection is organised into coherent columns, and these can generate a strong large-scale magnetic field. It has been conjectured that dynamos such as the Earth's, that maintain one polarity for a long period but also undergo intermittent reversals, lie on the boundary between these small- and large-scale dynamos. Currently little is known about the nature of the transition between these two types of dynamo. Our first aim is to understand this transition in a plane-layer geometry, which is computationally very efficient and will allow a thorough exploration of the three-dimensional parameter space governing the problem. Then, with the knowledge afforded by the plane layer problem, we shall conduct a series of focused computations in the more realistic, but computationally more demanding, spherical shell geometry.One of the crucial aspects of any dynamo calculation concerns the nature of the imposed boundary conditions -- on the temperature, the velocity and the magnetic field. In the Earth itself these are complex, and it is therefore very important to understand the implications of the various conditions. For example, will a slowly changing heat flux affect the nature of the dynamo mechanism and maybe the pattern of reversals?Finally, with the considerable computational power now available, we hope to be able to perform sufficiently long runs so as to produce statistics of reversals, thus allowing a direct comparison with the true statistics of the Earth's magnetic field.

了解负责产生地球磁场的物理机制的理解是当今杰出的科学挑战之一。 Paleomagnetic records provide a long history of the Earth's field, revealing long epochs in which the magnetic field is of a certain polarity, interspersed with relatively short periods during which the field reverses.Understanding why the Earth's magnetic field exhibits this characteristic behaviour can only come from a full understanding of the processes that maintain the magnetic field against its tendency otherwise to decay -- the geodynamo mechanism.地壳下面的地球内部有三个不同的区域：一个固体，主要是铁，内核；液态金属外芯；和电动执行地幔，其中运动只能在非常长的时间尺度上发生。因此，发电机位于外部芯中，并由该区域的运动通过诱导维持磁场。对于外部核的动作，最广泛接受的理论是，它们是由热对流和组成对流的组合引起的。尽管众所周知，控制地球核心动力学的方程式，由于无需尺寸的参数的极端值，因此无法轻易求解它们。但是，借助当今极其强大的，平行的处理器计算机，有可能采取某种方式朝着真实的参数制度迈进，至关重要的是，以获取对所涉及物理学的新见解，并随后，随后导致新的物理解释。因此，我们建议通过大规模的平行计算机进行数值模拟，并提出通过大量平行的计算机进行调查，并驱动旋转热量驱动旋转的热量驱动。对该问题的先前研究表明，在某些参数方面，磁场是小规模的，因此不让人联想到地球的偶极场，而如果旋转速率足够快，则将对流组织成一致的柱，并且这些对流可以产生强大的大型磁场。已经猜想的是，像地球这样的发电机长期保持一种极性，但也会发生间歇性逆转，位于这些小规模和大型发电机之间的边界上。目前，这两种类型的发电机之间的过渡性质知之甚少。我们的第一个目的是在平面层的几何形状上了解这种过渡，该几何形状在计算上非常有效，并且可以彻底探索有关该问题的三维参数空间。然后，借助平面层问题提供的知识，我们将在更现实但更苛刻的球形壳几何形状上进行一系列集中计算。任何发电机计算的关键方面的一个涉及强加边界条件的性质 - 在温度，速度，速度和磁场上。在地球本身中，这些都是复杂的，因此了解各种条件的含义非常重要。例如，缓慢变化的热通量会影响发电机机制的性质，也许会影响逆转模式吗？最后，由于现在可用的相当大的计算能力，我们希望能够进行足够的长期运行，从而产生逆转的统计，从而可以直接与地球磁场的真实统计数据进行直接比较。

项目成果

Generation of magnetic fields by large-scale vortices in rotating convection.

• DOI: 10.1103/physreve.91.041001
• 发表时间: 2015-03
• 期刊: Physical review. E, Statistical, nonlinear, and soft matter physics
• 作者: C. Guervilly;D. Hughes;C. Jones

Effect of metallic walls on dynamos generated by laminar boundary-driven flow in a spherical domain.

金属壁对球形域中层流边界驱动流生成的发电机的影响。

• DOI: 10.1103/physreve.88.053010
• 发表时间: 2013
• 期刊: Physical review. E, Statistical, nonlinear, and soft matter physics
• 作者: Guervilly C

Strong-field dynamo action in rapidly rotating convection with no inertia.

无惯性快速旋转对流中的强场发电机作用。

• DOI: 10.1103/physreve.93.061101
• 发表时间: 2016
• 期刊: Physical review. E
• 作者: Hughes DW

Large-scale-vortex dynamos in planar rotating convection

平面旋转对流中的大型涡旋发电机

• DOI: 10.1017/jfm.2017.56
• 发表时间: 2017
• 期刊: Journal of Fluid Mechanics
• 影响因子: 3.7
• 作者: Guervilly C

Jets and large-scale vortices in rotating Rayleigh-Bénard convection

旋转瑞利-贝纳德对流中的射流和大尺度涡旋

• DOI: 10.1103/physrevfluids.2.113503
• 发表时间: 2017
• 期刊: Physical Review Fluids
• 影响因子: 2.7
• 作者: Guervilly C