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.

获得对地球磁场产生的物理机制的理解是当今突出的科学挑战之一。古地磁记录提供了地球磁场的漫长历史，揭示了磁场具有一定极性的漫长时期，其间穿插着磁场反转的相对较短的时期。要理解地球磁场为何表现出这种特征性行为，只有充分了解维持磁场不受衰减趋势影响的过程-地球发电机机制。地球内部，在地壳之下，有三个不同的区域：一个固体，主要是铁，内核;一个液态金属外核;和一个导电地幔，其中运动只能发生在非常长的时间尺度。因此，发电机位于外核中，并且是由该区域中的运动通过感应维持磁场而产生的。最广为接受的外核运动理论认为，它们是热对流和成分对流共同作用的结果。虽然控制地核动力学的方程是已知的，但由于所涉及的无量纲参数的极端值，这些方程不容易求解。然而，与今天的非常强大的，并行处理器的计算机，它是可能的去某种方式对真正的参数制度，至关重要的是，然后获得新的见解所涉及的物理，并随后导致新的物理解释。因此，我们建议调查，通过大规模并行计算机上的数值模拟，发电机动作驱动的旋转热对流。以前对这个问题的研究表明，在某些参数范围内，磁场是小尺度的，因此不会让人联想到地球的偶极场，而如果旋转速度足够快，那么对流就会组织成相干柱，这些可以产生强大的大尺度磁场。已经证明，像地球这样的发电机，长期保持一个极性，但也经历间歇性的逆转，位于这些小型和大型发电机之间的边界。目前，人们对这两种发电机之间的过渡性质知之甚少。我们的第一个目标是了解这种过渡在一个平面层几何，这是计算非常有效的，将允许一个彻底的探索三维参数空间的问题。然后，利用平面层问题所提供的知识，我们将在更现实但计算要求更高的球壳几何中进行一系列有重点的计算。任何发电机计算的关键方面之一是所施加的边界条件的性质--温度、速度和磁场。在地球本身，这些是复杂的，因此理解各种条件的含义非常重要。例如，缓慢变化的热通量是否会影响发电机机制的性质，也许还会影响反转的模式？最后，随着相当大的计算能力，现在，我们希望能够执行足够长的运行，以便产生反转的统计数据，从而允许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