Three-Dimensional Rotational Dynamics and Coupling of the Core-Mantle System

对地球深部的性质和动力学的调查必然依赖于间接观测。地震学研究提供了有关地球深部及其边界的物理性质的信息，在某些情况下，重复观测可以揭示地球深部的动力学；例如，观测到的固体内核的“超级自转”。对地磁长期变化的研究提供了对液核表面流体流动性质的洞察，并提供了关于地球深处物理性质的间接信息。这颗行星的自转动力学提供了有关地球内部深处的额外信息来源，发展对行星自转动力学的更全面了解是该项目的目标。地球自转的变化既涉及自转速度的变化(观测为一天的长度变化，与流体核心中的所谓扭转振荡流动相关)，也涉及自转轴相对于天体参考系的方向的变化(与太阳、月球和行星的引力相互作用产生的周期性波动称为章动)。日长变化和章动都涉及地幔、外核和内核之间的角动量交换。这些区域之间耦合的强度取决于地球的物理性质，如核心内磁场的强度和几何形状，核心和下地幔的导电性，以及外层和内部核心的粘性。尽管日长变化和章动都涉及类似的动态效应，并为地球深处的性质提供了补充证据，但以前的工作往往是分别分析这些现象。该项目的第一个目标是完善和协调章动模型和日长变化模型中核-地幔耦合的理论描述。在这样做的过程中，我们将利用已经发生的理论进步来改进其中一种类型的模型，但这些进展尚未应用于另一种类型的模型。例如，章动模型中的粘性耦合理论比日长模型中的粘性耦合理论更加完善。另一方面，最近的工作改进了对日长模型中磁场几何形状的描述，并认识到了一种通常被忽视的效应的重要性，这种效应增加了核-地幔边界的欧姆耗散。利用更新后的模型，我们将重新分析现有的自转数据集，以改进对地球深处物理性质的估计。在这个项目的最后阶段，我们将开发一个能够自洽地描述章动和扭转振荡的单一模型。这将允许对独立的数据集进行联合反演，从而进一步改善对地球深处物理性质的限制。联合模型将被用来研究系统内的交叉耦合动力学，包括扭转振荡激发被称为马科维茨摆动的旋转轴方向的观察到的十年周期变化的可能性，以及钱德勒摆动(周期为433天)中相位跳跃的时间与所谓的地磁抖动(也与扭转振荡有关)之间的拟议关联。我们获得的有关地球核-地幔系统的物理性质和短时间尺度动力学的新信息将有助于检验地球发电机过程的数值模型，该过程负责地球磁场的产生。

Investigation of the properties and dynamics of the deep interior of the Earth necessarily relies on indirect observations. Seismological studies provide information on the physical properties of the deep Earth and its boundaries and in some cases repeat observations can reveal the dynamics of the deep Earth; for example, the observed 'super-rotation' of the solid inner core. Study of geomagnetic secular variation provides insight on the nature of fluid flow at the surface of the fluid core, and indirect information on the physical properties of the deep Earth. The planet's rotational dynamics provide an additional source of information on the Earth's deep interior and developing a more complete understanding of planetary rotational dynamics is the goal of this project. Variation in the Earth's rotation involves changes in both the rate of rotation (observed as a change in the length of day and correlated with so-called torsional oscillation flow in the fluid core) and the orientation of the rotation axis with respect to the celestial reference frame (periodic fluctuations arising from gravitational interaction with the sun, moon and planets are referred to as nutations). Both length-of-day variations and nutations involve angular momentum exchange between the mantle, outer core and inner core. The strength of the coupling between these regions depends on physical properties of the Earth such as the strength and geometry of the magnetic field within the core, the electrical conductivity of the core and lower mantle, and the viscosities of the outer and inner cores. Although both length-of-day variations and nutations involve similar dynamic effects and provide complementary evidence on the nature of the deep Earth, previous work has tended to analyse these phenomena separately. The first goal of this project is to refine and harmonise the theoretical descriptions of core-mantle coupling in models of nutation and of length-of-day variation. In so doing, we will take advantage of theoretical advances that have occurred to improve one of the types of model, but that have not yet been applied to the other. For example, the theory of viscous coupling in nutation models is more fully developed than that in models of length-of-day. On the other hand, recent work has led to improved descriptions of the geometry of the magnetic field in length-of-day models, and an appreciation for the importance of a commonly neglected effect that adds to ohmic dissipation at the core-mantle boundary. Using the updated models we will reanalyse the existing rotation data sets to obtain improved estimates of the physical properties of the deep Earth. In the final stage of this project we will develop a single model that can self-consistently describe both nutations and torsional oscillations. This would allow for joint inversion of the independent data sets, providing further improvements in the constraints on the physical properties of the deep Earth. The joint model will be used to investigate the dynamics of cross-coupling within the system, including the possibility that torsional oscillations excite an observed decadal-period variation in the orientation of the rotation axis known as the Markowitz Wobble, and a proposed correlation between the timing of phase jumps in the Chandler Wobble (which has a period of 433 days) and so-called geomagnetic jerks (which have also been linked to torsional oscillations). The new information that we gain concerning the physical properties and short time scale dynamics of the Earth's core-mantle system will be useful for testing numerical models of the geodynamo process that is responsible for generation of the Earth's magnetic field.