Probing the Earth's subdecadal core-mantle dynamics based on satellite geomagnetic field models

CHAMP使命从2000年到2010年提供了大量的全球地磁数据。其密集的数据覆盖范围使我们能够建立GRIMM| GFZ参考内部磁性模型|它在空间和时间上都具有有史以来最高的核心场分辨率。我们已经模拟了地球外核的流体流动，将无扩散磁感应方程应用到最新版本的GRIMM中，发现流体流动在亚十年的时间尺度上演化，与观测到的地球自转波动有显着的相关性。这些流动模型证实了外核流体中存在六年的扭转振荡。扭振（TO）是一种磁流体波，理论上被认为是十年或亚十年核心动力学中最重要的元素。它由刚性流体环的相对方位角旋转与地幔的旋转同轴，并与地幔和内核动力耦合。在以前的作品中，TO已经研究了数值模拟，无论是与全数字发电机，或解决特征值问题，理想地代表TO系统。虽然这些研究提出了关于模拟的TO的动力学方面的见解，但它们没有直接考虑到地磁场和地球自转的观测。特别是，还没有使用卫星磁数据或模型对TO进行基于观测的研究。在拟议的项目中，我们的目标是揭示的subdecadal动力学和能量的基础上，卫星磁观测的地球核幔系统。为此，我们将执行四个工作包（1）到（4），所有这些工作包都使用GRIMM。(1)我们进行时间序列分析的核心场和流模型，仔细提取的信号，从TO在不同的纬度。(2)我们改进了传统的流动建模方案参数化的核心表面的磁扩散。这里，在磁感方程中恢复了扩散项，通过将其与纳维-斯托克斯方程中的洛伦兹项联系起来，动态约束了磁感方程。(3)我们开发了一种方法来计算电磁核-幔耦合扭矩的核心流体环空，从而由于焦耳加热的能量耗散进行评估，为每个环空。这种分析将提供关于地球的TO是自由振荡还是强迫振荡的见解。(4)结合（1）~（3）的物理意义和计算工具，我们最终建立了一个与GRIMM和地球自转观测相一致的地球TO和核幔耦合动力学模型。该模型的独特之处在于，在时域中研究了关于TO的力平衡，并且该模型还旨在通过考虑核心动力学来改进基于观测的核心流模型。

The CHAMP mission provided a great amount of geomagnetic data all over the globe from 2000 to 2010. Its dense data coverage has allowed us to build GRIMM | GFZ Reference Internal Magnetic Model | which has the highest ever resolution for the core field in both space and time. We have already modeled the fluid flow in the Earth's outer core by applying the diffusionless magnetic induction equation to the latest version of GRIMM, to find that the flow evolves on subdecadal timescales, with a remarkable correlation to the observed fluctuation of Earth rotation. These flow models corroborated the presence of six-year torsional oscillations in the outer core fluid. Torsional oscillation (TO) is a type of hydromagnetic wave, theoretically considered to form the most important element of decadal or subdecadal core dynamics. It consists of relative azimuthal rotations of rigid fluid annuli coaxial with the mantle's rotation and dynamically coupled with the mantle and inner core. In preceding works, the TOs have been studied by numerical simulations, either with full numerical dynamos, or solving eigenvalue problems ideally representing the TO system. While these studies drew insights about dynamical aspects of the modeled TOs, they did not directly take into account the observations of geomagnetic field and Earth rotation. Particularly, there have been no observation-based studies for the TO using satellite magnetic data or models. In the proposed project, we aim at revealing the subdecadal dynamics and energetics of the Earth's core-mantle system on the basis of satellite magnetic observations. To that end, we will carry out four work packages (1) to (4), for all of which we use GRIMM. (1) We perform time-series analyses of core field and flow models, to carefully extract the signals from TOs at different latitudes. (2) We refine the conventional flow modeling scheme by parameterizing the magnetic diffusion at the core surface. Here, the diffusion term is reinstated in the magnetic induction equation, which is dynamically constrained by relating it to the Lorentz term in the Navier-stokes equation. (3) We develop a method to compute the electromagnetic core-mantle coupling torque on the core fluid annuli, whereby the energy dissipation due to the Joule heating is evaluated for each annulus. This analysis would provide insights on whether the Earth's TOs are free or forced oscillations. (4) Bringing together physical implications and computational tools obtained by (1) to (3), we finally construct a dynamical model for the Earth's TOs and core-mantle coupling such that they are consistent with GRIMM and Earth rotation observation. This modeling is unique in that the force balances concerning the TOs are investigated in time domain, as well as that the modeling also aims at improving the observation-based core flow model by considering the core dynamics.