从geomagnetic field模型反演的堆芯流变率模式
Variability modes in core flows inverted from geomagnetic field models
摘要：
液态金属在地核内的流动产生了地磁场及其时间变化。了解这些深层电流的变化对于改善影响人类空间和航空活动的地磁场变化的预测至关重要。此外，它还可以提供关于核心动态的相关信息。本研究的主要目标是提取和表征核心流百年期间的主要变异模式，并评估其统计鲁棒性。为此，我们使用的流量，我们从两个地磁场模型（gufm 1和COV-OBS）反演，并应用主成分分析和奇异值分解的耦合场。由两种地磁场模型反演的准地转的准地转的（QG）流具有相似的特征。然而，COV-OBS具有较低的能量平均值和较大的时间变异性。流量分量的统计意义进行了测试，从分析上进行的子领域的整个域。自举方法也被用来提取显着的流动功能所需的gufm 1和COV-OBS。三个主要的经验环流模式出现，同时受到两个地磁场模型，并预计将是强大的对特定的先验用于建立它们（大规模QG动态）。模式1在中/高纬度地区有三个大涡旋，在大西洋和太平洋半球有相反的环流。有趣的是，模式2解释了地球核心角动量的大部分变化。在这种模式中，切向圆柱体和赤道附近的区域是相关的，并且以80到90年的周期振荡。这两种模式的能量都足以改变平均气流，有时会加强偏心环流，有时则会将其分解为较小的环流。三种主要环流模式加在一起的平均流量占约70%的流量变异，90%的均方根总速度，和95%的长期变化引起的总流量。计算模式的直接物理解释并不简单。然而，在不同的核心动力学研究中发现的两个第一模式和时间/空间特征之间的相似性表明，我们的方法可以帮助确定核心内的相关物理过程的百年时间尺度。
英文摘要：
The flow of liquid metal inside the Earth's core produces the geomagnetic field and its time variations. Understanding the variability of those deep currents is crucial to improve the forecast of geomagnetic field variations, which affect human spacial and aeronautic activities. Moreover, it may provide relevant information on the core dynamics. The main goal of this study is to extract and characterize the leading variability modes of core flows over centennial periods, and to assess their statistical robustness. To this end, we use flows that we invert from two geomagnetic field models (gufm1 and COV-OBS), and apply Principal Component Analysis and Singular Value Decomposition of coupled fields. The quasi geostrophic (QG) flows inverted from both geomagnetic field models show similar features. However, COV-OBS has a less energetic mean and larger time variability. The statistical significance of flow components is tested from analyses performed on subareas of the whole domain. Bootstrapping methods are also used to extract significant flow features required by both gufm1 and COV-OBS. Three main empirical circulation modes emerge, simultaneously constrained by both geomagnetic field models and expected to be robust against the particular a priori used to build them (large scale QG dynamics). Mode 1 exhibits three large vortices at medium/high latitudes, with opposite circulation under the Atlantic and the Pacific hemispheres. Mode 2 interestingly accounts for most of the variations of the Earth's core angular momentum. In this mode, the regions close to the tangent cylinder and to the equator are correlated, and oscillate with a period between 80 and 90 years. Each of these two modes is energetic enough to alter the mean flow, sometimes reinforcing the eccentric gyre, and other times breaking it up into smaller circulations. The three main circulation modes added together to the mean flow account for about 70% of the flows variability, 90% of the root mean square total velocities, and 95% of the secular variation induced by the total flows. Direct physical interpretation of the computed modes is not straightforward. Nonethe-less, similarities found between the two first modes and time/spatial features identified in different studies of core dynamics, suggest that our approach can help to pinpoint the relevant physical processes inside the core on centennial timescales.
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