Abstract

Geomagnetic observations from satellites have highlighted interannual variations in the rate of change of the magnetic field originating from Earth’s core. Downward continued to the core surface, these variations primarily show up in the equatorial belt. First, we recall the main characteristics of these patterns, addressing their spatio-temporal resolution, as seen from field models. We then review the several dynamical frameworks proposed so far to understand and model these observations, which populate the frequency spectrum on time scales close to the Alfvén time \(\tau _A\approx 2\) yr, much shorter than the vortex turnover time \(\tau _U\approx 150\) yr in Earth’s core. Magnetic–Archimedes–Coriolis (MAC) waves in a stratified layer below the core surface constitute a first possibility in the case of a sub-adiabatic heat flux at the top of the core. Their period may reach the interannual range for a layer thickness less than \(\approx 30\) km, for a buoyancy frequency of the order of the Earth’s rotation rate. An alternative has been proposed in a context where the Coriolis force dominates the momentum balance, rendering transient motions almost invariant along the rotation axis (quasi-geostrophy, QG). Torsional Alfvén waves, consisting of axisymmetric QG motions, operate at periods similar to the Alfvén time, but are not sufficient to explain the interannual field changes, which require non-axisymmetric motions. QG Alfvén waves (involving the Coriolis and magnetic forces) constitute another possibility, with inertia playing an important role. They have been detected in the latest generation of geodynamo simulations, propagating in a ubiquitous manner at a speed slightly less than the Alfvén velocity. They are localized in longitude and as a result their description requires high azimuthal wavenumber. But the branch of QG waves with large extent in azimuth is also worth considering, as it reaches interannual periods as their radial wavenumber is increased. The excitation of such high-frequency dynamics is discussed with respect to the temporal spectrum of the core field, which presents a slope \(\sim f^{-4}\) for periods approximately between \(\tau _A\) and \(\tau _U\). We finally summarize the main geophysical implications of the existence of this interannual dynamics on core and lower mantle structure, properties, and dynamics.

中文翻译：

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地磁场年际变化的动力学展望

摘要

卫星的地磁观测突出了源自地核的磁场变化率的年际变化。继续向下到核心表面，这些变化主要出现在赤道带。首先，我们回顾这些模式的主要特征，解决它们的时空分辨率，如从场模型中看到的。然后，我们回顾了迄今为止提出的几个动力学框架来理解和模拟这些观察结果，这些框架在接近阿尔芬时间\(\tau _A\approx 2\) yr 的时间尺度上填充频谱，远短于涡旋周转时间\ (\tau _U\约 150\)年在地球的核心。在核心顶部亚绝热热通量的情况下，核心表面下方分层层中的磁-阿基米德-科里奥利 (MAC) 波构成了第一种可能性。对于小于\(\approx 30\) 的层厚，它们的周期可能达到年际范围km，对于地球自转速率数量级的浮力频率。在科里奥利力主导动量平衡的情况下提出了另一种选择，使瞬态运动沿旋转轴几乎不变（准地转，QG）。扭转阿尔文波由轴对称 QG 运动组成，其运行周期与阿尔文时间相似，但不足以解释需要非轴对称运动的年际磁场变化。QG 阿尔文波（涉及科里奥利力和磁力）构成了另一种可能性，其中惯性起着重要作用。它们已在最新一代的地球发电机模拟中被检测到，以略低于阿尔文速度的速度以无处不在的方式传播。它们被定位在经度上，因此它们的描述需要高方位波数。但是方位角范围大的QG波分支也值得考虑，因为随着径向波数的增加，它达到了年际周期。讨论了这种高频动力学的激发是关于核心场的时间谱，它呈现了一个斜率\(\sim f^{-4}\)大约在\(\tau _A\)和\(\tau _U\) 之间。我们最后总结了这种年际动力学的存在对地核和下地幔的结构、性质和动力学的主要地球物理意义。