Ground observatory and satellite-based determinations of temporal variations in the geomagnetic field probe a decadal to annual time scale range where Earth's core slow, inertialess convective motions and rapidly propagating, inertia-bearing hydromagnetic waves are in interplay. Here we numerically model and jointly investigate these two important features with the help of a geodynamo simulation that (to date) is the closest to the dynamical regime of Earth's core. This model also considerably enlarges the scope of a previous asymptotic scaling analysis. Three classes of hydrodynamic and hydromagnetic waves are identified in the model output, all with propagation velocity largely exceeding that of convective advection: axisymmetric, geostrophic Alfvén torsional waves, and non-axisymmetric, quasi-geostrophic Alfvén and Rossby waves. The contribution of these waves to the geomagnetic acceleration amounts to an enrichment and flattening of its energy density spectral profile at decadal time scales, thereby providing a constraint on the extent of the $f^{-4}$ range observed in the geomagnetic frequency power spectrum. The flow and magnetic acceleration energies carried by waves both linearly increase with the ratio of the magnetic diffusion time scale to the Alfvén time scale, highlighting the dominance of Alfvén waves in the signal and the stabilising control of magnetic dissipation at non-axisymmetric scales. Extrapolation of the results to Earth's core conditions supports the detectability of Alfvén waves in geomagnetic observations, either as axisymmetric torsional oscillations or through the geomagnetic jerks caused by non-axisymmetric waves. In contrast, Rossby waves appear to be too fast and carry too little magnetic energy to be detectable in geomagnetic acceleration signals of limited spatio-temporal resolution.
Reconciliation of experiments and theory on transport properties of iron and the geodynamo
来自核心的热量传输量，限制了该地区的动力学和热演化，取决于铁的传输特性。Ohta等人（2016）和Konopkova et al.（2016）在相关的地球核心压力-温度（P-T）条件下，分别在激光加热的金刚石对顶砧单元（DAC）中测量铁的电阻率和热导系数，并获得了非常矛盾的结果。在这里，我们测量hcp-铁的电阻率高达~170 GPa和~3,000 K使用四探针范德堡方法耦合均匀平顶激光加热DAC。我们还计算了它的电导率和热导系数的第一性原理方法，包括电子-声子和电子-电子散射。我们发现，测量的hcp-铁的电阻率随温度的升高几乎呈线性增加，并与当前的第一性原理计算是一致的。hcp-铁的电阻率和热导系数之间的比例系数（洛伦兹数）与理想值（2.44*10^-8 W Ω·K^-2）不同，因此使用~（2.0-2.1）*10^-8 W Ω·K^-2的非理想洛伦兹数将实验电阻率转换为hcp-铁在高P-T下的热导系数。结果表明，在核幔边界附近，hcp铁的电阻率和热导系数分别为80（5）u Ω cm和100（10）W/mK。我们的研究结果表明，通过核-幔边界的液态铁合金外核的绝热热流约为10（1）TW，支持当今地球发电机驱动的热对流通过核心的长期冷却和成分对流通过潜热和重力能在内核的凝固。
