Department of Earth and Planetary Science, University of California, Berkeley, California 94720, USA.
Nature. 2014 Mar 27;507(7493):484-7. doi: 10.1038/nature13122.
Modern observations of the geomagnetic field reveal fluctuations with a dominant period of about 60 years. These fluctuations are probably a result of waves in the liquid core, although the precise nature of the waves is uncertain. Common suggestions include a type of magnetic wave, known as a torsional oscillation, but recent studies favour periods that are too short to account for a 60-year fluctuation. Another possibility involves MAC waves, which arise from the interplay between magnetic, Archimedes and Coriolis forces. Waves with a suitable period can emerge when the top of the core is stably stratified. Here I show that MAC waves provide a good description of time-dependent zonal flow at the top of the core, as inferred from geomagnetic secular variation. The same wave motion can also account for unexplained fluctuations in the dipole field. Both of these independent predictions require a 140-kilometre-thick stratified layer with a buoyancy frequency comparable to the Earth's rotation rate. Such a stratified layer could have a thermal origin, implying a core heat flow of about 13 terawatts. Alternatively, the layer could result from chemical stratification. In either case, the existence of a stratified layer at the top of the core obscures the nature of flow deeper in the core, where the magnetic field is continually regenerated.
现代对地磁场的观测显示出具有约 60 年主导周期的波动。这些波动可能是液体核心中波动的结果，尽管波动的确切性质尚不确定。常见的建议包括一种称为扭转振荡的磁波，但最近的研究倾向于周期太短，无法解释 60 年的波动。另一种可能性涉及 MAC 波，它源于磁、阿基米德和科里奥利力之间的相互作用。当核心顶部稳定分层时，就会出现具有合适周期的波。在这里，我表明 MAC 波很好地描述了从地磁长期变化推断出的核心顶部随时间变化的纬向流。相同的波动运动也可以解释偶极场中无法解释的波动。这两个独立的预测都需要一个 140 公里厚的分层层，其浮力频率与地球自转速度相当。这样的分层层可能具有热起源，这意味着核心热流约为 13 太瓦。或者，该层可能是由于化学分层造成的。在这两种情况下，核心顶部的分层层掩盖了核心深处不断产生磁场的流动性质。
