Equatorially trapped waves in Earth's core
Equatorially trapped waves in Earth's core
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DOI：
10.1093/gji/ggz233
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发表时间：
2019-08-01
影响因子：
2.8
通讯作者：
Matsui, Hiroaki
中科院分区：
文献类型：
--
作者：
Buffett, Bruce;Matsui, Hiroaki
在最近的卫星观测中发现了地磁加速度的短期波动。这一信号的一个主要组成部分被限制在赤道地区，表明赤道波的存在，但这些波的确切性质是未知的。我们探讨的可能性，这些波产生的相互作用的磁，阿基米德和科里奥利力在分层层的核心（有时称为MAC波）的顶部。我们采用β-平面近似，并表明，低频MAC波不被困在赤道附近时，均方根（RMS）径向磁场是恒定的核幔边界的表面。然而，赤道捕获MAC波出现时，均方根径向磁场增加向两极。进一步限制MAC波发生时，我们占主导地位的球形几何形状的影响。由此产生的MAC波传播到东部的相速度，强烈依赖于分层层的厚度。当层厚小于30 km时，预报了周期小于10年的波浪。这些波具有低品质因数，Q近似为1，这意味着它们在被欧姆损耗耗散之前仅传播几千公里。观测中东向和向西向传播扰动的证据可能反映了波传播和激发源强迫运动的叠加。如果源主要是由于赤道地区向西漂移的羽流，则源与波传播的分离是可能的。
Short-period fluctuations in geomagnetic acceleration are detected in recent satellite observations. A major component of this signal is confined to the equatorial region, suggesting the presence of equatorial waves, but the precise nature of these waves is not known. We explore the possibility that these waves arise from an interplay of magnetic, Archimedes and Coriolis forces in a stratified layer at the top of the core (sometimes called MAC waves). We adopt a beta-plane approximation and show that low-frequency MAC waves are not trapped near the equator when the root-mean-square (rms) radial magnetic field is constant over the surface of the core-mantle boundary. However, equatorial trapped MAC waves emerge when the rms radial magnetic field increases towards the poles. Further confinement of MAC waves occurs when we account for the leading-order effects of spherical geometry. The resulting MAC waves propagate to the east with phase velocities that depend strongly on the thickness of the stratified layer. Waves with periods less than 10 yr are predicted when the layer thickness is less than 30 km. These waves have low quality factors, Q approximate to 1, which means that they propagate only a few thousand kilometers before being dissipated by ohmic losses. Evidence for eastward and westward propagating disturbances in the observations may reflect a superposition of wave propagation and forced motion by an excitation source. Separation of the source from the wave propagation may be possible if the source is due mainly to westward drifting plumes in the equatorial region.