Constraints on core‐mantle electromagnetic coupling from torsional oscillation normal modes
Constraints on core‐mantle electromagnetic coupling from torsional oscillation normal modes
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DOI：
10.1029/2007jb005135
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发表时间：
2008-03
影响因子：
--
通讯作者：
M. Dumberry;J. Mound
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文献类型：
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作者：
M. Dumberry;J. Mound
[1]地核中的十年轴向角动量变化被认为是由扭转振荡的正常模式携带的。与地幔的耦合将角动量传递给后者，导致日长（LOD）的变化。核幔边界的电磁应力可能是一种重要的耦合机制，也是扭转振荡的耗散源。在这项工作中，我们调查所观察到的频谱的流体核心速度和LOD的变化是否都可以解释的正常模式的扭转振荡时，唯一的耦合与地幔是通过电磁应力。我们表明，这种解释可能是真实的，当在CMB的磁场是基于一个向下延续的表面观测，在地幔底部的电导不大大超过10 - 8 S和小波长场的功能不贡献超过约25%的总径向磁场在CMB。更大的电导或更高幅度的径向磁场导致扭转振荡的正常模式的阻尼，该阻尼足够大以至于它们不应该被检测到。特别是，我们表明，这是从地球的强迫章动推断的电导和径向磁场的情况下。如果这些限制是正确的，在流体速度和LOD的十年周期性，然后必须代表的激励机制的扭转振荡，而不是签名的自由模式的首选频率。引用：Dumberry，M.，和J.E.。Mound（2008），来自扭转振荡正常模式的对核幔电磁耦合的约束，J. Geophys.结果：113，B03102，doi：10.1029/2007JB 005135。
[1] Decadal axial angular momentum variations in the Earth’s core are believed to be carried by the normal modes of torsional oscillations. Coupling with the mantle transfers angular momentum to the latter, leading to changes in length of day (LOD). Electromagnetic stresses at the core-mantle boundary (CMB) may be an important coupling mechanism as well as a source of dissipation for torsional oscillations. In this work, we investigate whether the observed spectra of fluid core velocities and LOD variations can be both explained in terms of the normal modes of torsional oscillations when the only coupling with the mantle is through electromagnetic stresses. We show that this explanation may be true when the magnetic field at the CMB is based on a downward continuation of surface observations, provided the conductance at the bottom of the mantle does not greatly exceed 10 8 S and small wavelength field features do not contribute more than approximately 25% of the total radial field at the CMB. A larger conductance or a higher amplitude radial magnetic field results in a damping of the normal modes of torsional oscillation that is sufficiently large that they should not be detectable. In particular, we show that this is the case for the conductance and radial magnetic field that are inferred from the Earth’s forced nutations. If these constraints are correct, the decadal periodicities in the fluid velocity and LOD must then represent the preferred frequencies of the excitation mechanism of torsional oscillations rather than the signature of the free modes. Citation: Dumberry, M., and J. E. Mound (2008), Constraints on core-mantle electromagnetic coupling from torsional oscillation normal modes, J. Geophys. Res., 113, B03102, doi:10.1029/2007JB005135.