Mechanism of the interannual oscillation in length of day and its constraint on the electromagnetic coupling at the core–mantle boundary
Mechanism of the interannual oscillation in length of day and its constraint on the electromagnetic coupling at the core–mantle boundary
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DOI：
10.1016/j.epsl.2017.11.007
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发表时间：
2018-01
影响因子：
5.3
通讯作者：
Pengshuo Duan;Genyou Liu;Xiaogang Hu;Jin Zhao;Chengli Huang
中科院分区：
文献类型：
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作者：
Pengshuo Duan;Genyou Liu;Xiaogang Hu;Jin Zhao;Chengli Huang
年际尺度上的日长(LOD)存在明显的6年振荡。目前主要有两种模型来解释这种振荡，即地幔-内核引力(MicG)耦合模式和流体外核内的快速扭转波。前者一直受到怀疑，而后者的激发来源尚不清楚。因此，这种6年振荡的机制仍不清楚。在考虑地幔和内核角动量的情况下，我们研究了MicG耦合模式及其固有周期(T0)。考虑到在最近的约束范围内引力核幔耦合(Γ)的强度比先前估计的要弱得多，6年振荡的机制仍然可以归结为MicG耦合模式(即T0等于6年)，但我们要求参与6年振荡的切柱内流体的惯性矩小于1.2 3×10 35 kg m2。这一解释可用于约束核内边界的电磁耦合。为了定量研究观测到的具有品质因子Q(∼51.6)的6年振荡对核幔边界电磁耦合的制约，我们进一步推导了基于观测的Q值与核幔边界电磁耦合之间的数学表达式。根据最新估计的Γ‘值，假定6年振荡是自由衰变的，我们得到了当中心磁导为108S时，中心磁核的径向磁场为0.52mT∼0.62mT。
A significant 6 yr oscillation exists in the length of day (LOD) on the interannual scales. There are mainly two models currently to explain this oscillation, ie, mantle–inner core gravitational (MICG) coupling mode and the fast torsional waves within the fluid outer core. The former has been doubted, while the source of the excitation of the latter is not yet understood. Therefore, the mechanism of the 6 yr oscillation is still not clear. Here, by considering the mantle and inner core angular momentum, we investigate the MICG coupling mode and its natural period (T 0). Given that the strength of gravitational core–mantle coupling (Γ¯) within a recently constrained range is quite weaker than that estimated previously, the mechanism of the 6 yr oscillation still can be attributed to MICG coupling mode (ie, T 0 equals to 6 yrs), but, we require the inertia moment of fluid within the tangent cylinder involved in the 6 yr oscillation to be smaller than 1.23× 10 35 kg m 2. This interpretation can be used to constrain the electromagnetic (EM) coupling at the inner core boundary (ICB). In order to study quantitatively the constraints on the EM coupling at the core–mantle boundary (CMB) from the observed 6 yr oscillation with a quality factor Q (∼ 51.6), we further develop the mathematical expression between the Q value based on the observations and EM coupling at the CMB. According to the Γ¯ value from recent estimates and assuming that the 6 yr oscillation is in a free decay, we can obtain the radial magnetic field at the CMB is 0.52 mT∼ 0.62 mT when conductance at the CMB is 10 8 S.