Electromagnetically driven westward drift and inner-core superrotation in Earth’s core
Electromagnetically driven westward drift and inner-core superrotation in Earth’s core
复制标题
地核电磁驱动向西漂移和内核超级旋转
DOI：
--
复制
发表时间：
2013
影响因子：
11.1
通讯作者：
A. Jackson
中科院分区：
文献类型：
--
作者：
P. Livermore;Rainer Hollerbach;A. Jackson
意义对地球深层内部的地震探测表明，内核，即我们星球的固体核心，旋转得稍快（即，比地球上的其他地方。对地磁场的观测提供了液态外核边缘向西漂移特征的证据，这是相当独立的。本文描述了一个计算机模型，表明地磁场本身可能提供了它们之间的联系：相关的电磁转矩目前在最外面的外核中是向西的，而相等和相反的转矩施加到内核。地磁场的十年变化可能会导致这两种效应的波动，这与最近观察到的准振荡内核旋转速率一致。一个地球核心的三维数值模型，其粘度比现有技术低两个数量级，表明观测到的磁场向西漂移与内核的超旋转之间存在联系。在我们的模型中，轴向电磁转矩仅在表面和核心的最深处具有主导影响，在那里它分别驱动一个向西的流上升到轴对称赤道射流，并在固体内核上施加一个向东的转矩。内部磁场结构的细微变化不仅会改变力矩的大小，还会改变力矩的方向。这不仅表明内核超旋转的准振荡性质[Tkalčić H，Young M，Bodin T，Ngo S，Sambridge M（2013）The shuffling rotation of the earth's inner core revealed by earthquake doublets. Nat Geosci 6：497-502.]可能是由磁场的十年变化驱动的，但进一步指出，磁场向东漂移的历史时期与向西的内核旋转是同时发生的。该模型进一步表明，一个强大的内部剪切层上的切线圆柱体，可能是一个源的核心内的扭转波。
Significance Seismic probing of the earth’s deep interior has shown that the inner core, the solid core of our planet, rotates slightly faster (i.e., eastward) than the rest of the earth. Quite independently, observations of the geomagnetic field provide evidence of westward-drifting features at the edge of the liquid outer core. This paper describes a computer model that suggests that the geomagnetic field itself may provide a link between them: The associated electromagnetic torque currently is westward in the outermost outer core, whereas an equal and opposite torque is applied to the inner core. Decadal changes in the geomagnetic field may cause fluctuations in both these effects, consistent with recent observations of a quasi-oscillatory inner-core rotation rate. A 3D numerical model of the earth’s core with a viscosity two orders of magnitude lower than the state of the art suggests a link between the observed westward drift of the magnetic field and superrotation of the inner core. In our model, the axial electromagnetic torque has a dominant influence only at the surface and in the deepest reaches of the core, where it respectively drives a broad westward flow rising to an axisymmetric equatorial jet and imparts an eastward-directed torque on the solid inner core. Subtle changes in the structure of the internal magnetic field may alter not just the magnitude but the direction of these torques. This not only suggests that the quasi-oscillatory nature of inner-core superrotation [Tkalčić H, Young M, Bodin T, Ngo S, Sambridge M (2013) The shuffling rotation of the earth’s inner core revealed by earthquake doublets. Nat Geosci 6:497–502.] may be driven by decadal changes in the magnetic field, but further that historical periods in which the field exhibited eastward drift were contemporaneous with a westward inner-core rotation. The model further indicates a strong internal shear layer on the tangent cylinder that may be a source of torsional waves inside the core.