Bottom-up control of geomagnetic secular variation by the Earth's inner core
Bottom-up control of geomagnetic secular variation by the Earth's inner core
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DOI：
10.1038/nature12574
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发表时间：
2013-10-10
期刊：
影响因子：
64.8
通讯作者：
Fournier, Alexandre
中科院分区：
文献类型：
--
作者：
Aubert, Julien;Finlay, Christopher C.;Fournier, Alexandre
地球磁场的时间变化，即地磁长期变化，在大西洋半球的低纬度地区（即从东经290度到东经90度）最为显著（1,2），而在太平洋半球，活动相对较少。这是地核表面强烈的、向西漂移的赤道磁通量斑块地理定位的结果(3)。尽管成功地解释了地磁场的形态(4)，但地球动力学的数值模型迄今未能系统地解释这种显著的地磁长期变化模式。在这里，我们表明，如果联合考虑依赖于内核的两种机制，则可以再现它。首先，引力耦合(5)使内核与地幔对齐，迫使外核中的液态金属流动成为一个巨大的、向西漂移的片状环流(6)。由此产生的剪切将方位磁通量集中在靠近地核-地幔边界的低纬度地区，在那里被地核对流排出并随后向西输送。其次，内核的差异增长（7,8），在印度尼西亚（6,9）以下速度最快，导致外核不对称的浮力释放，这反过来扭曲了环流，迫使它变得偏心，与最近的核心流动反转一致（6,10,11）。这种由下而上的非均质驱动的地核对流主导了由地幔热非均质驱动的自上而下的地核对流，并将磁场变化定位在以大西洋为中心的纵向扇区，在那里偏心环流到达地核表面。为了匹配观测到的地磁长期变化模式，形成内核的固体物质现在必须处于微分生长状态，而不是由对流平移引起的生长和融化状态（7,8）。
Temporal changes in the Earth's magnetic field, known as geomagnetic secular variation, occur most prominently at low latitudes in the Atlantic hemisphere(1,2) (that is, from 290 degrees east to 90 degrees east), whereas in the Pacific hemisphere there is comparatively little activity. This is a consequence of the geographical localization of intense, westward drifting, equatorial magnetic flux patches at the core surface(3). Despite successes in explaining the morphology of the geomagnetic field(4), numerical models of the geodynamo have so far failed to account systematically for this striking pattern of geomagnetic secular variation. Here we show that it can be reproduced provided that two mechanisms relying on the inner core are jointly considered. First, gravitational coupling(5) aligns the inner core with the mantle, forcing the flow of liquid metal in the outer core into a giant, westward drifting, sheet-like gyre(6). The resulting shear concentrates azimuthal magnetic flux at low latitudes close to the core-mantle boundary, where it is expelled by core convection and subsequently transported westward. Second, differential inner-core growth(7,8), fastest below Indonesia(6,9), causes an asymmetric buoyancy release in the outer core which in turn distorts the gyre, forcing it to become eccentric, in agreement with recent core flow inversions(6,10,11). This bottom-up heterogeneous driving of core convection dominates top-down driving from mantle thermal heterogeneities, and localizes magnetic variations in a longitudinal sector centred beneath the Atlantic, where the eccentric gyre reaches the core surface. To match the observed pattern of geomagnetic secular variation, the solid material forming the inner core must now be in a state of differential growth rather than one of growth and melting induced by convective translation(7,8).