标题： 地核-地表运动控制的地磁场极性反转在高级地磁发电机模拟中的作用 显示英文标题

标题： Core-surface kinematic control of polarity reversals in advanced geodynamo simulations

摘要： 地磁场在地球历史过程中经历了数百次极性反转，其频率变化不定。在地球外核动力学的数值模型中，当对流强迫超过某一临界水平时，反转发生的频率增加。此前，这种转变与力平衡中的惯性影响有关。由于这种力在地球外核中是次要的，因此有人对该范式的地质物理适用性提出了质疑。重现过去一百万年的反转率也需要一些强迫条件，而这些条件并不能保证再现地磁场变化谱的其余部分。这些问题促使人们寻找替代的反转机制。使用一系列数值模型，在这些模型中地核底部通过内核冻结提供浮力，我们发现磁偶极振幅由浅层上涌和地核表面水平环流的相对强度控制。上涌的相对减弱使系统从稳定的偶极状态转变为反转偶极状态。这一机制纯粹是运动学的，因为它独立于内部力平衡而运作。因此，预计它适用于地球外核的物理条件。浅层上涌可能受到最外层地核中稳定分层的阻碍。我们证明，当分层水平较弱，对应于近绝热的核心表面热流时，单一模型能够再现从几十年到数百万年的观测地磁场变化。\rev{In contrast with}根据现有范式，由这种稳定的顶核机制引起的反转在分层程度增加时变得更加频繁，即当核心热流减少时。这表明需要重新审视地幔动力学与磁反转频率之间的联系。 显示英文摘要

摘要： The geomagnetic field has undergone hundreds of polarity reversals over Earth's history, at a variable pace. In numerical models of Earth's core dynamics, reversals occur with increasing frequency when the convective forcing is increased past a critical level. This transition has previously been related to the influence of inertia in the force balance. Because this force is subdominant in Earth's core, concerns have been raised regarding the geophysical applicability of this paradigm. Reproducing the reversal rate of the past million years also requires forcing conditions that do not guarantee that the rest of the geomagnetic variation spectrum is reproduced. These issues motivate the search for alternative reversal mechanisms. Using a suite of numerical models where buoyancy is provided at the bottom of the core by inner-core freezing, we show that the magnetic dipole amplitude is controlled by the relative strength of subsurface upwellings and horizontal circulation at the core surface. A relative weakening of upwellings brings the system from a stable to a reversing dipole state. This mechanism is purely kinematic because it operates irrespectively of the interior force balance. It is therefore expected to apply at the physical conditions of Earth's core. Subsurface upwellings may be impeded by stable stratification in the outermost core. We show that with weak stratification levels corresponding to a nearly adiabatic core surface heat flow, a single model reproduces the observed geomagnetic variations ranging from decades to millions of years. \rev{In contrast with} the existing paradigm, reversals caused by this stable top core mechanism become more frequent when the level of stratification increases i.e. when the core heat flow decreases. This suggests that the link between mantle dynamics and magnetic reversal frequency needs to be reexamined.