Bloxham Jeremy, Zatman Stephen, Dumberry Mathieu
Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.
Nature. 2002 Nov 7;420(6911):65-8. doi: 10.1038/nature01134.
Geomagnetic jerks, which in the second half of the twentieth century occurred in 1969 (refs 1, 2), 1978 (refs 3, 4), 1991 (ref. 5) and 1999 (ref. 6), are abrupt changes in the second time-derivative (secular acceleration) of the Earth's magnetic field. Jerks separate periods of almost steady secular acceleration, so that the first time-derivative (secular variation) appears as a series of straight-line segments separated by geomagnetic jerks. The fact that they represent a reorganization of the secular variation implies that they are of internal origin (as has been established through spherical harmonic analysis), and their short timescale implies that they are due to a change in the fluid flow at the surface of the Earth's core (as has also been established through mapping the time-varying flow at the core surface). However, little is understood of their physical origin. Here we show that geomagnetic jerks can be explained by the combination of a steady flow and a simple time-varying, axisymmetric, equatorially symmetric, toroidal zonal flow. Such a flow is consistent with torsional oscillations in the Earth's core, which are simple oscillatory flows in the core that are expected on theoretical grounds, and observed in both core flow models and numerical dynamo models.
地磁急变是地球磁场二阶时间导数（长期加速度）的突然变化，在20世纪下半叶发生于1969年（参考文献1、2）、1978年（参考文献3、4）、1991年（参考文献5）和1999年（参考文献6）。急变将几乎稳定的长期加速度时期分隔开来，使得一阶时间导数（长期变化）呈现为由地磁急变分隔的一系列直线段。它们代表长期变化的一种重新组织这一事实意味着它们起源于内部（这已通过球谐分析得到证实），而它们较短的时间尺度意味着它们是由地球核心表面流体流动的变化引起的（这也已通过绘制核心表面随时间变化的流动得到证实）。然而，人们对它们的物理起源了解甚少。在这里，我们表明地磁急变可以通过稳定流动与简单的随时间变化、轴对称、赤道对称的环形纬向流动的组合来解释。这样的流动与地核中的扭转振荡一致，扭转振荡是地核中基于理论预期的简单振荡流动，并且在核心流动模型和数值发电机模型中都有观测到。
