Torsional oscillations in dynamo simulations
Torsional oscillations in dynamo simulations
复制标题
发电机模拟中的扭转振荡
DOI：
10.1111/j.1365-246x.2010.04581.x
复制
发表时间：
2010
影响因子：
2.8
通讯作者：
U. Christensen
中科院分区：
文献类型：
--
作者：
Johannes Wicht;U. Christensen
总结
与行星自转轴对齐的行星轮在行星发电机区域的动力学中具有特殊的意义。作用在这些地转圆柱体上的方位洛伦兹力预计会在很大程度上抵消，建立所谓的泰勒状态。与该状态的偏差采取扭转振荡（TO）的形式，这些振荡应该代表重要的快速流量变化。据报道，这些振荡已经在来自地球核心顶部的长期变化信号中被识别出来。我们已经在不同的参数下进行了几次发电机模拟，以检查是否泰勒状态和TO也可以在数值模型中识别。当Ekman数E = 3 × 10−5或更低时，粘性效应很小，并且通过选择适当的Rayleigh数来保持较低的雷诺应力时，接近泰勒状态。一维磁阿尔芬波的边界旅行，然后成为突出的地转柱体的运动。这些波服从TO理论，但也受到其他效应的阻尼和修改。例如，在我们所有的模拟中，可能的对流起源的快速变化仍然很重要。雷诺应力可能发挥更大的作用，在地球的发电机区域的动态比通常假设的。它们也可能对地转柱体的运动有贡献，并严重降低了TO对快核动力学的意义。在模拟中，TO的幅度不超过总流量幅度的百分之几，这使得这些运动对于长期发电机过程来说是微不足道的。
SUMMARY
Cylinders aligned with the planetary rotation axis have a special significance in the dynamics of planetary dynamo regions. The azimuthal Lorentz forces on these geostrophic cylinders is expected to cancel to a large degree, establishing the so-called Taylor state. Deviations from this state take the form of torsional oscillations (TOs) that are supposed to represent important fast flow variations. These oscillations have reportedly been identified in the secular variation signal from the top of Earth's core. We have performed several dynamo simulations at different parameters to check whether Taylor state and TOs can also be identified in a numerical model. Taylor states are approached when viscous effects are small at Ekman numbers of E = 3 × 10−5 or below and Reynolds stresses are kept low by choosing moderate Rayleigh numbers. One-dimensional magnetic Alfven waves that travel towards the boundaries then become prominent in the motion of the geostrophic cylinders. These waves obey the TO theory but are also damped and modified by other effects. For example, fast variations of likely convective origin remain important in all our simulations. Reynolds stresses may play a more sizable role for the dynamics in Earth's dynamo region than commonly assumed. They may also contribute to the motions of geostrophic cylinders and severely reduce the significance of TOs for the fast core dynamics. The amplitude of TOs amounts to not more than a few percent of the total flow amplitude in the simulations, which renders these motions insignificant for the long-term dynamo process.