Physical Mechanism of Generation and Reversals of Geomagnetic Dynamo

地球磁场有两个有趣的特征；其一是它的偶极子性质；另一个是它的南北极性反转。本研究的目的是通过大规模的计算机模拟来模拟地磁场的这些特征，并了解其背后的物理机制。模拟模型如下：考虑两个同心球之间的球壳区。球面边界温度保持恒定和均匀；内球是热的，外球是冷的。导电流体被限制在这个球壳区域，与地球外核的液态铁相对应。当温差足够大时，导电流体就开始对流运动。其基本方程是磁流体动力学方程和温度场的时间发展方程。通过计算机模拟，成功地再现了偶极子磁场的自发产生及其间歇倒转。对仿真数据的详细分析表明，发电机产生的磁场具有两种稳定状态；一个是高能态，另一个是低能态。在这个模拟中，一个有趣的发现是，偶极子逆转只发生在高能态。另一个有趣的发现是四极矩增长，超过了反转前的偶极矩。模拟中使用的计算网格系统是球面经纬度网格，可以成功地再现偶极子场的产生和反转。当我们将此代码应用于地球模拟器时，我们发现经纬度网格不适合这种矢量并行架构。因此，我们设计了一种基于重叠（或嵌合）网格方法的球形网格系统，并将其命名为阴阳网格。将阴阳网格成功地应用于球壳MHD发电机仿真中。少

The Earth's magnetic field has two intriguing features ; one is its dipole nature ; and the other is its reversal of the north-south polarity. The purpose of this study is to simulate these features of the geomagnetic field and to understand the physical mechanism behind it by means of large scale computer simulation. The simulation model is as follows : We consider a spherical shell region between two concentric spheres. The temperatures of the spherical boundaries are kept constant and uniform ; the inner sphere is hot and outer sphere is cold. An electrically conducting fluid is confined in this spherical shell region, corresponding to the liquid iron in the Earth's outer core. When the temperature difference is sufficiently large, convection motion of the electrically conducting fluid sets about. The fundamental equations are the magnetohydrodynamic (MHD) equations with time development equations of temperature field. As a result of our computer simulation, spontaneous generation o … More f dipole magnetic field and its intermittent reversals are successfully reproduced. detail analysis of the simulation data indicates that the dynamo-generated magnetic field has two stable states ; one is high energy state and the other is low energy state. One of interesting findings in this simulation is hat the dipole reversals take place only in the high energy state. Another interesting finding is that quadrupole moment grows, exceeding the dipole moment before the reversal. The computational grid system used in our simulation, by which we could successfully reproduced the dipole field generation and reversals, were spherical latitude-longitude grid. When we applied this code to the Earth Simulator, we found that the latitude-longitude grid is not suitable to this vector-parallel architecture. We, therefore, devised a new kind of spherical grid system based on the overset (or Chimera) grid methodology, and named it Yin-Yang grid. The Yin-Yang grid is successfully applied to the spherical shell MHD dynamo simulation. Less