Kageyama Akira, Miyagoshi Takehiro, Sato Tetsuya
Earth Simulator Center, Japan Agency for Marine-Earth Science and Technology, Yokohama, 236-0001, Japan.
Nature. 2008 Aug 28;454(7208):1106-9. doi: 10.1038/nature07227.
Computer simulations have been playing an important role in the development of our understanding of the geodynamo, but direct numerical simulation of the geodynamo with a realistic parameter regime is still beyond the power of today's supercomputers. Difficulties in simulating the geodynamo arise from the extreme conditions of the core, which are characterized by very large or very small values of the non-dimensional parameters of the system. Among them, the Ekman number, E, has been adopted as a barometer of the distance of simulations from real core conditions, in which E is of the order of 10(-15). Following the initial computer simulations of the geodynamo, the Ekman number achieved has been steadily decreasing, with recent geodynamo simulations performed with E of the order of 10(-6). Here we present a geodynamo simulation with an Ekman number of the order of 10(-7)-the highest-resolution simulation yet achieved, making use of 4,096 processors of the Earth Simulator. We have found that both the convection flow and magnetic field structures are qualitatively different from those found in larger-Ekman-number dynamos. The convection takes the form of sheet plumes or radial sheet jets, rather than the columnar cell structures that are usually found. We have found that this sheet plume convection is an effective dynamo and the generated current is organized as a set of coils in the shape of helical springs or at times as a torus.
计算机模拟在我们对地球发电机的认识发展过程中发挥了重要作用，但在现实参数范围内对地球发电机进行直接数值模拟仍超出了当今超级计算机的能力。模拟地球发电机的困难源于地核的极端条件，其特征是系统的无量纲参数值非常大或非常小。其中，埃克曼数E已被用作衡量模拟与实际地核条件距离的指标，实际地核条件下的E约为10^(-15)。在最初的地球发电机计算机模拟之后，所达到的埃克曼数一直在稳步下降，最近的地球发电机模拟中E约为10^(-6)。在此，我们展示了一次埃克曼数约为10^(-7)的地球发电机模拟——这是迄今实现的最高分辨率模拟，使用了地球模拟器的4096个处理器。我们发现，对流流动和磁场结构在性质上与在较大埃克曼数的发电机中发现的情况不同。对流呈现为片状羽流或径向片状射流的形式，而不是通常发现的柱状单元结构。我们发现这种片状羽流对流是一种有效的发电机，所产生的电流被组织成一组螺旋弹簧形状的线圈，有时也呈环形。
