Formation of current coils in geodynamo simulations

Abstract

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.

Earth's dynamo limit of predictability

“…This common main dependence of l = t e −1 and t SV −1 on Rm finally suggests that we also plot the ratio t e / t SV as a function of Rm. Figure 3 shows that for high Rm (low 1/Rm), t e /t SV converges towards a constant value, fairly independently of the exact value of E (at least within the range E = 10 −3 − 10 −5 of values reasonably accessible for such a multi-run numerical study, and close to the E = 10 −6 value currently achieved in the most advanced dynamo simulations [Kageyama et al, 2008;Christensen et al, 2009]). This convergence is achieved all the faster as E is small.…”

Theory and Modeling of Planetary Dynamos

“…A recent simulations by Kageyama et al (2008) at E = 10 −6 indicates a more drastic regime change: Sheetlike convective columns are restricted to the region closer to the tangent cylinder and the magnetic field is rather small scale and non-dipolar. However, the results are inconclusive since the magnetic field is still developing and may only be transient.…”

“…The Kageyama type model describes the dynamo medium as an ideal gas and retains weak compressibility effects Sato 1995, 1997;Kageyama et al , 2008Li et al 2002;Kageyama and Yoshida 2005;Nishikawa and Kusano 2008).…”

Virtual Reality Visualization of Frozen-in Vector Fields

“…7. The flow in this case is an output of an MHD dynamo simulation to understand the Earth's magnetic field generation process, or geodynamo simulation for short [8,9]. In this case, the timeline can be interpreted as a magnetic field line in the Earth's core that is frozen in the convection flow of the liquid metal of the core.…”

“…To study the origin of the magnetic field of planets and stars, large scale MHD simulations have been extensively performed on high performance computers [8,9].…”