Numerical Simulation of Plumes and Small-Scale Dynamics in the Earth's Fluid Core

地球核心的小尺度动力学PI：保罗·H·罗伯茨人们普遍认为，地球的主磁场是在其核心产生的，其产生方式与发电机在发电站产生电流的方式大致相同。因为地幔是一个很差的导电体，这些电流不会到达地球表面，但不可避免地伴随着任何电流的磁场确实会穿透地幔，形成主要的地球磁场。发电机通过一种称为“电磁感应”的过程，使导电材料在磁场中运动，从而产生电流和磁场。当伴随电流的感应磁场与产生电流的感应磁场相同时，发电机就称为“自励”。人们普遍认为地球发电机就是这种类型的。导体所需的运动是流体核心内的运动，流体核心似乎是铁合金，因此是良好的电导体。目前，该发电机的耗电量估计约为1011至1012瓦。这必须以动能的形式提供给地核流动，否则运动将停止，磁场将在几千年后消失，这与表明地球磁场与地球一样古老的古地磁证据相矛盾。有几个迹象表明，液核正在运动。地球表面的地磁场地图显示出不断增长和消亡的焦点，这让人想起天气图，尽管它是朝着相反的方向漂移，演化速度要慢得多。此外，成功的地球内部模型要求流体核心处于近乎绝热的状态，如果充分混合，这是自然发生的。核心搅拌最有可能是由于流体的化学成分和温度的微小差异所驱动的对流运动。这些最有可能是地球持续演化的副产品，在这种过程中，固体内核的表面以10-12到10-11米/S的速度逐渐向上移动，因为流体冻结在它上面，释放出铁合金的潜热和光成分。从内核边界释放的热和光物质可能远不均匀，以浮力羽流的形式搅动流体核心。这些羽流的命运尚不确定，已经提出了两种截然相反的情况。在较老的“传统情景”中，羽流迅速分解成越来越小的小块，使流体均匀，从而产生一种略重的状态，这种状态的不稳定性驱动了为地球发电机提供动力的大规模对流。较新的洛珀-莫法特方案特别注意到热量的低小扩散系数，特别是化学不均匀，这可能允许包裹保持其浮力，直到它变得真的很小。因此，有人认为，这些地块及其碎片停留的时间将比传统情况下认识到的要长得多，并将填满靠近核心表面的稳定分层的层。这个项目的一个主要目标是对混合过程进行计算建模，并确定这两种情况中的哪一种(如果有)与核心更相关。正在使用一种新的数值技术，允许包括地球物理上更真实的扩散系数，特别是化学不均匀的小扩散系数。这些计算可能会澄清岩心的小尺度动力学，以便在未来的地球发电机模拟中能够更好地将它们参数化。岩心动力学的长度和时间尺度的巨大范围使得参数化是不可避免的，但人们普遍认为目前使用的参数化是非物理的。这构成了当今地球发电机模拟中比较严重的缺陷之一。换言之，考虑到它们可疑的基础，目前对岩心磁流体动力学和地球发电机的整合是不合理的成功。这一成功有时被称为地球发电机悖论，它的解决是这个项目的目标之一。

Small-scale dynamics of the Earths corePI: Paul H. RobertsIt is generally accepted that the main magnetic field of the Earth is created in its core in much the same way that dynamos generate electric currents in power stations. Because the Earths mantle is a poor conductor of electricity, these currents do not reach the Earths surface, but the magnetic fields that inevitably accompany any flow of electricity do penetrate the mantle, and form the main geomagnetic field. A dynamo creates electric currents and magnetic fields by the motion of electrically conducting material across a magnetic field, by a process called `electromagnetic induction. When the induced magnetic field accompanying the electric currents is the same as the inducing magnetic field creating the currents, the dynamo is said to be `self-exciting. It is widely believed that the geodynamo is of this type. The required motions of the conductor are movements within the fluid core, which is plausibly a ferric alloy and therefore a good electrical conductor. The power consumed by the geodynamo is currently estimated to be about 1011 to 1012 watts. This has to be provided as kinetic energy to the core flow, otherwise motion would cease and the magnetic field would disappear in a few thousand years, in contradiction to paleomagnetic evidence that shows the geomagnetic field is as old as the Earth. There are several indications that the fluid core is in motion. Maps of the geomagnetic field at the Earths surface show growing and dying foci reminiscent of weather maps, though drifting in the reverse direction and evolving much more slowly . Also, successful models of the Earths interior require the fluid core to be in a nearly adiabatic state, which happens naturally if it is well mixed. Core stirring is most plausibly due to convective motions driven by small differences in the chemical composition and temperature of the fluid. Most likely these are the by-product of the continuing evolution of the Earth, in which the surface of the solid inner core gradually advances upwards, at a speed between 10-12 and 10-11 m/s, as fluid freezes onto it, releasing latent heat and light constituents of the ferric alloy as it does so. The release of heat and light material from the inner core boundary is probably far from uniform, taking the form of buoyant plumes that stir the fluid core. The fate of these plumes is uncertain, and two rival scenarios have been proposed. In the older `traditional scenario, the plumes quickly break up into ever smaller parcels that homogenize the fluid, so generating a slightly top-heavy state, the instability of which drives the large-scale convection powering the geodynamo. The newer Loper-Moffatt scenario, takes particular note of the low small diffusivities of heat and particularly of chemical inhomogeneities, which may allow a parcel to retain its buoyancy until it becomes really small. It is argued that the parcels and their debris will therefore linger for much longer than the traditional scenario recognizes, and will fill a stably stratified layer adjacent to the core surface. A main objective of this project is to model the mixing process computationally, and to determine which (if either) of the two scenarios is more relevant to the core. A fresh numerical technique is being used that allows geophysically more realistic diffusivities to be included, and particularly the small diffusivity of chemicalinhomogeneities. These calculations may clarify the small-scale dynamics of the core so that they can be better parameterized in future geodynamo simulations. The vast range of length and time scales of core dynamics makes parameterization unavoidable, but it is generally accepted that the parameterizations currently being used are unphysical. This constitutes one of the more serious deficiencies in geodynamo simulations today. Stated another way, current integrations of core magnetohydrodynamics and the geodynamo are unreasonably successful, bearing in mind their dubious basis. This success is sometimes called the the geodynamo paradox, and its resolution is an aim of this project.