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

’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’’人们普遍认为，地球的主磁场是在地核形成的，就像发电站的发电机产生电流一样。由于地球的地幔是电的不良导体，这些电流无法到达地球表面，但任何电流不可避免地会产生磁场，穿透地幔，形成主要的地磁场。发电机通过导电材料在磁场中的运动产生电流和磁场，这一过程被称为“电磁感应”。当伴随电流的感应磁场与产生电流的感应磁场相同时，发电机被称为“自激”。人们普遍认为地球发电机就是这种类型。导体所需的运动是在流体核心内的运动，流体核心似乎是铁合金，因此是良好的电导体。地球发电机消耗的功率目前估计约为1011至1012瓦。这必须作为动能提供给地核流动，否则运动将停止，磁场将在几千年后消失，这与古地磁证据相矛盾，古地磁证据表明地磁场与地球一样古老。有几个迹象表明，流体核心在运动。地球表面的地磁场图显示出增长和消失的焦点，这让人想起了天气图，尽管方向相反，演变得慢得多。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，如果混合良好，这是自然发生的。岩心搅拌最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。这些极有可能是地球持续演化的副产品，其中固体内核的表面以10-12至10-11米/秒的速度逐渐向上移动，因为液体在其上冻结，释放出潜热和铁合金的光成分。从内核边界释放的热和光物质可能远不是均匀的，它们以浮力羽流的形式搅动着流体内核。这些羽流的命运尚不确定，人们提出了两种截然相反的设想。在旧的传统场景中，羽流迅速分解成更小的包裹，使流体均匀化，从而产生轻微的头重体状态，这种状态的不稳定性驱动了大规模对流，为地球发电机提供动力。较新的Loper-Moffatt方案，特别注意到低的小热扩散率，特别是化学不均匀性，这可能使一个包裹保持其浮力，直到它变得非常小。有人认为，这些包裹及其碎片因此将停留的时间比传统情景所认识的要长得多，并将填满靠近核心表面的稳定分层层。这个项目的一个主要目标是对混合过程进行计算建模，并确定两种情况中哪一种（如果有的话）与核心更相关。目前正在使用一种新的数值技术，使地球物理学上更真实的扩散系数，特别是化学不均匀性的小扩散系数得以包括在内。这些计算可以澄清地核的小尺度动力学，以便在未来的地球动力学模拟中更好地参数化。岩心动力学的长度和时间尺度的巨大范围使得参数化不可避免，但人们普遍认为，目前使用的参数化是非物理的。这构成了当今地球动力学模拟中较为严重的缺陷之一。换句话说，目前地核磁流体动力学和地球动力学的结合是不合理的成功，记住它们可疑的基础。这种成功有时被称为地球发电机悖论，解决它是这个项目的目标。

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.