Seismological probes of Earth's outer core

外核是地球磁场的来源，也是地幔对流以及最终板块构造的大部分热量的来源，被认为是地球内部最具活力的区域。虽然地球上许多其他区域的性质随深度的变化是比较为人所知的，但对于最上层外核的物理性质的深度变化，特别是关于层状的、富含轻元素的层的可能存在，仍然没有达成共识。这一层的存在对于理解混合良好的外核与地幔之间的热关系具有重要意义；该层也可能影响地球动力学和地磁观测。该项目将使用两种互补的地震工具，体波和低频全地球振动，来研究最外层核的性质。作为该项目的一部分，将为STEM学生提供本科和研究生阶段的教育、培训和研究经验，并为地球物理学中代表性不足的群体成员提供PI支持，鼓励多样性。这个多方法项目将利用体波和地球的正常振荡模式来更好地了解最上层的外核。使用对精细速度结构敏感的多个SmKS相位，将允许对外核顶部的压缩波速度结构进行分辨率，在那里已经有几位研究人员提出了一层轻元素层。先前的体波研究表明，与混合良好的外核相比，存在一定范围的层厚、径向剖面和差异程度。一些体波研究报告说，这一层的地震速度比预期的要慢，这通常与外核顶部的致密物质相对应，而不是轻物质。通过增加来自整个地球振荡中心频率的信息，这些信息对外核的速度和密度都很敏感，这项工作将为表面上矛盾的最外层核提供新的见解，而不必对现有液体的矿物学性质做出任何假设。该项目还将结合体波观测来调查外核切线柱上任何可观测到的地震差异，这是之前存在分歧的问题。这个项目不仅对我们理解这一层的地震性质有影响，而且对地磁现象，如MAC和罗斯比波，对检查地球的热收支，包括地幔对流和地球发电机的动力源，也有影响。它还可以将观测到的外核性质与矿物物理学的预测进行比较，以更好地了解那里存在的液态铁合金的化学成分。

The outer core, the source of Earth's magnetic field and much of the heat that powers mantle convection, and ultimately plate tectonics, is thought to be the most dynamic region of our planet's interior. While the change in properties with depth of many other regions of the Earth is relatively well known, there is still no consensus on the depth variation of physical properties in the uppermost outer core, especially with regard to the possible existence of a stratified, light element enriched layer. The presence of such a layer is important for understanding the thermal relationship between the well-mixed outer core and the mantle; the layer could also affect the geodynamo and geomagnetic observables. This project will use two complementary seismic tools, body-waves and low-frequency whole Earth vibrations, to investigate the properties of the uppermost outer core. Education, training and research experience for STEM students at the undergraduate and graduate level will be provided as part of this project as well as support for a PI who is a member of an underrepresented group in geophysics, encouraging diversity.This multi-method project will exploit both body-waves and the Earth's normal modes of oscillation to better understand the uppermost outer core. Using multiple SmKS phases, sensitive to fine velocity structure, will allow for the resolution of compressional-wave velocity structure at the top outer core, where a stratified layer of light elements has been proposed by several researchers. Previous body-wave studies have suggested a range of layer thickness, radial profiles and degree of difference from a well-mixed outer core. Some body-wave studies have reported that the layer is seismically slower than expected, which would normally be expected to correspond to dense, rather that light, material at the top of the outer core. By adding information from whole Earth oscillation center frequencies, which are sensitive to both velocity and density in the outer core, this work will provide new insights into the apparently paradoxical uppermost outer core without having to make any assumptions about the mineralogical properties of the liquid present. The project will also use a combination of body-wave observations to investigate any seismologically observable distinction across the outer core's tangent cylinder, a matter of previous disagreement. This project will have implications not only for our understanding of the seismic properties of this layer, but also for geomagnetic phenomena, such as MAC and Rossby waves, for examining the heat budget of the planet, including the power sources for mantle convection and the geodynamo. It will also permit the comparison of the observed properties of the outer core with predictions from mineral physics to better understand the chemical composition of the liquid iron alloy present there.