Helical waves in Earth's outer core
type: CreativeWork
description: This thesis addresses the generation and dispersion of wave packets in Earth's outer core. The waves of interest feed off the buoyancy field, and are supported by strong rotation and/or a large-scale magnetic field, two key features of the dynamics in Earth's core. We aim to better understand the role of wave packets in shaping the turbulent convection, the process of magnetic induction, and maintaining Earth's magnetic field. Our numerical experiments focus on the emission of wave packets from localised sources, on the small length-scales of the convection, which stirs the fluid iron. The fluid flow in Earth's outer core is characterised by a small Rossby number, the ratio of nonlinear inertia and the Coriolis acceleration. However, due to computational constraints, many geodynamo simulations lie in a regime where the convective structures have a moderate Rossby number. These simulations, that over-estimate the influence of nonlinear inertia, undergo an abrupt transition from a columnar flow structure with a strongly dipolar magnetic field, to a state of disorganised flow accompanied by a multipolar field. This transition is commonly termed the \emph{dipolar-multipolar} transition, and the collapse of the dipole occurs when the \emph{`local'} Rossby number \citep[as introduced by][]{christensen2006} is greater than $0.1$. Separately, in the rotating turbulence literature, there is preliminary evidence of a similarly sharp transition from a columnar flow structure to incoherent turbulence when the Rossby number is $ Ro \sim 0.2-0.6$. We show, that when $Ro > Ro^{\text{crit}} \approx 0.4$, inertial wave packets are suppressed, and columnar flow structures break down. Furthermore, we highlight a relationship between the `local' Rossby number used to describe the dipolar-multipolar transition and our $Ro$, which places both transitions at approximately $Ro^{\text{crit}}$. Based on this evidence, we conjecture that the breakdown of columnar structures, followed by the d…
abstract: This thesis addresses the generation and dispersion of wave packets in Earth's outer core. The waves of interest feed off the buoyancy field, and are supported by strong rotation and/or a large-scale magnetic field, two key features of the dynamics in Earth's core. We aim to better understand the role of wave packets in shaping the turbulent convection, the process of magnetic induction, and maintaining Earth's magnetic field. Our numerical experiments focus on the emission of wave packets from localised sources, on the small length-scales of the convection, which stirs the fluid iron. The fluid flow in Earth's outer core is characterised by a small Rossby number, the ratio of nonlinear inertia and the Coriolis acceleration. However, due to computational constraints, many geodynamo simulations lie in a regime where the convective structures have a moderate Rossby number. These simulations, that over-estimate the influence of nonlinear inertia, undergo an abrupt transition from a columnar flow structure with a strongly dipolar magnetic field, to a state of disorganised flow accompanied by a multipolar field. This transition is commonly termed the \emph{dipolar-multipolar} transition, and the collapse of the dipole occurs when the \emph{`local'} Rossby number \citep[as introduced by][]{christensen2006} is greater than $0.1$. Separately, in the rotating turbulence literature, there is preliminary evidence of a similarly sharp transition from a columnar flow structure to incoherent turbulence when the Rossby number is $ Ro \sim 0.2-0.6$. We show, that when $Ro > Ro^{\text{crit}} \approx 0.4$, inertial wave packets are suppressed, and columnar flow structures break down. Furthermore, we highlight a relationship between the `local' Rossby number used to describe the dipolar-multipolar transition and our $Ro$, which places both transitions at approximately $Ro^{\text{crit}}$. Based on this evidence, we conjecture that the breakdown of columnar structures, followed by the d…
author: McDermott, Ben Robert
language: eng
keywords: Planetary dynamos, Rotating fluids, MHD, Earth's core, Geomagnetic induction, Inertial waves
license: https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/cc6df081-814e-423c-90aa-10223f49a038/download
license: https://creativecommons.org/licenses/by-nc-nd/4.0/
date published: 2020-09-12
date modified: 2026-07-22T22:24:11Z
provider: Cambridge University
publisher: Networked Digital Library of Theses and Dissertations
type: WebPage
source organization: University of Cambridge

PropertyValue
property ID: DOI
value: 10.17863/CAM.65609