Fast Quasi‐Geostrophic Magneto‐Coriolis Modes in the Earth's Core

• Published in: Geophysical research letters Vol. 48; no. 4
• Main Authors: Gerick, F., Jault, D., Noir, J.
• Format: Journal Article
• Language: English
• Published: American Geophysical Union 28-02-2021
• Subjects: core dynamics; Earth Sciences; Geophysics; Magneto‐Coriolis modes; MHD modes; Sciences of the Universe; secular variations
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL090803

Fast changes of Earth's magnetic field could be explained by inviscid and diffusion‐less quasi‐geostrophic (QG) Magneto‐Coriolis modes. We present a hybrid QG model with columnar flows and three‐dimensional magnetic fields and find modes with periods of a few years at parameters relevant to Earth's core. For the simple poloidal magnetic field that we consider here they show a localization of kinetic and magnetic energy in the equatorial region. This concentration of energy near the equator and the high frequency make them a plausible mechanism to explain similar features observed in recent geomagnetic field observations. Our model potentially opens a way to probe the otherwise inaccessible magnetic field structure in the Earth's outer core.
Plain Language Summary
Earth's magnetic field is evolving on many time scales. Here, we show that the changes of periods of a few years could possibly be explained by standing waves (modes) within the fluid outer core, which we model as not stratified. These modes are mostly in a balance between the magnetic and rotational forces. The numerically calculated modes show agreement with features of recent magnetic field observations through satellites, for example a stronger amplitude of the magnetic field near the equator. In a next step, our model might allow us to probe the magnetic field inside the liquid and conducting outer core.
Key Points
Magneto‐Coriolis modes of periods close to torsional Alfvén modes could be present in Earth's core model without stratification
The magnetic field changes of such modes show properties similar to geomagnetic observations, with fast changes localized near the equator
Our model could allow core‐flow inversions from geomagnetic field data to the flow and simultaneously the magnetic field within the core