Stochastic generation of MAC waves and implications for convection in Earth’s core

Buffett, B. A.; Knezek, Nicholas

doi:10.1093/gji/ggx492

Cited by 11 publications

(8 citation statements)

References 31 publications

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“…Labbé et al 2015), motions within a stratified layer (e.g. Buffett & Knezek 2017), or any other interpretation through a reduced model. We also lack suitable long coverage by high quality satellite records to perform spectral analyses with a refined sampling in the frequency domain, which would allow us to isolate possible waves at interannual periods.…”

Section: Future Workmentioning

confidence: 99%

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

Barrois

Hammer

Finlay

et al. 2018

Geophysical Journal International

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Section: Future Workmentioning

confidence: 99%

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

Barrois

Hammer

Finlay

et al. 2018

Geophysical Journal International

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“…Excitation of MAC waves in a stably stratified layer at the top of the core remains largely an open question. Buffett and Knezek (2018) investigated linear excitation and found that Maxwell stresses yield an efficient generation mechanism with most power in the period range 30 to 100 years, long compared to the timescales discussed here. We may learn also from other fluid systems where a stratified layer is adjacent to a turbulent layer such as the Earth's stratosphere or the Sun's tachocline (Le Bars et al 2020).…”

Section: Mac Waves In a Stratified Layermentioning

confidence: 89%

A Dynamical Prospective on Interannual Geomagnetic Field Changes

2021

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Geomagnetic observations from satellites have highlighted interannual variations in the rate of change of the magnetic field originating from Earth's core. Downward continued to the core surface, these variations primarily show up in the equatorial belt. First, we recall the main characteristics of these patterns, addressing their spatio-temporal resolution, as seen from field models. We then review the several dynamical frameworks proposed so far to understand and model these observations, which populate the frequency spectrum on time scales close to the Alfvén time A ≈ 2 yr, much shorter than the vortex turnover time U ≈ 150 yr in Earth's core. Magnetic-Archimedes-Coriolis (MAC) waves in a stratified layer below the core surface constitute a first possibility in the case of a sub-adiabatic heat flux at the top of the core. Their period may reach the interannual range for a layer thickness less than ≈ 30 km, for a buoyancy frequency of the order of the Earth's rotation rate. An alternative has been proposed in a context where the Coriolis force dominates the momentum balance, rendering transient motions almost invariant along the rotation axis (quasi-geostrophy, QG). Torsional Alfvén waves, consisting of axisymmetric QG motions, operate at periods similar to the Alfvén time, but are not sufficient to explain the interannual field changes, which require non-axisymmetric motions. QG Alfvén waves (involving the Coriolis and magnetic forces) constitute another possibility, with inertia playing an important role. They have been detected in the latest generation of geodynamo simulations, propagating in a ubiquitous manner at a speed slightly less than the Alfvén velocity. They are localized in longitude and as a result their description requires high azimuthal wavenumber. But the branch of QG waves with large extent in azimuth is also worth considering, as it reaches interannual periods as their radial wavenumber is increased. The excitation of such high-frequency dynamics is discussed with respect to the temporal spectrum of the core field, which presents a slope ∼ f −4 for periods approximately between A and U . We finally summarize the main geophysical implications of the existence of this interannual dynamics on core and lower mantle structure, properties, and dynamics.

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“…An example of such radial motion is that associated with zonal Magnetic Archimedes Coriolis (MAC) waves, which possibly account for the decadal zonal flows at the CMB (Buffett 2014;Buffett and Knezek 2018). With a layer 140 km thick, and assuming N is linearly increasing from zero at the base of the layer to a maximum approximately equal to the Earth rotation frequency o at the top, zonal MAC waves have periods of a few decades.…”

Section: Radial Motion In a Stably Stratified Layermentioning

confidence: 99%

Gravity Variations and Ground Deformations Resulting from Core Dynamics

Dumberry

Mandea

2021

Surv Geophys

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Fluid motion within the Earth’s liquid outer core leads to internal mass redistribution. This occurs through the advection of density anomalies within the volume of the liquid core and by deformation of the solid boundaries of the mantle and inner core which feature density contrasts. It also occurs through torques acting on the inner core reorienting its non-spherical shape. These in situ mass changes lead to global gravity variations, and global deformations (inducing additional gravity variations) occur in order to maintain the mechanical equilibrium of the whole Earth. Changes in Earth’s rotation vector (and thus of the global centrifugal potential) induced by core flows are an additional source of global deformations and associated gravity changes originating from core dynamics. Here, we review how each of these different core processes operates, how gravity changes and ground deformations from each could be reconstructed, as well as ways to estimate their amplitudes. Based on our current understanding of core dynamics, we show that, at spherical harmonic degree 2, core processes contribute to gravity variations and ground deformations that are approximately a factor 10 smaller than those observed and caused by dynamical processes within the fluid layers at the Earth’s surface. The larger the harmonic degree, the smaller is the contribution from the core. Extracting a signal of core origin requires the accurate removal of all contributions from surface processes, which remains a challenge. Article Highlights Dynamical processes in Earth's fluid core lead to global gravity variations and surface ground deformations We review how these processes operate, how signals of core origin can be reconstructed and estimate their amplitudes Core signals are a factor 10 smaller than the observed signals; extracting a signal of core origin remains a challenge

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Stochastic generation of MAC waves and implications for convection in Earth’s core

Cited by 11 publications

References 31 publications

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

Assimilation of ground and satellite magnetic measurements: inference of core surface magnetic and velocity field changes

A Dynamical Prospective on Interannual Geomagnetic Field Changes

Gravity Variations and Ground Deformations Resulting from Core Dynamics

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