The Viscous and Ohmic Damping of the Earth's Free Core Nutation

Triana, S. A.; Trinh, Antony; Rekier, J.; Zhu, Paikun; Déhant, V.

doi:10.1029/2020jb021042

Cited by 9 publications

(13 citation statements)

References 44 publications

(78 reference statements)

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“…The restoring force for inertial waves is the Coriolis force (Rekier et al 2019(Rekier et al , 2020. There are many waves that are more or less damped, and that show different repartitioning of the energy between the core and the mantle (Triana et al 2019(Triana et al , 2021.…”

Section: Cmb From Geodesymentioning

confidence: 99%

Structure, Materials and Processes in the Earth’s Core and Mantle

et al. 2022

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This paper reviews current knowledge about the Earth’s core and the overlying deep mantle in terms of structure, chemical and mineralogical compositions, physical properties, and dynamics, using information from seismology, geophysics, and geochemistry. High-pressure experimental techniques that can help to interpret and understand observations of these properties and compositions in the deep interior are summarized. The paper also examines the consequences of core flows on global observations such as variations in Earth’s rotation and orientation or variations in the Earth’s magnetic field. Processes currently active at the core-mantle boundary and the various coupling mechanisms between the core and the mantle are discussed, together with some evidence from magnetic field observations.

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Section: Cmb From Geodesymentioning

confidence: 99%

Structure, Materials and Processes in the Earth’s Core and Mantle

et al. 2022

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“…However, for short length scales, where the viscous forces start to exceed inertial forces, and especially in the boundary layers, the effect of viscous dissipation may become important (Schaeffer et al 2017). Further analysis of waves associated with rapid SV observations can help determine where the energy of the wave is dissipated, whether it be within a boundary layer (Triana et al 2021) or in the internal shear layers as claimed by Buffett (2010). This latter mechanism was found to be inefficient by Lin and Ogilvie (2020), who observe in numerical simulations (for values of Ek as low as 10 −11 ) a too weak dissipation in conical shear layers, in order to explain the damping of the free inner core nutation.…”

Section: Excitation and Dissipation Of Waves/modesmentioning

confidence: 99%

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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“…However, we note from figures 2 and 4 that the decay rates from Ohmic The effect of the magnetic field on these modes has been the subject of several recent studies. Focusing on the spin-over node, [47] find that for Λ ≥ 1 and low Pm, Ohmic and viscous dissipation contribute nearly equally to the decay rate when the imposed field is uniform and the fluid is contained in a Earth-like shell with rigid boundary conditions. Working with stress-free boundary conditions in a spherical shell, [48] found that Ohmic dissipation was the predominant mechanism when P m = 10 −6 as in the Earth, Λ ≥ 10 −1 and the Ekman number E was 10 −9 .…”

Section: (G) Damping Of Fast Modesmentioning

confidence: 99%

Waves in the Earth's core. II. Magneto–Coriolis modes

2022

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We investigate the normal modes of a rapidly rotating, electrically conducting, inviscid fluid sphere in the presence of a background magnetic field. Neglecting the fast modes that are essentially slightly modified inertial waves, we focus on the slower branch of modes traditionally called magneto–Coriolis (MC) modes. We focus on the magnetostrophic limit, an approximation that neglects the fluid inertia for these modes by setting the magnetic Rossby number E η equal to zero, converting the momentum equation from a prognostic to a diagnostic equation. In contrast to a local analysis and contrary to previous assertions, this inertialess-inviscid double limit is perfectly well behaved and shows that E η → 0 and E η = 0 are essentially identical. With applied axisymmetric background fields, we find the modes are critically damped, with decay rates greater than or equal to the eigenfrequency, in contrast to the local theory. We find evidence for both eastward and westward propagating columnar modes, in contradiction of commonly perceived wisdom. The results bode well for numerical time integrations based on the magnetostrophic approximation, which is highly suited to the core of the Earth and holds the potential for more realistic solutions of the dynamo equations.

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The Viscous and Ohmic Damping of the Earth's Free Core Nutation

Cited by 9 publications

References 44 publications

Structure, Materials and Processes in the Earth’s Core and Mantle

Structure, Materials and Processes in the Earth’s Core and Mantle

A Dynamical Prospective on Interannual Geomagnetic Field Changes

Waves in the Earth's core. II. Magneto–Coriolis modes

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