Large-Scale Flow in the Core

Holme, Richard

doi:10.1016/b978-0-444-53802-4.00138-x

Cited by 75 publications

(55 citation statements)

References 123 publications

(128 reference statements)

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“…Estimating U based on the RMS velocity just below the CMB obtained from core flow inversions gives U = 3.8-5 × 10 −4 m s −1 (Holme, 2007). Together with η = 0.7 m 2 s −1 (Pozzo et al, 2012(Pozzo et al, , 2013, Rm ≈ 1200-1500.…”

Section: Geodynamo Modelsmentioning

confidence: 99%

Insights from geodynamo simulations into long-term geomagnetic field behaviour

Davies

Constable

2014

Earth and Planetary Science Letters

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8Detailed knowledge of the long-term spatial configuration and temporal variability of the geomagnetic field is lacking because of insufficient data for times prior to 10 ka. We use realisations from suitable numerical simulations to investigate three important questions about stability of the geodynamo process: is the present field representative of the past field; does a time-averaged field actually exist; and, supposing it exists, how long is needed to define such a field. Numerical geodynamo simulations are initially selected to meet existing criteria for morphological similarity to the observed magnetic field. A further criterion is introduced to evaluate similarity of long-term temporal variations. Allowing for reasonable uncertainties in the observations, observed and synthetic axial dipole moment frequency spectra for time series of order a million years in length should be fit by the same power law model. This leads us to identify diffusion time as the appropriate time scaling for such comparisons. In almost all simulations, intervals considered to have good morphological agreement between synthetic and observed field are shorter than those of poor agreement. The time needed to obtain a converged estimate of the time-averaged field was found to be comparable to the length of the simulation, even in non-reversing Preprint submitted to Earth and Planetary Science LettersJuly 25, 2014 models, suggesting that periods of stable polarity spanning many magnetic diffusion times are needed to obtain robust estimates of the mean dipole field. Long term field variations are almost entirely attributable to the axial dipole; non-zonal components converge to long-term average values on relatively short timescales (15 − 20 kyr). In all simulations, the time-averaged spatial power spectrum is characterised by a zigzag pattern as a function of spherical harmonic degree, with relatively higher power in odd degrees than in even degrees. We suggest that long-term spatial characteristics of the observed field may emerge on averaging times that are within reach for the next generation of global time-varying paleomagnetic field models.

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Section: Geodynamo Modelsmentioning

confidence: 99%

Insights from geodynamo simulations into long-term geomagnetic field behaviour

Davies

Constable

2014

Earth and Planetary Science Letters

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“…Temporal changes in the geomagnetic field termed secular variation (SV) provide vital insight into the fluid dynamics and dynamo action at the top of the core. Indeed, geomagnetic field and SV models (e.g., Jackson et al 2000;Olsen and Mandea 2008) have been used as constraints on numerical dynamo simulations (e.g., Christensen et al 1998Christensen et al , 2010Aubert et al 2013) or to infer various aspects of Earth's core dynamics (e.g., Finlay and Jackson 2003), in particular the fluid flow just below the CMB (for a review, see Holme 2007).…”

Section: Introductionmentioning

confidence: 99%

“…Global core flow models inverted from the geomagnetic SV can be constructed with and without poloidal flow (Holme 2007). Based on various theoretical arguments, most models assume some relation between the toroidal and poloidal flows.…”

Section: Introductionmentioning

confidence: 99%

Magnetic field stretching at the top of the shell of numerical dynamos

2016

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The process of magnetic field stretching transfers kinetic energy to magnetic energy and by that maintains dynamos against Ohmic dissipation. Stretching at the top of the outer core may play an important role at specific regions. High-latitude intense magnetic flux patches may be concentrated by flow convergence. Reversed flux patches may emerge due to expulsion of toroidal field advected to the core-mantle boundary by fluid upwelling. Here we analyze snapshots from self-consistent 3D numerical dynamos to unravel the nature of field-flow interactions that induces stretching secular variation at the top of the core. We find that stretching at the top of the shell has a significant influence on the secular variation despite the relatively weak poloidal flow. In addition, locally stretching is often more effective than advection in particular at regions of significant field-aligned flow. Magnetic flux patches are concentrated by fluid downwelling and dispersed by fluid upwelling. Stretching is more efficient than advection in intensifying magnetic flux patches. Both stretching and the poloidal flow mostly depend on the magnetic Prandtl number Pm. Decreasing Pm gives smaller poloidal flow but stronger stretching. Accounting for field-flow interactions in both the advection and stretching terms suggests that the magnetic Reynolds number overestimates the actual ratio of magnetic advection to diffusion by ∼50 %. Morphological resemblance between local stretching in our dynamo models and local observed geomagnetic secular variation may suggest the presence of stretching at the top of the Earth's core. Our results shed light on the kinematic origin of intense geomagnetic flux patches and may have implications to the convective state of the upper outer core.

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“…The frozen flux approximation is widely applied to the several decade data that include the satellite observation. Further assumptions are made such as tangentially geostrophy or quasi-geostrophy (see Bloxham and Jackson 1991;Holme 2015;Finlay et al 2010, for review). In the tangentially geostrophic assumption, core surface flow has been estimated by assuming that the Lorentz force is negligibly weak at the core surface and the geostrophic balance holds.…”

Section: Geomagnetic Secular Variation and Westward Driftmentioning

confidence: 99%

“…The uniform drift rate, one of the characteristic features of the drifting field, implies that the drift velocity is nondispersive. It has long been a matter of controversy whether the westward drift is due to a material flow or a hydromagnetic wave (Bullard et al 1950;Hide 1966;Holme 2015). The uniform drift rate seems to favor the material flow, in which the field is generated and conveyed together.…”

Section: Outline Of the Generating Processmentioning

confidence: 99%

A generating process of geomagnetic drifting field

Yukutake

Shimizu

2018

Earth Planets Space

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The geomagnetic field is comprised of drifting and standing fields. The drifting field has two remarkable features. One is predominance of sectorial harmonics when the field is expressed in a spherical harmonic series, and the other is uniform drift rate irrespective of harmonics. We consider that the drifting field is a product of interaction of the core flow with the axial dipole field near the surface of the core. The key to the predominance of sectorial harmonics is in the boundary condition on the electric current at the core-mantle boundary. If we take the mantle to be an electrical insulator, the electric current normal to the boundary must vanish. This strongly constrains the surface flow. The toroidal flow becomes the flow with the sectorial harmonics predominant. Then, the sectorial toroidal flow, interacting with the axial dipole field, induces the poloidal field in which the sectorial harmonics are predominant. This is the observed type of drifting field. The uniform drift rate, the second nature of the drifting field, seems to suggest that the surface part of the core is rotating westwards as a whole. Subsequently, the sectorial type toroidal flow embedded in the westward-rotating surface layer is considered as the cause of the drifting field.

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Large-Scale Flow in the Core

Cited by 75 publications

References 123 publications

Insights from geodynamo simulations into long-term geomagnetic field behaviour

Insights from geodynamo simulations into long-term geomagnetic field behaviour

Magnetic field stretching at the top of the shell of numerical dynamos

A generating process of geomagnetic drifting field

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