Advances in geodynamo modelling

Wicht, Johannes; Sanchez, Sabrina

doi:10.1080/03091929.2019.1597074

Cited by 50 publications

(43 citation statements)

References 131 publications

(242 reference statements)

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“…Here, g o is the outer boundary reference gravity, α the thermal expansivity, F the mass anomaly flux, ν the kinematic viscosity, κ the thermal diffusivity and the magnetic diffusivity. More details of the model are given in Christensen and Wicht (2015) or Wicht and Sanchez (2019). We used the numerical code Parody (Dormy 1997;Aubert et al 2008) with the additional implementation of the assimilation scheme PDAF (Fournier et al 2013).…”

Section: Background Dynamo Modelmentioning

confidence: 99%

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Sanchez

Bärenzung

2020

Earth Planets Space

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The IGRF offers an important incentive for testing algorithms predicting the Earth’s magnetic field changes, known as secular variation (SV), in a 5-year range. Here, we present a SV candidate model for the 13th IGRF that stems from a sequential ensemble data assimilation approach (EnKF). The ensemble consists of a number of parallel-running 3D-dynamo simulations. The assimilated data are geomagnetic field snapshots covering the years 1840 to 2000 from the COV-OBS.x1 model and for 2001 to 2020 from the Kalmag model. A spectral covariance localization method, considering the couplings between spherical harmonics of the same equatorial symmetry and same azimuthal wave number, allows decreasing the ensemble size to about a 100 while maintaining the stability of the assimilation. The quality of 5-year predictions is tested for the past two decades. These tests show that the assimilation scheme is able to reconstruct the overall SV evolution. They also suggest that a better 5-year forecast is obtained keeping the SV constant compared to the dynamically evolving SV. However, the quality of the dynamical forecast steadily improves over the full assimilation window (180 years). We therefore propose the instantaneous SV estimate for 2020 from our assimilation as a candidate model for the IGRF-13. The ensemble approach provides uncertainty estimates, which closely match the residual differences with respect to the IGRF-13. Longer term predictions for the evolution of the main magnetic field features over a 50-year range are also presented. We observe the further decrease of the axial dipole at a mean rate of 8 nT/year as well as a deepening and broadening of the South Atlantic Anomaly. The magnetic dip poles are seen to approach an eccentric dipole configuration.

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Section: Background Dynamo Modelmentioning

confidence: 99%

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Sanchez

Bärenzung

2020

Earth Planets Space

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“…As quoted the previous section ('Observational Constraints'), the secular variations of the spatial pattern of the geomagnetic field (geographical path of the polarity reversals and westward drift) may be interpreted by the geodynamo action with the core-mantle thermal coupling. There are a few good review articles concerning the core-mantle thermal coupling in the geodynamo modeling (Olson 2016;Wicht and Sanchez 2019). In this section, I introduce the summary of those reviews.…”

Section: Spatial Pattern Of the Cmb Heat Flux: Short-term Evolutionmentioning

confidence: 99%

A coupled core-mantle evolution: review and future prospects

Nakagawa

2020

Prog Earth Planet Sci

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In this review, I provide the current status and future prospects for the coupled core-mantle evolution and specifically summarize the constraints arising from geomagnetism and paleomagnetism on the long-term secular variations of the geomagnetic field. The heat flow across the core-mantle boundary (CMB) is essential for determining the best-fit scenario that explains the observational data of geomagnetic secular variations (e.g., onset timing of the inner core growth, geomagnetic polarity reversals, and westward drift) and should include the various origins of the heterogeneous structures in the deep mantle that have affected the heat transfer across the core-mantle boundary for billions of years. The coupled core-mantle evolution model can potentially explain the onset timing of the inner core and its influence on the long-term geomagnetic secular variations, but it is still controversial among modeling approaches on the core energetics because the paleomagnetic data contains various uncertainties. Additionally, with the coupled core-mantle evolution model in geodynamo simulations, the frequency of the geomagnetic polarity reversals can be explained with the time variations of the heat flow across the CMB. Additionally, the effects of the stable region in the outermost outer core to the magnetic evolution are also crucial but there would be still uncertain for their feasibility. However, despite this progress in understanding the observational data for geomagnetic secular variations, there are several unresolved issues that should be addressed in future investigations: (1) initial conditions—starting with the solidification of the global magma ocean with the onset timing of plate tectonics and geodynamo actions and (2) planetary habitability—how the dynamics of the Earth’s deep interior affects the long-term surface environment change that has been maintained in the Earth’s multisphere coupled system.

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“…Recent simulations have been able to probe more Earth-like parameter regimes than previously possible, achieving very low Ekman numbers of E = 10 −7 −10 −8 (Schaeffer et al 2017;Aubert 2019). However despite this progress, these simulations remain in parameter regimes vastly different to that of the Earth (Christensen &Wicht2015), posing the inescapable question of how representative of the Earth they really are, as force balances can still vary significantly between the simulation regime and the correct regime of the Earth (Wicht & Sanchez 2019), with the ability to simultaneously reproduce Earth-like field morphology and reversal frequency still beyond current capabilities (Christensen et al 2010). The assessments conducted by Sprain et al (2019) highlight that present geodynamo models are unable to satisfactorily reproduce all aspects of Earth's long-term field behaviour.…”

Section: The Earth-like Magnetostrophic Force Balancementioning

confidence: 99%

Enhanced magnetic fields within a stratified layer

Hardy

Livermore

Niesen

2020

Geophysical Journal International

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SUMMARY Mounting evidence from both seismology and numerical experiments on core composition suggests the existence of a layer of stably stratified fluid at the top of Earth’s outer core. In such a layer, a magnetostrophic force balance and suppressed radial motion lead to stringent constraints on the magnetic field, named Malkus constraints, which are a much more restrictive extension of the well known Taylor constraints. Here, we explore the consequences of such constraints for the structure of the core’s internal magnetic field. We provide a new simple derivation of these Malkus constraints, and show solutions exist which can be matched to any external potential field with arbitrary depth of stratified layer. From considerations of these magnetostatic Malkus constraints alone, it is therefore not possible to uniquely infer the depth of the stratified layer from external geomagnetic observations. We examine two models of the geomagnetic field defined within a spherical core, which obey the Taylor constraints in an inner convective region and the Malkus constraints in an outer stratified layer. When matched to a single-epoch geomagnetic potential field model, both models show that the toroidal magnetic field within the outer layer is about 100 times stronger compared to that in the inner region, taking a maximum value of 8 mT at a depth of 70 km. The dynamic regime of such a layer, modulated by suppressed radial motion but also a locally enhanced magnetic field, may therefore be quite distinct from that of any interior dynamo.

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Advances in geodynamo modelling

Cited by 50 publications

References 131 publications

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

Predictions of the geomagnetic secular variation based on the ensemble sequential assimilation of geomagnetic field models by dynamo simulations

A coupled core-mantle evolution: review and future prospects

Enhanced magnetic fields within a stratified layer

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