COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations

Huder, Loïc; Gillet, Nicolas; Finlay, Christopher C.; Hammer, Magnus D.; Tchoungui, Hervé

doi:10.1186/s40623-020-01194-2

Cited by 38 publications

(39 citation statements)

References 58 publications

(128 reference statements)

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“…For the main field at epochs 2015.0 and 2020.0, some teams derived their candidate models from parent models describing the magnetic field over multi-decadal periods by combining datasets from several satellite missions and ground observatories Huder et al 2020;Ropp et al 2020;Sabaka et al 2020;Wardinski et al 2020). Several teams built parent models covering the full Swarm satellite mission era (November 2013 to present) (Brown et al 2020;Rother et al 2020;Vigneron et al 2020).…”

Section: Introductionmentioning

confidence: 99%

“…Other teams built dedicated main field candidate models for each of the epochs requested by the call, thus using data within smaller time windows centered on 2015.0, or immediately preceding 2020.0 (Pavón-Carrasco et al 2020;Yang et al 2020;Alken et al 2020a;Petrov and Bondar 2020), which required less complex parameterization in time. Some teams co-estimated a low-degree external field model representing magnetospheric sources (Brown et al 2020;Yang et al 2020;Pavón-Carrasco et al 2020;Rother et al 2020;Ropp et al 2020;Huder et al 2020;Sabaka et al 2020;Vigneron et al 2020). Two teams co-estimated additional geomagnetic source fields in their parent models (Ropp et al 2020;Sabaka et al 2020).…”

Section: Introductionmentioning

confidence: 99%

“…For the predictive secular variation models covering 2020.0 to 2025.0, the candidates broadly fall into two main categories: (1) computing SV solely from the latest available satellite and ground data (Alken et al 2020a;Pavón-Carrasco et al 2020;Huder et al 2020;Petrov and Bondar 2020;Rother et al 2020), or (2) applying physics-based modeling, combined with recent satellite and ground data to forecast future field changes based on underlying core dynamics (Brown et al 2020;Fournier et al 2020;Minami et al 2020;Metman et al 2020;Sanchez et al 2020;Tangborn et al 2020 et al 2020). Accurately forecasting the temporal evolution of the main geomagnetic field is nontrivial, due to challenges such as the low resolution of the recoverable magnetic field at the core-mantle boundary, the occurrence of unpredictable geomagnetic jerks or the uncertainty associated with diffusion of the field over short timescales (e.g.…”

Section: Introductionmentioning

confidence: 99%

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Evaluation of candidate models for the 13th generation International Geomagnetic Reference Field

Alken

Thébault

Beggan

et al. 2021

Earth Planets Space

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In December 2019, the 13th revision of the International Geomagnetic Reference Field (IGRF) was released by the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group V-MOD. This revision comprises two new spherical harmonic main field models for epochs 2015.0 (DGRF-2015) and 2020.0 (IGRF-2020) and a model of the predicted secular variation for the interval 2020.0 to 2025.0 (SV-2020-2025). The models were produced from candidates submitted by fifteen international teams. These teams were led by the British Geological Survey (UK), China Earthquake Administration (China), Universidad Complutense de Madrid (Spain), University of Colorado Boulder (USA), Technical University of Denmark (Denmark), GFZ German Research Centre for Geosciences (Germany), Institut de physique du globe de Paris (France), Institut des Sciences de la Terre (France), Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (Russia), Kyoto University (Japan), University of Leeds (UK), Max Planck Institute for Solar System Research (Germany), NASA Goddard Space Flight Center (USA), University of Potsdam (Germany), and Université de Strasbourg (France). The candidate models were evaluated individually and compared to all other candidates as well to the mean, median and a robust Huber-weighted model of all candidates. These analyses were used to identify, for example, the variation between the Gauss coefficients or the geographical regions where the candidate models strongly differed. The majority of candidates were sufficiently close that the differences can be explained primarily by individual modeling methodologies and data selection strategies. None of the candidates were so different as to warrant their exclusion from the final IGRF-13. The IAGA V-MOD task force thus voted for two approaches: the median of the Gauss coefficients of the candidates for the DGRF-2015 and IGRF-2020 models and the robust Huber-weighted model for the predictive SV-2020-2025. In this paper, we document the evaluation of the candidate models and provide details of the approach used to derive the final IGRF-13 products. We also perform a retrospective analysis of the IGRF-12 SV candidates over their performance period (2015–2020). Our findings suggest that forecasting secular variation can benefit from combining physics-based core modeling with satellite observations.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Evaluation of candidate models for the 13th generation International Geomagnetic Reference Field

Alken

Thébault

Beggan

et al. 2021

Earth Planets Space

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“…Being constructed upon adjusted regularizations, CHAOS-7 is not provided with uncertainties. • COV-OBS-x2 (Huder et al 2020) covers the entire observatory era since 1840. Satellite data are considered by means of virtual observatories (Hammer et al 2021a).…”

Section: Observational Constraint On Rapid Magnetic Field Changesmentioning

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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“…9 to produce a forecast, by resorting instead to a "calibrated" stochastic equation to advance B in time, in the form of a collection of auto-regressive processes whose characteristic time scales are precisely the time scales discussed here. (In the IGRF context, see the candidates proposed by Gillet et al 2015;Baerenzung et al 2020;Huder et al 2020)…”

Section: Time Scales Of the Secular Variation And Secular Accelerationmentioning

confidence: 99%

Physics-based secular variation candidate models for the IGRF

Fournier

Aubert

Lesur

et al. 2021

Earth Planets Space

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Each International Geomagnetic Reference Field (IGRF) model released under the auspices of the International Association of Geomagnetism and Aeronomy comprises a secular variation component that describes the evolution of the main magnetic field anticipated for the 5 years to come. Every Gauss coefficient, up to spherical harmonic degree and order 8, is assumed to undergo its own independent linear evolution. With a mathematical model of the core magnetic field and its time rate of change constructed from geomagnetic observations at hand, a standard prediction of the secular variation (SV) consists of taking the time rate of change of each Gauss coefficient at the final time of analysis as the predicted rate of change. The last three generations of the IGRF have additionally witnessed a growing number of candidate SV models relying upon physics-based forecasts. This surge is motivated by satellite data that now span more than two decades and by the concurrent progress in the numerical modelling of Earth’s core dynamics. Satellite data reveal rapid (interannual) geomagnetic features whose imprint can be detrimental to the quality of the IGRF prediction. This calls for forecasting frameworks able to incorporate at least part of the processes responsible for short-term geomagnetic variations. In this letter, we perform a retrospective analysis of the performance of past IGRF SV models and candidates over the past 35 years; we emphasize that over the satellite era, the quality of the 5-year forecasts worsens at times of rapid geomagnetic changes. After the definition of the time scales that are relevant for the IGRF prediction exercise, we cover the strategies followed by past physics-based candidates, which we categorize into a “‘core–surface flow” family and a “dynamo” family, noting that both strategies resort to “input” models of the main field and its secular variation constructed from observations. We next review practical lessons learned from our previous attempts. Finally, we discuss possible improvements on the current state of affairs in two directions: the feasibility of incorporating rapid physical processes into the analysis on the one hand, and the accuracy and quantification of the uncertainty impacting input models on the other hand.

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COV-OBS.x2: 180 years of geomagnetic field evolution from ground-based and satellite observations

Cited by 38 publications

References 58 publications

Evaluation of candidate models for the 13th generation International Geomagnetic Reference Field

Evaluation of candidate models for the 13th generation International Geomagnetic Reference Field

A Dynamical Prospective on Interannual Geomagnetic Field Changes

Physics-based secular variation candidate models for the IGRF

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