ALBERT

Description: We analyse models describing time changes of the Earth’s core magnetic field (secular variation) covering the historical period (several centuries) and the more recent satellite era (previous decade), and we illustrate how both the information contained in the data and the a priori information (regularisation) affect the result of the ill-posed geomagnetic inverse problem. We show how data quality, frequency and selection procedures govern part of the temporal changes in the secular variation norms and spectra, which are sometimes difficult to dissociate from true changes of the core state. We highlight the difficulty of resolving the time variability of the high degree secular variation coefficients (i.e. the secular acceleration), arising for instance from the challenge to properly separate sources of internal and of external origin. In addition, the regularisation process may also result in artificial changes in the model norms and spectra. Model users should keep in mind that such features can be miss-interpreted as the signature of physical mechanisms (e.g. diffusion). Finally, we present perspectives concerning core field modelling: imposing dynamical constraints (e.g. by means of data assimilation) reduces the non-uniqueness of the geomagnetic inverse problem. The article is also available electronically on SpringerLink: http://www.springerlink.com/openurl.asp?genre=article&id= doi:1.0.1007/s11214-009-9586-6

Description: In December 2019, the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group (V-MOD) adopted the thirteenth generation of the International Geomagnetic Reference Field (IGRF). This IGRF updates the previous generation with a definitive main field model for epoch 2015.0, a main field model for epoch 2020.0, and a predictive linear secular variation for 2020.0 to 2025.0. This letter provides the equations defining the IGRF, the spherical harmonic coefficients for this thirteenth generation model, maps of magnetic declination, inclination and total field intensity for the epoch 2020.0, and maps of their predicted rate of change for the 2020.0 to 2025.0 time period.

Description: 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.

Description: The variations of the Earth’s length of day (LOD) exhibit several interannual oscillations. The first detected one, with a period of 5.9-year, is better seen after removing the atmospheric contribution. More recently was isolated an oscillation of period ~8.5-year, distinct from the previous one. Torsional waves could explain both signals, although recently discovered non-axisymmetric magneto-Coriolis waves may also be involved. With a much smaller amplitude, a 7.3-year fluctuation was also detected in LOD data but no physical mechanism has been proposed. We show using a continuous wavelet transform analysis of synthetic oscillators embedded into a random noise, the limits of isolating damped signals with nearby periods in time series of limited duration. In particular, we emphasize the possibility that the 7.3-year oscillation could originate from an artifact due to the restricted length of the LOD series. Finally, we perform a wavelet coherence analysis between geodetically observed LOD variations and predicted LOD changes from geomagnetically inferred core flow models. Observed coherence at 5.9-year and ~8.5-year periods most probably confirms the fluid outer core origin for these two oscillations. Two coherent oscillations around 3.5 and 5-year periods are also conspicuous. Torsional Alfvén waves and/or Quasi-geostrophic magneto-Coriolis waves are natural explanations to the presence of various quasi-periodic signals in the LOD at interannual time-scales.

Description: We now benefit of more than two decades of global geomagnetic surveys from low-Earth orbiting satellites. They have revealed repeated interannual changes, more intense towards the equator. In order to reconstruct the dynamics at the surface of the core, these observational constraints are introduced in the “pygeodyn” data assimilation tool. This algorithm incorporates spatio-temporal constraints derived from geodynamo numerical simulations approaching Earth’s conditions. We discovered recurrent non-axisymmetric flow patterns presenting a period of about 7 yr. They propagate equatorward throughout the fluid core, and present their strongest amplitude (~3 km/yr) at the equator, where they show a coherent westward drift at phase speeds of about 1,500 km/yr. We interpret and model these flows as the signature of Magneto-Coriolis waves. Using synthetic data from dynamo simulations, we show that the recovery of such transient motions depends mostly on the data coverage and on their magnitude. The identification of Magneto-Coriolis waves offers a way to probe the cylindrical radial component of the dynamo field inside Earth’s core, and possibly to sample lateral variations in the electrical conductivity of the lower mantle. It follows from our work that there is no need for a stratified layer at the top of the core to account for these geomagnetic field changes. Such waves most likely also exist on longer time-scales, calling for long-lived magnetic records from space.

Description: One of the crucial observational quantities leading to information about the core is the geomagnetic field and its temporal variations. Over the satellite era, precise magnetic field measurements from satellites in low Earth orbits have provided novel information on small-scale field features exhibiting short time scale variations. Could add a sentence here: Satellite measurements compliment ground observatory measurements by providing global coverage of data collection, new data products (such as Geomagnetic Virtual Observatories), and the Swarm mission has allowed us to consider along- and across-track gradiometry to extract different geomagnetic sources more easily. Many models describing the core magnetic field, the secular variation and secular acceleration exist, and are maintained by different research groups. However, no clear general overview of the cross-compared quality of these models at the core-mantle boundary (top of the source region) is available. An evaluation of the core field and its variations as estimated by a three geomagnetic models with global coverage is presented here. The considered models are CHAOS, COVOBS and KALMAG, with an emphasis over the last decades. The differences between these models and the implications of choosing these compared to IGRF are presented and discussed.

Description: Geomagnetic jerks - abrupt changes in the acceleration of Earth's magnetic field that punctuate geomagnetic records - have been richly documented over the past decades by taking advantage of the complementary strengths of ground observatory and satellite measurements. Here we explore the morphological properties and generation mechanisms of 14 synthetic events obtained in a 14000-yr long temporal sequence from a numerical geodynamo simulation operating in the vicinity of Earth's core conditions. The majority of jerk events are found to arise from intermittent local disruptions of the leading-order force balance between the pressure, Coriolis, buoyancy and Lorentz forces (the QG-MAC balance), that leads to an inertial compensation through the emission of rapid Alfvén waves from the region where this force balance is disrupted. Jerk events of moderate strength arise from the arrival at low latitudes of waves emitted from convective plumes rooted at the inner core boundary. Significantly stronger jerks arise from waves generated near the surface of the core by arriving buoyancy plumes and magnetic field rearrangement. Most well-known features of geomagnetic jerks identified in ground-based observatory records as well as in recent satellite data are reproduced by the ensemble of synthetic events. In particular, low-latitude pulses of geomagnetic acceleration associated to rapid core surface flows, and nearly synchronous 'V-shaped' magnetic variation patterns seen over a wide portion of Earth's surface are systematically observed. Irrespectively of the event strength, our results support the hypothesis of a single physical root cause for jerks observed throughout the geomagnetic record.