Geomagnetic jerks as core surface flow acceleration pulses – observations and simulations.

The International Geomagnetic Reference Field (IGRF) has been regularly updated since its inception in 1965. Every recent iteration contains an estimate of the geomagnetic secular variation (SV), for the intermediate years between iterations. We submit a candidate model for the geomagnetic secular variation (SV) for the period 2025–2030 for the 14th generation of the IGRF. Given the recent evidence in the geomagnetic SV record for core surface waves, we forecast SV based on the periodic behaviour of core surface flow acceleration. We obtain an advective core surface flow model, in terms of poloidal and toroidal flow coefficients, from spatial gradients of SV geomagnetic virtual observatory data from the low-Earth orbiting CHAMP and Swarm missions from January 2001 to January 2010, and April 2014 to January 2024, respectively. From these, we calculate the flow acceleration coefficients from the first time-derivative. This assumes the flow is spatio-temporally simple, without imposing any physical constraints on its geometry. We fit each acceleration coefficient with a sinusoidal function, which is used to extrapolate 6 years into the future. These sinusoidally varying acceleration time series are integrated over time to obtain the core flow coefficients, which are then used to predict the average advected SV over the 5-year IGRF period. We recreate previous IGRF predictions using our CHAMP-based flows to validate our methodology, which we find to outperform previous IGRF iterations, and use the Swarm-based flows to forecast the SV for IGRF-14. Our Swarm-based model predicts sudden changes in SV—also known as geomagnetic jerks—in 2024 in the Equatorial Pacific, and in 2028 in the region around central Africa. Although the IGRF SV is a snapshot over a 5-year period, allowing for periodic behaviour offers potential improvements over other methods of prediction.Graphical Supplementary InformationThe online version contains supplementary material available at 10.1186/s40623-025-02347-x.

Geomagnetic field data from four observatories and annual field surveys between 2005 and 2015 provide a detailed description of Earth’s magnetic field changes over South Africa, Namibia and Botswana on time scales of less than 1 year. The southern African area is characterized by rapid changes in the secular variation pattern and lies in close proximity to the South Atlantic Anomaly (SAA) where the geomagnetic field intensity is almost 30 % weaker than in other regions at similar latitudes around the globe. Several geomagnetic secular acceleration (SA) pulses (geomagnetic jerks) around 2007, 2010 and 2012 could be identified over the last decade in southern Africa. We present a new regional field model for declination and horizontal and vertical intensity over southern Africa (Southern African REGional (SAREG)) which is based on field survey and observatory data and covering the time interval from 2005 to 2014, i.e. including the period between 2010 and 2013 when no low Earth-orbiting vector field satellite data are available. A comparative evaluation between SAREG and global field models like CHAOS-5, the CHAMP, Orsted and SAC-C model of the Earth's magnetic field and International Geomagnetic Reference Field (IGRF-12) reveals that a simple regional field model based on a relatively dense ground network is able to provide a realistic representation of the geomagnetic field in this area. We particularly note that a global field model like CHAOS-5 does not always indicate similar short-period patterns in the field components as revealed by observatory data, while representing the general secular variation reasonably well during the time interval without near-Earth satellite vector field data. This investigation further shows the inhomogeneous occurrence and distribution of secular variation impulses in the different geomagnetic field components and at different locations in southern African.

Investigating the 2003 geomagnetic jerk by simultaneous inversion of the secular variation and acceleration for both the core flow and its acceleration

Secular variation of the Earth's main magnetic field is believed to originate in the Earth's core. (The main field is operationally defined as comprising spherical harmonics of degree l⩽10.) I propose a different mechanism of secular variation: ocean water being a conductor of electricity, the magnetic field induced by the ocean as it flows through the Earth's main field may depend on time and manifest itself globally as secular variation. This proposal is supported by calculation of secular variation using the induction equation of magnetohydrodynamics, the observed main field and the ocean flow field. The predicted secular variation is in rough agreement with that observed. Additional support is provided by the striking temporal correlation (hitherto unsuspected) between the intensity of the North Atlantic oceanic circulation and the rate of secular variation in Western Europe; this explains, in particular, the geomagnetic jerks, and the recently discovered correlation between secular variation and climate. Spatial correlation between ocean currents and secular variation is also strong.

Detection of secular acceleration pulses from magnetic observatory data

SUMMARYWe make use of recent geodynamo simulations to propose a reduced stochastic model of the dynamics at the surface of Earth’s core. On decadal and longer periods, this model replicates the most energetic eigen directions of the geodynamo computation. Towards shorter timescales, it proposes a compensation for weaknesses of these simulations. This model furthermore accounts for the signature, in the geomagnetic secular variation, of errors of representativeness associated with unresolved processes. We incorporate the reduced stochastic model into a geomagnetic data assimilation algorithm—an augmented state ensemble Kalman filter—and apply it to re-analyse magnetic field changes over the period 1880–2015. Errors of representativeness appear to be responsible for an important fraction of the observed changes in the secular variation, as it is the case in the dynamo simulation.Recovered core surface motions are primarily symmetric with respect to the equator. We observe the persistence of the eccentric westward gyre over the whole studied era and vortices that partly follow isocontours of the radial magnetic field at the core surface. Our flow models provide a good fit to decadal changes in the length-of-day and predict its interannual variations over the period 1940–2005. The largest core flow acceleration patterns are found in an equatorial belt below 10° in latitude and are associated with non-axisymmetric features. No systematic longitudinal drift of acceleration patterns is found, even over the past decades where satellite data are available. The acceleration of the high-latitude westward jet in the Pacific hemisphere is, during the satellite era, a factor 5 smaller than previously reported and its structure shows some evidence for equatorial asymmetry. The era of continuous satellite records provides enhanced contrast on the rapid core flow variations. The proposed assimilation algorithm offers the prospect of evaluating Earth-likeness of geodynamo simulations.

By means of a Monte Carlo simulation, we explored the extent to which large earthquakes may induce fluctuations on the secular variation of the Earth's magnetic field. We speculate that the energy released by large earthquakes may perturb the dynamical interaction between the outer core and the lower mantle, and consequently give rise to rapid variations of the main field detectable on the Earth's surface (geomagnetic jerks). In order to test the existence of a possible correlation between the seismic activity and the occurrence of magnetic jerks, we examined both the number of earthquakes and the total moment released in a lapse of time preceding the known geomagnetic jerks of this century. Our results are compatible with the hypothesis of a causal relationship between periods of time characterized by an anomalous global seismic activity and rapid changes of the main magnetic field. Several possible mechanisms accounting for such a correlation are discussed.

Improving forecasts of the temporal and spatial changes of the Earth's main magnetic field over periods of less than 5 years has important scientific and economic benefits. Various methods for forecasting the rate of change, or secular variation, have been tried over the past few decades, ranging from the extrapolation of trends in ground observatory measurements to computational geodynamo modeling with data assimilation from historical magnetic field models. We examine the utility of an intermediate approach, using temporally varying core surface flow models derived from relatively short periods of magnetic field data to produce, by advection, secular variation estimates valid for the Earth's surface. We describe a new method to compute a core flow changing linearly with time from magnetic secular variation and acceleration data. We invert a combination of data from the CHAMP satellite mission and ground observatories over the period 2001.0 to 2010.0 for a series of such models. We assess their ability to forecast magnetic field changes by comparing them to CHAOS‐4, a state‐of‐the‐art model using data from 1997 to 2014.5. We show that the magnetic field predictions tend to have a lower root‐mean‐square difference from CHAOS‐4 than the International Geomagnetic Reference Field or World Magnetic Map series of secular variation models.

SUMMARY The Earth’s magnetic field is mainly produced within the Earth’s liquid and electrically conducting core, as a result of a process known as the geodynamo. Many other sources also contribute to the magnetic signal accessible to observation at the Earth’s surface, partly obscuring the main core magnetic field signal. Thanks to a series of very successful satellites and to advances in magnetic field modelling techniques, considerable progress has, however, been made in the recent years toward better identifying the signal of each of these sources. In particular, temporal changes in the field of internal origin happen to be detectable now in spherical harmonic degrees up to, perhaps, 16. All of these changes are usually attributed to changes in the core field itself, the secular variation, on the ground that the lithospheric magnetization cannot produce such signals. It has, however, been pointed out, on empirical grounds, that temporal changes in the field of internal origin produced by the induced part of the lithospheric magnetization could dominate the core field signal beyond degree 22. This short note revisits this issue by taking advantage of our improved knowledge of the small-scale field changes and of the likely sources of the lithospheric field. We rely on a simple extrapolation of the observed spatial spectrum of the field changes beyond degree 16 and use a forward approach based on a recent geological model of lithospheric magnetization. This leads us to confirm that the main cause of the observed changes in the field of internal origin up to some critical degree, N C, is indeed likely to be the secular variation of the core field, but that the signal produced by the time-varying lithospheric field is bound to dominate and conceal the time-varying core signal beyond that critical degree, in very much the same way the permanent component of the lithospheric field dominates and conceals the permanent component of the core field beyond degree 14. All uncertainties taken into account, we estimate N C to lie between 22 and 24. We, however, also note that in practice, the main limitation to the observation of the core field small-scale secular variation is not so much its concealing by the field of lithospheric origin but its fast changing nature and small magnitude. This leads us to conclude that whereas cumulative small-scale lithospheric field changes might be detected some day, detection of core-field secular variation beyond degree 18 is likely to remain a severe challenge for some time.

The secular variation of the core field is generally characterized by smooth variations, sometimes interrupted by abrupt changes, named geomagnetic jerks. The origin of these events, observed and investigated for over three decades, is still not fully understood. Many fundamental features of geomagnetic jerks have been the subject of debate, including their origin internal or external to the Earth, their occurrence dates, their duration and their global or regional character. Specific tools have been developed to detect them in geomagnetic field or secular variation time series. Recently, their investigation has been advanced by the availability of a decade of high-quality satellite measurements. Moreover, advances in the modelling of the core field and its variations have brought new perspectives on the fluid motion at the top of the core, and opened new avenues in our search for the origin of geomagnetic jerks. Correlations have been proposed between geomagnetic jerks and some other geophysical observables, indicating the substantial interest in this topic in our scientific community. This paper summarizes the recent advances in our understanding and interpretation of geomagnetic jerks.

Based on an expanded network of world magnetic observatory data, a comprehensive analysis of secular geomagnetic variations observed over a 100-year period has been carried out to determine the role of jerks in the formation of secular variations belonging to the high-frequency range of the spectrum of variations of the Earth's main magnetic field. Jerks – sharp changes in the direction of the linear trend characteristic of time series of values of secular variations of the magnetic field generated in the Earth's liquid core over decades – have been identified. Epochs of global jerks, manifesting on the scale of the entire Earth's surface with an interval of about ten years, have been identified and their spherical harmonic models have been constructed. Analysis of the morphology of the global jerk fields confirmed the idea that their source are processes occurring in the Earth's liquid core. Spectral analysis of series of secular variations, performed by the Fourier series decomposition method, made it possible to identify the main periods constituting the spectrum of secular variations with T < 10 years. Taking into account the jerk phenomena showed that all identified periods of secular variations can be interpreted within the framework of the concept of geomagnetic jerks, stochastic in the variability of manifestation both in regional and temporal aspects. Based on the analysis of the results of the conducted comprehensive research, a hypothesis is put forward that the dynamics of the Earth's main magnetic field, within centennial intervals, is formed due to the spatio-temporal superposition of global jerks caused by a sharp restructuring of the flow system on the surface of the Earth's liquid core.

Aiming to recover the fluid flow at the Earth's Core–Mantle Boundary (CMB) and the possible relation with geomagnetic jerks, we use the global "gufm1" model which makes use of the expansion of each element of the geomagnetic field in spherical harmonics, covering the last 400 years. Under the frozen–flux approximation, the induction equation for the radial component of the geomagnetic field (Br) is employed due to the continuity of this component through the CMB. The velocity field at the CMB is essentially two-dimensional and is separated into toroidal and poloidal parts. Then these parts as well as the radial component of geomagnetic field and its secular variation are expanded in spherical harmonics series and are substituted in the induction equation. By involving the Elsasser and Gaunt integrals, a system of algebraic equations is obtained. Solving this system of equations we recover the spherical harmonics coefficients of the two parts of the velocity field. The jerks, sharp change in the trend of secular variation of the geomagnetic field, are thought to be related to sharp changes in the velocity field at the CMB. There are jerks for which such changes are obtained, while in other cases this relation is not confirmed. Thus jerks may not be related to the velocity field at the CMB or small-scale velocity field (not studied here) may have its contribution.

The 1969 geomagnetic impulse and spin-up of the Earth's liquid core

We relax the steady-motions theorem by solving for a steady velocity field at the surface of the core in a frame of reference drifting at a linear rate with respect to an observer fixed in the mantle frame of reference. We make the frozen-flux approximation, and compare the misfit of the secular variation (SV) predicted by the drifting velocity field with that from a steady velocity field fixed to the mantle frame of reference. The decrease of the misfit to the geomagnetic SV across the period 1960–80 is substantial, but is marginal across the interval 1930–60. The drift rate changes sign at the 1970 geomagnetic‘jerk—epoch, indicating a change in phase speed between the mantle and core flow. The marginal decrease in misfit prior to 1960 is inadequate to fit the SV data, necessitating a more complex drift function or perhaps a fully time-dependent flow. The results suggest that the SV is driven by deep-seated convection rather than from the core-mantle boundary.

Reversal of drift direction during the Laschamp geomagnetic excursion