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Paleomagnetic records provide us with information about the extreme geomagnetic events known as excursions and reversals, but the sparsity of available data limits detailed knowledge of the process and timing. To date there are no agreed on criteria for categorizing such events in terms of severity or longevity. In an analogy to categorizing storms in weather systems, we invoke the magnitude of the global (modified) paleosecular variation index |$P_{i_D}$| to define the severity of the magnetic field state, ranging in level from 0 to 3, and defined by instantaneous values of |$P_{i_D}$| with level 0 being normal (⁠|$P_{i_D}\lt 0.5$|⁠) to extreme (⁠|$P_{i_D}\ge 15$|⁠). We denote the time of entry to an excursional (or reversal) event by when |$P_{i_D}$| first exceeds 0.5, and evaluate its duration by the time at which |$P_{i_D}$| first returns below its median value, termed the end of event threshold. We categorize each excursional event according to the peak level of |$P_{i_D}$| during the entire event, with a range from Category-1 (Cat-1) to Cat-3. We explore an extended numerical dynamo simulation containing more than 1200 events and find that Cat-1 events are the most frequent (72 per cent), but only rarely lead to actual field reversals where the axial dipole, |$g_1^0$|⁠ , has reversed sign at the end of the event. Cat-2 account for about 20 per cent of events, with 34 per cent of those leading to actual reversals, while Cat-3 events arise about 8 per cent of the time but are more likely to produce reversals (43 per cent). Higher category events take as much as 10 times longer than Cat-1 events. Two paleomagnetic field models separately cover the Laschamp excursion and Matuyama–Brunhes (M-B) reversal which are Cat-2 events with respective durations of 3.6 and 27.4 kyr. It seems likely that Cat-2 may be an underestimate for M-B due to limitations in the paleomagnetic records. Our overall results suggest no distinction between excursions and reversals other than a reversal having the ending polarity state opposite to that at the start. [ABSTRACT FROM AUTHOR]

Abstract A technique for predicting the secular variation (SV) of the geomagnetic field based on the echo state network (ESN) model is proposed. SV is controlled by the geodynamo process in the Earth’s outer core, and modeling its nonlinear behaviors can be challenging. This study employs an ESN to represent the short-term temporal evolution of the geomagnetic field on the Earth’s surface. The hindcast results demonstrate that the ESN enables us to predict SV for a duration of several years with satisfactory accuracy. It is also found that the prediction is robust to the length of the the training data period. This suggests that the recent features of the SV are important for short-term prediction and that the ESN effectively learns these features. Graphical Abstract

A technique for predicting the secular variation (SV) of the geomagnetic field based on the echo state network (ESN) model is proposed. SV is controlled by the geodynamo process in the Earth's outer core, and modeling its nonlinear behaviors can be challenging. This study employs an ESN to represent the short-term temporal evolution of the geomagnetic field on the Earth's surface. The hindcast results demonstrate that the ESN enables us to predict SV for a duration of several years with satisfactory accuracy. It is also found that the prediction is robust to the length of the the training data period. This suggests that the recent features of the SV are important for short-term prediction and that the ESN effectively learns these features. [ABSTRACT FROM AUTHOR]

Recent studies on the behavior of geomagnetic secular acceleration (SA) pulses have provided a basis for understanding the dynamic processes in the Earth's core. This analysis statistically evaluates the evolution of the SA pulse amplitude and position since 2000 by computing the three-year difference in SA with the CHAOS-7 geomagnetic field model (CHAOS-7.17 release). Furthermore, the study explores the correlation between the acceleration pulse amplitude and geomagnetic jerks and the dynamic processes of alternating variation and polarity reversal of pulse patches over time. Research findings indicate that the variation in pulse amplitude at the Core Mantle Boundary (CMB) closely resembles that observed at the Earth's surface, with an average period of 3.2 years. The timing of peak pulse amplitude aligns with that of the geomagnetic jerk, suggesting its potential utility as a novel indicator for detecting geomagnetic jerk events. The acceleration pulses are the strongest near the equator (2°N) and more robust in the high-latitude region (68°S) of the Southern Hemisphere, indicating that the variation is more dramatic in the Southern Hemisphere. The acceleration pulses fluctuate unevenly in the west-east direction, with characteristics of local variation. In the Western Hemisphere, the pulse patches are distributed near the equator, exhibiting an evident westward drifting mode. The positive and negative patches alternate in time, displaying a polarity reversal in the west-east direction, with an average interval of approximately 32°. These characteristics can be attributed to the rapid magnetic field fluctuations disclosed by the model of stratification at the top of the Earth's core. In the Eastern Hemisphere, the pulses are weaker between 10°E and 60°E, with the most active pulses occurring around 80°E to 105°E and near 150°E. The pulse patches exhibit a broader distribution in the north-south direction, with relatively strong patches still occurring near 40°N and 40°S. These local variation characteristics match the actual cases of zonal flows and geostrophic Alfven waves in the Earth's core. [ABSTRACT FROM AUTHOR]

The rich and extensively studied archaeological record of the Near East provides an opportunity to develop a comprehensive archaeomagnetic dataset for exploring the behavior of the geomagnetic field with high precision. The Levantine archaeomagnetic curve (LAC) project is an ongoing effort to develop a continuous high‐resolution geomagnetic intensity curve for the Levant and Mesopotamia. The first version of the LAC covered the period between 3000 and 550 BCE. Here, we report archaeointensity data from 169 samples compiled into 32 groups dating between the 7th and the 1st centuries BCE aiming at extending the LAC up to the end of the first millennium BCE. Twenty‐two groups are assembled from storage jar handles bearing different types of royal seal impressions, which were used in Judah as part of a taxation administrative system. These groups are combined with 10 other groups of pottery assemblages, three of which are from Hellenistic destruction layers dated using radiocarbon and coins. The new curve shows that the Levantine Iron Age Anomaly (LIAA) spanned 550 years (1100 ‐ 550 BCE) and that the rate of decline during the last spike around 600 BCE could have reached ∼0.6 μT/year. During the 6th century, the virtual axial dipole moment (VADM) dropped from 160 ZAm2 to 125 ZAm2 after which field intensity only slightly increased to 135 ZAm2, until another considerable decline to ∼90 ZAm2 during the 3rd to the 1st centuries BCE. We highlight the archaeomagnetic implication of the new curve in inferring the relative chronological relationship between different stamp types. Plain Language Summary: The Earth's magnetic field is continuously changing both in time and space in an unpredictable manner. A detailed description of how the magnetic field has changed throughout Earth's history offers constraints on our understanding of the mechanism generating the field in Earth's core. In this study, we reconstruct the intensity of the past field using an assemblage of well‐dated archaeological materials from Israel, dated to the Assyrian, Babylonian, Persian, Hellenistic and Hasmonean periods. This work is part of an ongoing effort to procure a high‐resolution curve describing the changes in field intensity for the Levant and Mesopotamia over the past several millennia. With the new data, we calculate the curve for the first three millennia BCE. The curve provides further details on an anomalous behavior of the field between 1100 BCE and 550 BCE, termed the Levantine Iron Age Anomaly (LIAA), during which the field intensity and its rate of change were significantly higher than today's. In addition, the curve forms the basis for an archaeomagnetic dating tool, which can be especially useful for periods when traditional archaeological dating methods fail to provide precise ages due to large uncertainties in radiocarbon dates. Key Points: Archaeomagnetic intensity data from 32 groups of pottery in Israel dated between the 7th and the 1st centuries BCEThe second generation of the Levantine Archaeomagnetic Curve (LAC.v.2.0) covering the last three millennia BCEThe new data constrain the duration of the Levantine Iron Age Anomaly (LIAA) from 1100 BCE to 550 BCE [ABSTRACT FROM AUTHOR]

Recent studies on the behavior of geomagnetic secular acceleration (SA) pulses have provided a basis for understanding the dynamic processes in the Earth’s core. This analysis statistically evaluates the evolution of the SA pulse amplitude and position since 2000 by computing the three-year difference in SA with the CHAOS-7 geomagnetic field model (CHAOS-7.17 release). Furthermore, the study explores the correlation between the acceleration pulse amplitude and geomagnetic jerks and the dynamic processes of alternating variation and polarity reversal of pulse patches over time. Research findings indicate that the variation in pulse amplitude at the Core Mantle Boundary (CMB) closely resembles that observed at the Earth’s surface, with an average period of 3.2 years. The timing of peak pulse amplitude aligns with that of the geomagnetic jerk, suggesting its potential utility as a novel indicator for detecting geomagnetic jerk events. The acceleration pulses are the strongest near the equator (2°N) and more robust in the high-latitude region (68°S) of the Southern Hemisphere, indicating that the variation is more dramatic in the Southern Hemisphere. The acceleration pulses fluctuate unevenly in the west-east direction, with characteristics of local variation. In the Western Hemisphere, the pulse patches are distributed near the equator, exhibiting an evident westward drifting mode. The positive and negative patches alternate in time, displaying a polarity reversal in the west-east direction, with an average interval of approximately 32°. These characteristics can be attributed to the rapid magnetic field fluctuations disclosed by the model of stratification at the top of the Earth’s core. In the Eastern Hemisphere, the pulses are weaker between 10°E and 60°E, with the most active pulses occurring around 80°E to 105°E and near 150°E. The pulse patches exhibit a broader distribution in the north-south direction, with relatively strong patches still occurring near 40°N and 40°S. These local variation characteristics match the actual cases of zonal flows and geostrophic Alfvén waves in the Earth’s core.

Abstract The rich and extensively studied archaeological record of the Near East provides an opportunity to develop a comprehensive archaeomagnetic dataset for exploring the behavior of the geomagnetic field with high precision. The Levantine archaeomagnetic curve (LAC) project is an ongoing effort to develop a continuous high‐resolution geomagnetic intensity curve for the Levant and Mesopotamia. The first version of the LAC covered the period between 3000 and 550 BCE. Here, we report archaeointensity data from 169 samples compiled into 32 groups dating between the 7th and the 1st centuries BCE aiming at extending the LAC up to the end of the first millennium BCE. Twenty‐two groups are assembled from storage jar handles bearing different types of royal seal impressions, which were used in Judah as part of a taxation administrative system. These groups are combined with 10 other groups of pottery assemblages, three of which are from Hellenistic destruction layers dated using radiocarbon and coins. The new curve shows that the Levantine Iron Age Anomaly (LIAA) spanned 550 years (1100 ‐ 550 BCE) and that the rate of decline during the last spike around 600 BCE could have reached ∼0.6 μT/year. During the 6th century, the virtual axial dipole moment (VADM) dropped from 160 ZAm2 to 125 ZAm2 after which field intensity only slightly increased to 135 ZAm2, until another considerable decline to ∼90 ZAm2 during the 3rd to the 1st centuries BCE. We highlight the archaeomagnetic implication of the new curve in inferring the relative chronological relationship between different stamp types.

We use advanced numerical geodynamo series to derive a reduced stochastic model of the dynamics at the surface of Earth's core. Considering order 3 autoregressive (AR-3) processes allows to replicate the simulated spatiotemporal spectrum over a broad range of time-scales, spanning millennia to a fraction of year, including the cut-off found for periods shorter than approximately 2 years and associated with magnetic dissipation. We show how to derive such a forward model from a variety of input simulation series, and present its implementation into the pygeodyn data assimilation algorithm, based on a sequential ensemble method. The updated scheme is applied to perform magnetic field hindcasts and core flow reanalyses. For all observable length-scales, the rate of change of the observed magnetic field is most of the time accounted for within the spread of the forward model trajectories. AR-3 predictions on average supersede by about 35 % linear extrapolations on short (2 yr) time-scales, reducing high-frequency spurious variations in reanalysed flow motions. This improvement is reduced to ≈ 10 % for 5 yr increments, with a large variability from one epoch to the other depending on the overall curvature of the magnetic field evolution. We perform a reanalysis over the period 1880–2023 covered by observatory and satellite records. We find enhanced kinetic energy in three period ranges around 12.5, 6.5 and 3.5 years. At all three periods, fluid motions share geometrical properties compatible with quasi-geostrophic magneto-Coriolis waves: equatorial symmetry, larger amplitude near the equator, flow dominated by low azimuthal wave number and modulated in longitude, phase speed much faster than the fluid velocity and decreasing with the period. At 6.5 yr period we trace back to the mid-1990's the patterns previously detected from satellite data. We also find in the 1960–70's a similar wave-train, possibly in link with the 1969 geomagnetic jerk. The AR-3 model, in conjunction with early satellite records, likely helps isolate such coherent features on interannual time-scales. Similar wave-like motions also show up at 3.5 yr period around 1970 and during the past decades. At periods around 12.5 yr we detect recurrent patterns starting as far back as 1920, and modulated over decadal time-scales. Our results show growing evidence for core dynamics governed by the presence of hydro-magnetic waves over a wide range of periods. This may allow deterministic and/or empirical descriptions of the signal that may help sound deep Earth's properties, and improve predictions of the magnetic field evolution. • An order 3 stochastic model of the Earth's core surface dynamics is derived from advanced geodynamo simulations. • It mimics the cut-off seen in simulated PSD on periods shorter than the Alfvén time, related to magnetic dissipation. • The new model, plugged into a geomagnetic data assimilation algorithm, is beneficial for reanalysis and prediction purposes. • Application to secular variation data shows evidence for wave-like motions on various time-scales. • Flow patterns detected around 6.5 and 12.5 yr periods are coherent with Magneto-Coriolis waves. [ABSTRACT FROM AUTHOR]

Geomagnetic Secular Variation (SV) and acceleration (SA) can provide a wealth of information about fluid motions in the core. However, recovering this information from noisy data is complicated by the structure of the magnetic field at the core-mantle boundary. We demonstrate the challenge using a numerical model for equatorial waves to produce synthetic records of SV and SA. We show that interpreting magnetic oscillations directly as waves in Earth's core can lead to spurious conclusions due to coupling between non-dipole components of the magnetic field and wave structures. Using the structure of our modeled waves, we derive a set of methods to detect waves using magnetic field observations. We show that these methods can recover the wave period, longitudinal wavenumber, equatorial symmetry, latitudinal extent, and amplitude for synthetic equatorial waves in Earth's core when the wave fluid velocity is ∼0.5 km/yr or greater.

Geomagnetic Secular Variation (SV) and acceleration (SA) can provide a wealth of information about fluid motions in the core. However, recovering this information from noisy data is complicated by the structure of the magnetic field at the core-mantle boundary. We demonstrate the challenge using a numerical model for equatorial waves to produce synthetic records of SV and SA. We show that interpreting magnetic oscillations directly as waves in Earth's core can lead to spurious conclusions due to coupling between non-dipole components of the magnetic field and wave structures. Using the structure of our modeled waves, we derive a set of methods to detect waves using magnetic field observations. We show that these methods can recover the wave period, longitudinal wavenumber, equatorial symmetry, latitudinal extent, and amplitude for synthetic equatorial waves in Earth's core when the wave fluid velocity is ∼0.5 km/yr or greater.

Abstract 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.

Abstract We have produced a 5-year mean secular variation (SV) of the geomagnetic field for the period 2020–2025. We use the NASA Geomagnetic Ensemble Modeling System (GEMS), which consists of the NASA Goddard geodynamo model and ensemble Kalman filter (EnKF) with 400 ensemble members. Geomagnetic field models are used as observations for the assimilation, including gufm1 (1590–1960), CM4 (1961–2000) and CM6 (2001–2019). The forecast involves a bias correction scheme that assumes that the model bias changes on timescales much longer than the forecast period, so that they can be removed by successive forecast series. The algorithm was validated on the time period 2010-2015 by comparing with CM6 before being applied to the 2020–2025 time period. This forecast has been submitted as a candidate predictive model of IGRF-13 for the period 2020–2025. Graphical abstract

Abstract This paper describes the design of a candidate secular variation model for the 13th generation of the International Geomagnetic Reference Field. This candidate is based upon the integration of an ensemble of 100 numerical models of the geodynamo between epochs 2019.0 and 2025.0. The only difference between each ensemble member lies in the initial condition that is used for the numerical integration, all other control parameters being fixed. An initial condition is defined as follows: an estimate of the magnetic field and its rate-of-change at the core surface for 2019.0 is obtained from a year (2018.5–2019.5) of vector Swarm data. This estimate (common to all ensemble members) is subject to prior constraints: the statistical properties of the numerical dynamo model for the main geomagnetic field and its secular variation, and prescribed covariances for the other sources. One next considers 100 three-dimensional core states (in terms of flow, buoyancy and magnetic fields) extracted at different discrete times from a dynamo simulation that is not constrained by observations, with the time distance between each state exceeding the dynamo decorrelation time. Each state is adjusted (in three dimensions) in order to take the estimate of the geomagnetic field and its rate-of-change for 2019.0 into account. This methodology provides 100 different initial conditions for subsequent numerical integration of the dynamo model up to epoch 2025.0. Focussing on the 2020.0–2025.0 time window, we use the median average rate-of-change of each Gauss coefficient of the ensemble and its statistics to define the geomagnetic secular variation over that time frame and its uncertainties.

Variations of the Earth's geomagnetic field during the Holocene are important for understanding centennial to millennial-scale processes of the Earth's deep interior and have enormous potential implications for chronological correlations (e.g., comparisons between different sedimentary recording sequences, archaeomagnetic dating). Here, we present 21 robust archaeointensity data points from eastern China spanning the past ∼6 kyr. These results add significantly to the published data both regionally and globally. Taking together, we establish an archaeointensity reference curve for Eastern Asia, which can be used for archaeomagnetic dating in this region. Virtual axial dipole moments (VADMs) of the data range from a Holocene-wide low of ∼27 to "spike" values of ∼166 ZAm2 (Z: 1021). The results, in conjunction with our recently published data, confirm the existence of a decrease in paleointensity (DIP) in China around ∼2200 BCE. These low intensities are the lowest ever found for the Holocene and have not been reported outside of China. We also report a spike intensity of 165.8 ± 6.0 ZAm2 at ∼1300 BCE (±300 y), which is either a prelude to or the same event (within age uncertainties) as spikes first reported in the Levant.

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 τ A ≈ 2 yr, much shorter than the vortex turnover time τ 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 τ A and τ U . We finally summarize the main geophysical implications of the existence of this interannual dynamics on core and lower mantle structure, properties, and dynamics. [ABSTRACT FROM AUTHOR]

A spherical harmonic model of the magnetic field of Jupiter is obtained from vector magnetic field observations acquired by the Juno spacecraft during 32 of its first 33 polar orbits. These Prime Mission orbits sample Jupiter's magnetic field nearly uniformly in longitude (∼11° separation) as measured at equator crossing. The planetary magnetic field is represented with a degree 30 spherical harmonic and the external field is approximated near the origin with a simple external spherical harmonic of degree 1. Partial solution of the underdetermined inverse problem using generalized inverse techniques yields a model ("JRM33") of the planetary magnetic field with spherical harmonic coefficients reasonably well determined through degree and order 13. Useful information regarding the field extends through degree 18, well fit by a Lowes' spectrum with a dynamo core radius of 0.81 Rj, presumably the outer radius of the convective metallic hydrogen region. This new model provides a most detailed view of a planetary dynamo and evidence of advection of the magnetic field by deep zonal winds in the vicinity of the Great Blue Spot (GBS), an isolated and intense patch of flux near Jupiter's equator. Comparison of the JRM33 and JRM09 models suggests secular variation of the field in the vicinity of the GBS during Juno's nearly 5 years of operation in orbit about Jupiter. The observed secular variation is consistent with the penetration of zonal winds to a depth of ∼3,500 km where a flow velocity of ∼0.04 ms−1 is required to match the observations. Plain Language Summary: Characterizing the planetary magnetic field of Jupiter is one of the primary science objectives of the Juno Mission. Is the magnetic field generated within the outer envelope consisting mostly of molecular hydrogen, or is it generated at depth where hydrogen becomes metallic under great pressure? The Juno spacecraft, in polar orbit about Jupiter since July 2016, just completed its baseline mapping mission of 33 orbits, providing global coverage of Jupiter's magnetic field near the planet. A detailed representation of the field has emerged, suggesting that Jupiter's magnetic field is generated by dynamo action at depth (beneath 0.81 Rj) in convective metallic hydrogen. A change in Jupiter's magnetic field over time ("secular variation") was identified by comparison of the model field with that of an earlier model. The secular variation appeared on the flanks of an isolated magnetic patch (the "Great Blue Spot" (GBS)) and can be explained by the eastward motion of the field of the GBS, carried by zonal winds at a depth (∼3,500 km) where molecular hydrogen is sufficiently electrically conductive to grip the magnetic field. Key Points: The Juno spacecraft sampled Jupiter's vector magnetic field along 32 polar passes separated by ∼11° longitude at the equatorA degree 18 spherical harmonic model of Jupiter's magnetic field is obtained by partial solution of a degree 30 linear systemThe new model is consistent with dynamo action in metallic hydrogen, advection of the field by deep zonal winds, and secular variation [ABSTRACT FROM AUTHOR]

Abstract Observations of the geomagnetic field taken at Earth’s surface and at satellite altitude are combined to construct continuous models of the geomagnetic field and its secular variation from 1957 to 2020. From these parent models, we derive candidate main field models for the epochs 2015 and 2020 to the 13th generation of the International Geomagnetic Reference Field (IGRF). The secular variation candidate model for the period 2020–2025 is derived from a forecast of the secular variation in 2022.5, which results from a multi-variate singular spectrum analysis of the secular variation from 1957 to 2020.

Abstract 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.

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. [ABSTRACT FROM AUTHOR]

Localized regions of low geomagnetic intensity such as the South Atlantic Anomaly allow energetic particles from the Van Allen radiation belt to precipitate into the atmosphere and have been linked to a signature in the formof red aurora-like airglow visible to the naked eye. Smoothed global geomagnetic models predict a lowintensity West Pacific Anomaly (WPA) during the sixteenth to nineteenth centuries characterized by a simple time dependence. Here, we link the WPA to an independent database of equatorial aurorae recorded in Seoul, South Korea. These records show a complex fluctuating behavior in auroral frequency, whose overall trend from 1500 to 1800 AD is consistent with the locally weak geomagnetic field of the WPA, with a minimum at 1650 AD. We propose that the fluctuations in auroral frequency are caused by corresponding and hitherto unknown fluctuations in the regional magnetic intensity with peaks at 1590 and 1720 AD, a time dependence that has been masked by the smoothing inherent in regularized global geomagnetic models. A physical core flow model demonstrates that such behavior requires localized time-dependent upwelling flows in the Earth's core, possibly driven by regional lower-mantle anomalies. [ABSTRACT FROM AUTHOR]

We analyze the geomagnetic data recorded at 78 stations from 13 to 31 March 2015. Using the empirical mode decomposition (EMD) method, we focus our attention on geomagnetic signal due to sources which are external to the Earth, that is, due to current systems flowing in the ionosphere and magnetosphere. We analyze the short timescale fluctuations (τ < 200 min) of this magnetic signal, their dependence on magnetic latitude, magnetic local time, and geomagnetic activity. At high geomagnetic latitudes (>|60°|), these short timescale fluctuations constitute more than 30% of the external magnetic field. Their maximum contribution occurs along the auroral oval suggesting that they are mainly triggered by the ionospheric electric current systems active in these regions. A discussion of the relevance of these short timescale magnetic fluctuations to result in a more significant modeling and prediction of geomagnetically induced currents in the auroral zones is also provided. Key Points: The multiscale nature of geomagnetic field fluctuations recorded on the ground is analyzedShort‐timescale magnetic fluctuations constitute a significant contribution to the magnetic field of external origin at high latitudesThe main source of short timescale magnetic fluctuations seems to be the occurrence of magnetospheric substorms [ABSTRACT FROM AUTHOR]

We have produced a 5-year mean secular variation (SV) of the geomagnetic field for the period 2020–2025. We use the NASA Geomagnetic Ensemble Modeling System (GEMS), which consists of the NASA Goddard geodynamo model and ensemble Kalman filter (EnKF) with 400 ensemble members. Geomagnetic field models are used as observations for the assimilation, including gufm1 (1590–1960), CM4 (1961–2000) and CM6 (2001–2019). The forecast involves a bias correction scheme that assumes that the model bias changes on timescales much longer than the forecast period, so that they can be removed by successive forecast series. The algorithm was validated on the time period 2010-2015 by comparing with CM6 before being applied to the 2020–2025 time period. This forecast has been submitted as a candidate predictive model of IGRF-13 for the period 2020–2025. [ABSTRACT FROM AUTHOR]

This paper describes the design of a candidate secular variation model for the 13th generation of the International Geomagnetic Reference Field. This candidate is based upon the integration of an ensemble of 100 numerical models of the geodynamo between epochs 2019.0 and 2025.0. The only difference between each ensemble member lies in the initial condition that is used for the numerical integration, all other control parameters being fixed. An initial condition is defined as follows: an estimate of the magnetic field and its rate-of-change at the core surface for 2019.0 is obtained from a year (2018.5–2019.5) of vector Swarm data. This estimate (common to all ensemble members) is subject to prior constraints: the statistical properties of the numerical dynamo model for the main geomagnetic field and its secular variation, and prescribed covariances for the other sources. One next considers 100 three-dimensional core states (in terms of flow, buoyancy and magnetic fields) extracted at different discrete times from a dynamo simulation that is not constrained by observations, with the time distance between each state exceeding the dynamo decorrelation time. Each state is adjusted (in three dimensions) in order to take the estimate of the geomagnetic field and its rate-of-change for 2019.0 into account. This methodology provides 100 different initial conditions for subsequent numerical integration of the dynamo model up to epoch 2025.0. Focussing on the 2020.0–2025.0 time window, we use the median average rate-of-change of each Gauss coefficient of the ensemble and its statistics to define the geomagnetic secular variation over that time frame and its uncertainties. [ABSTRACT FROM AUTHOR]

Abstract 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.

Planetary scale interannual deformations of the Earth's surface, of millimetric amplitude, have recently been related to both geomagnetic field changes and motion within the fluid outer core. We calculate the temporal variations of the dynamical pressure at the surface of the core associated with core flow models inverted from geomagnetic observations. From these we compute predictions of the changes in Earth's topography in response to elastic deformations in the mantle. We show that at decadal periods, the predicted changes in Earth's topography are at most of the order of 0.3 mm. Focused at interannual periods between 4 and 9.5 yr, the predicted topography variations are smaller than 0.05 mm, at least an order of magnitude smaller than the reported observations. These amplitudes are only weakly sensitive to the choice of hypothesis used to reconstruct fluid motions at the core surface. We conclude that surface deformations induced by dynamical pressure changes in the core are below the detection level at present-day. Alternative geophysical sources must be sought to explain the observed millimetric interannual variations of the planetary scale topography, and its associated gravity variations. We currently see no justification for a physical relationship between interannual fluctuations of the geomagnetic field and of Earth's observed deformations. We conjecture that the largest gravity signal of core origin is potentially associated with decadal longitudinal oscillations of the inner core. It might be detectable as longer series will become available. [ABSTRACT FROM AUTHOR]

We present a revised stratigraphy of the proximal deposits around Ohachidaira volcano and their paleomagnetic directions obtained from 21 sites (193 samples). We identify four proximal pyroclastic members from Ohachidaira volcano that were produced after the early edifice-building effusive volcanism, from older to younger: (1) vent-opening mafic pyroclastic density current (PDC) deposits (Kobachidaira ignimbrite); (2) maar-forming mafic tephra-ring deposits (Mamiyadake tephra ring); (3) mafic to silicic caldera-forming deposits (Sounkyo Member); and (4) mafic PDC deposits (Kumonotaira ignimbrite). Well-grouped paleomagnetic directions obtained from the Kobachidaira ignimbrite and the Sounkyo Member indicate that each deposit was formed over a short time interval compared to geomagnetic secular variation―that is, less than a century. Conversely, scattered paleomagnetic directions of the Mamiyadake tephra ring record the secular variation of the geomagnetic field that took place over a longer time interval―that is, at least 750 yr, probably more. The Kumonotaira ignimbrite, which has a similar paleomagnetic direction to the underlying Sounkyo Member, may have been produced during the Sounkyo eruption. From comparison between our data and the previously reported paleomagnetic directions of two petrologically distinct distal ignimbrites, we conclude that the Sounkyo Member is a proximal correlative of the distal pyroxene-rich ignimbrite, and that the distal hornblende-rich ignimbrite may not be of an Ohachidaira origin. The revised stratigraphy suggests that Ohachidaira volcano is a maar-caldera complex volcano, and that the summit caldera is a flaring funnel formed incrementally by explosive erosion and syn-eruptive collapse of the vent walls. Volume estimates imply that the Sounkyo eruption fundamentally enlarged the vent and formed the caldera. Our data confirm that the comparison of paleomagnetic directions can offer a useful identification and correlation tool for the Ohachidaira pyroclastic sequences. [ABSTRACT FROM AUTHOR]

In this response, we address the comments by D. Bilardello on the paper by Brandt et al. (2019, https://doi.org/10.1029/2018JB016968). His main point is that the Mafra rhythmites are remagnetized as are supposed to be all Paleozoic sediments in the Paraná Basin in the context of a widespread Jurassic‐Cretaceous remagnetization in South America. In this way, the obtained results are not suitable for investigations of the paleosecular variation models. The authors do not agree with his conclusions and present counterevidence to support the approach and the conclusions of Brandt et al. (2019, https://doi.org/10.1029/2018JB016968). Key Point: We present counterarguments to demonstrate the validity of the Mafra results to study secular variation [ABSTRACT FROM AUTHOR]

Studies of the Earth's secular variation have blossomed in recent decades, due to the availability of brand‐new satellite data. In particular, flow inversions at the core surface that incorporate the effects of rapid rotation a priori have been able to provide new insight into flows with time scales as short as a few years. These models have been mostly developed in a cylindrical representation, while core inversion methods are based on a spherical surface representation of the flow. This technical brief links band‐limited expressions for the flow coefficients in a cylindrical representation and those in a spherical surface representation and demonstrates that these also remain band limited, with equal degrees of freedom. Correct regularity conditions on the stream function ensure a regular representation of these flows on the spherical surface. A triangular truncation rule for the cylindrical representation that is applicable for "isotropic resolution" is also presented. The equations we derive will be of use in future data‐assimilation work employing this cylindrical flow representation and may prove helpful for conventional core‐inversion strategies. Key Points: We present an exact band‐limited relationship between a cylindrical flow representation and its spherical surface counterpart [ABSTRACT FROM AUTHOR]

In Western Eurasia, the first millennium BCE is characterized by the fastest secular variation of the Earth Magnetic Field observed over the last millennia and by a geomagnetic anomaly centered on the Middle East. On the global scale, the variation of the dipolar field during this period remains poorly constrained because of the lack of data in other geographical areas. Here, we presented 23 new mean archaeointensity data on ceramic sherds dated between 1500 BCE and 200 CE from Chalcatzingo archaeological site in Central Mexico. Archaeointensities were determined using the classical Thellier‐Thellier protocol with corrections for TRM anisotropy and cooling rate effects. Our work doubles the number of high‐quality archaeointensity data in Mexico during the considered period. Using a Bayesian approach, a new secular variation curve was calculated at Mexico City between 1500 BCE and 200 CE after selection of Mexican archaeointensity data. After a period of oscillations of the intensity between 20 and 40 μT from 1500 to 300 BCE, the curve shows a large maximum '~65 μT in the second century BCE. The corresponding VADM varied between ~4.0 and ~11.0 × 1022 Am2, which highlights further that the intensity of the geomagnetic field could vary at regional scale over a larger range as previously thought. However, this amplitude variation may be overestimated, as it does not take into account the fast directional variation observed at this time. Plain Language Summary: Archaeological baked clays are the best material to reconstitute the past secular variation of the Earth magnetic field, because they acquired a generally stable thermoremanent magnetization usually parallel and proportional to the ambient field at the time of their baking. Global modeling of the geomagnetic field requires a spatial and temporal distribution of data as homogeneous as possible, which is still not the case yet. In spite of its rich archaeological heritage, high‐quality archaeointensity data are still very few in Mexico. The present study focuses on pottery sherds dated between 1500 BCE and 200 CE from Chalcatzingo archaeological site in Central Mexico. We obtained 21 new mean archaeointensities, which almost doubles the high‐quality Mexican dataset for this period. The new secular variation Mexican curve exhibits oscillations of the intensity between 20 and 40 μT from 1500 to 300 BCE, before a large maximum ~65 μT in the second century BCE. This work will help to have a better knowledge of the variation of the dipolar geomagnetic field during the first millennium BCE, when the fastest secular variation over the last millennia was reported in the Middle East ("geomagnetic spikes"). Key Points: Twenty‐three new archaeointensity data were obtained on pottery sherds from Central Mexico in the first millennium BCEA new Mexican secular variation curve between 1500 BCE and 200 CE is proposedThe curve exhibits a fast increase of the intensity (~250 nT/year) during the third century BCE and a large maximum ~65 μT circa 200–100 BCE [ABSTRACT FROM AUTHOR]

A small hydrovolcanic eruption occurred on Mt. Hakone in central Japan on 29 June 2015, ejecting small amounts of volcanic ash and gas. We have conducted continuous surveys of He isotopes in the Owakudani geothermal area close to Mt. Komagatake, the central cone of Mt. Hakone. Long‐term (decadal‐scale) data from the Black‐egg site show generally constant 3He/4He ratios with 6.37 ± 0.06 Ra, where Ra is the atmospheric 3He/4He ratio of 1.382 × 10‐6, after air correction. Short‐term (monthly) data of two fumarole sites indicate a rapid increase from May through August 2015 and a gradual decrease through February 2018, whereas seismic activity began to intensify in March 2015, reached the maximum in May, and decreased faster than the decrease in He isotopes. We also measured N and Ar isotopes in fumarole samples. A δ15N – N2/36Ar diagram shows that they are well explained by mixing of atmosphere and volcanic gas source components. Monthly variations of air‐corrected δ15N values shows a rapid decrease followed by a slow increase, contrary to the air‐corrected 3He/4He variations, suggesting a common mechanism of mixing between magma and crust during migration of fluids and gases. The time sequence of observed phenomena shows inflation of the volcanic edifice, maximum swarm activity, phreatic eruption, and maximum He and N isotope anomaly. Temporal variations of He and N isotopes were correlated with activation of the Owakudani geothermal system and with termination of volcanic unrest, which may provide useful information for observing and assessing geothermal activity around the world. Key Points: Secular variations of He isotopes reflected the 2015 volcanic unrest of Mt. Hakone in JapanCorrective method of N isotopes for air contamination was proposed by N2/Ar ratiosAir‐corrected N isotopes showed anticorrelation with that of He [ABSTRACT FROM AUTHOR]

Paleomagnetic secular variations in South Asia are not well understood due to insufficient records. In this study, we produce a new paleomagnetic secular variation curve covering the past 40,000 years by stacking four marine sediment records from the middle portion of Bay of Bengal. Age models are based on radiocarbon dating and core correlation. Stepwise alternating field demagnetization was used to isolate the primary remanent component. Rock magnetic measurements indicate that the primary magnetic remanence carriers are single domain and pseudo‐single domain magnetites with no sign of diagenetic alteration. Our Bengal relative paleointensity stack is broadly consistent with local records and the PISO‐1500 relative paleointensity stack (Channell et al., 2009; https://doi.org/10.1016/j.epsl.2009.03.012). Two geomagnetic excursions presented by paleointensity minima are recognized, which correlate to the Mono Lake (29–32 ka) and Laschamp excursion (38–40 ka), respectively. We conclude that the Mono Lake and Laschamp excursion are two separate events. Key Points: We produce a new paleomagnetic secular variation curve for the past 40 kyr from the Bay of BengalOur curve is consistent with other regional records and global modelsMono Lake and Laschamp geomagnetic excursions are recognized separately in our records [ABSTRACT FROM AUTHOR]

The scarcity of reliable paleomagnetic secular variation (PSV) records from East Asia especially from low‐latitude regions impedes better understanding of global PSV mechanisms. Here we report on a radiocarbon‐dated Holocene PSV record from a composite ~6.7‐m‐long core collected from the high‐sedimentation‐rate Huguangyan Maar Lake (HML) in subtropical‐tropical South China. Rock magnetic results demonstrate that the natural remanent magnetization resides in single‐domain magnetite. Alternating field demagnetization experiments at 1‐cm spacing on u‐channel samples reveal six distinct inclination highs at ~7,500 BCE, ~5,100 BCE, ~4,600 BCE, ~3,600–3,400 BCE, ~1,600–1,200 BCE, and 600–800 CE; three inclination lows at ~4,800 BCE, ~600–300 BCE, and ~1,000–1,300 CE; and three eastward declination trends at ~3,600–3,200 BCE, ~2,600–2,400 BCE, and 400 BCE to 200 CE. The similarity between the HML PSV record and other independently dated records from East Asia and geomagnetic field models corroborates the robustness of our age model and Holocene PSV record. Strikingly, centennial‐ to millennial‐scale PSV features of the HML are comparable, within age uncertainties, with other Holocene records from Europe, North America, and Canada, suggesting that such directional patterns are likely to be hemispheric in scale. Although relative paleointensity data of HML are affected by environmental factors (e.g., organic matter diagenesis), the record still provides a regionally important new PSV reference curve whose conspicuous features may serve as stratigraphic markers for East Asian paleorecords. Key Points: A well‐dated high‐resolution paleomagnetic secular variation record throughout the Holocene from Huguangyan Maar Lake is presentedThis PSV record broadly agrees with typical sediment records from East Asia, Europe, North America, and geomagnetic field model predictionsThis record may be used as a reliable PSV reference curve and a valuable tool for regional correlation and dating purpose in South China [ABSTRACT FROM AUTHOR]

Abstract Oceanic carbonate chemistry during the Cenozoic has affected the climatology, ecology, and marine geology of our planet; yet, we have limited means to know the evolution of that chemistry, due to a lack of preserved and unaltered seawater samples and a continuing paucity of proxies. Modeling is often used to address this problem; here, we offer a simple, data-driven, secular timescale, inverse model for the mean, Cenozoic, carbonate chemistry of the oceans. Inputs for the model include carbonate compensation depth (CCD), CaCO 3 burial, seawater temperature, atmospheric CO 2 and carbonate ion records, as well as a simple set of original, but justified, assumptions. The model retrodicts the total dissolved inorganic carbon (DIC), carbonate alkalinity (CAlk), and pH of the surface and deep waters of the ocean. The retrodicted DIC and CAlk records do not indicate any unusually elevated values in the early Cenozoic, as found in some past studies. If the CCD record from Lyle et al. (2008) is employed, the changes in DIC and CAlk appear entirely related to changes in the alkalinity input to the pelagic oceans and atmospheric CO 2 ; however, with the CCD from Pälike et al. (2012) , the increases in DIC and CAlk during the last 15 Ma reflect the effects of ocean cooling. Using either CCD-record, our model provides consistent retrodictions of the available pH record. Our results are not consistent with many past modeling assumptions, such as constancy of alkalinity in surface waters, or the ratio of shallow and deep carbonate ion concentrations. Finally, we use our results to provide new estimates of atmospheric CO 2 based on Boron isotopes and find significantly lower CO 2 values in the early Cenozoic than previous values. Highlights • Inverse model of the secular carbonate chemistry of the pelagic ocean during the Cenozoic. • Retrodicted carbonate alkalinity does not exhibit large changes suggested by others. • Retrodicted pH agrees with observations from proxies. • Increase in alkalinity in the past 15 Ma may be due to the effects of cooling. • Recalculated Boron-based p CO 2 is comparatively much lower in the early Cenozoic. Graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Thirteen new archaeointensity data on 650–900 CE potteries from Mexico. • Poor agreement of new data with global models. • Heterogeneous quality of Holocene palaeointensity data in Central America. • Criteria based on consistency and experimental quality discard 74% of data. • Data without cooling rate correction cause overestimation of global models. Abstract Thanks to its rich archaeological heritage, Central America is a key region to recover the past secular variation of the geomagnetic field. This article presents 13 new palaeointensity data on Epiclassic (650 – 900 CE) pottery sherds from Central Mexico. Archaeointensities were determined using the Thellier-Thellier protocol with anisotropy and cooling rate corrections. Average results between 25 and 42 µT reveals a fast secular variation in the second half of the first millennium CE but are not in agreement with global geomagnetic models that predict a higher geomagnetic field strength. To check the reasons of this discrepancy, we compiled all intensity data over the last millennia published in Central America. The Bayesian curve calculated from 194 data covering the last 4 millennia highlights a rapid succession of oscillations of the geomagnetic field strength between 20 and 80 µT. But a critical analysis of the dataset shows a large influence of data quality, 74% of them having a poor cooling unit consistency and experimental quality. The small number of specimens per cooling unit and the anisotropy correction absent or incorrectly made increase the scatter between data, whereas the absence of cooling rate correction biases the dataset towards higher palaeointensity. Discarding these data results in a lower secular variation by removing most extreme values and several intensity oscillations. The weaknesses of the dataset are likely the main reason of the limitations of global models in Central America. Pending the acquisition of new high-quality data, archaeomagnetic dating seems premature in Central America. [ABSTRACT FROM AUTHOR]

Here, we present a synthetic validation for the inversion of transient fluid motions at the surface of Earth's core. It is based on a numerical simulation of the geodynamo in which the main time-scales (based on rotation, magnetic field and velocity) are sufficiently separated to give rise to a variety of hydro-magnetic waves. We focus the study on wave-like motions with periods commensurate to the Alfvén time, which is based on the strength of the magnetic field in the core interior. Synthetic magnetic data are generated over 90 Alfvén times, representative of the era covered by observatory and satellite measurements. These synthetic data are inverted to estimate a magnetic field model. Thereafter, we apply the pygeodyn data assimilation tool to recover core surface flows. We investigate the quality of their reconstruction as a function of their time scale. The success of the reconstruction depends on the data accuracy and coverage and on the magnitude of the flow. We also retrieve axi-symmetric torsional Alfvén waves, despite their relatively weak magnitude. We use the synthetic data to investigate the exchanges of angular momentum between core and mantle that induce length-of-day (LOD) changes. These exchanges result from the electromagnetic torque between the fluid core and the mantle and the gravitational torque between the inner core and the mantle. The inverted flows convincingly predict LOD variations in the dynamo calculation. We find that core surface zonal motions match well with the geostrophic (axially invariant) motions at the origin of the LOD changes, on all considered time-scales. We also investigate the different contributions to the electro-magnetic torque. In the dynamo simulation, only a small part can be attributed to the leakage torque caused by electrical currents flowing from the core to the mantle. The relative contribution from the poloidal field induced in the mantle, which amounts to about 1/3 of the total torque, is significantly larger than estimated in previous studies, based on geomagnetic observations. The remaining torque, which is associated with the toroidal induced field, mostly stems from the solid body rotation interacting with the radial magnetic field up to spherical harmonic degree 30. • Torsional Alfven waves and quasi-geostrophic magneto-Coriolis waves are recovered from simulated geomagnetic data. • The ability to recover transient flows depends on their magnitude, geometry and period, plus the data coverage and accuracy. • Extrapolating throughout the core the inverted zonal surface flows we predict well core angular momentum changes. • The core-mantle electromagnetic torque mainly arises from the axial rotation part of the core surface motion. • The electrical currents, part of the dynamo process, leaking from the core interior yield a negligible torque on the mantle. [ABSTRACT FROM AUTHOR]

The data from observation of the geomagnetic field variations at the Midlatitude Mikhnevo Geophysical Observatory of Institute of Geosphere Dynamics of the Russian Academy of Sciences, Mikhnevo village, Moscow oblast, Russia (coordinates 54.959° N; 37.766° E) and at the INTERMAGNET international magnetic network station of the Belsk Geophysical Observatory of Geophysical Institute of the Polish Academy of Sciences, Belsk, Poland (coordinates 51.837° N, 20.792° E) in 2008−2016 are analyzed. A long-term trend related to secular variation in the magnetic field of the Earth is studied by the daily-mean values. Annual variation is distinguished in the north horizontal component of the magnetic field. The reliability of the recent version of the International Geomagnetic Reference Field (IGRF-12) model, which was released in December 2014 to describe variations in the main magnetic field at the Mikhnevo and Belsk observatories, is estimated. The 2011 and 2014 jerks are identified. [ABSTRACT FROM AUTHOR]

Apart from modifying the total amount of rain, another emerging impact of global warming on the rainfall regime could be an increase in the frequency and intensity of events, as well as their greater variability over time and space. The objectives of this work were to evaluate the long-term climatic variation of the precipitation regime in the central region of Argentina, determining the intensity and nature of the changes, and also examining the two typical modalities of representing the trend of change: linear and periodic models. Rainfall records were analyzed of the Córdoba Observatorio and Pilar Observatorio weather stations, as from 1910 and 1925 respectively, which belong to the Servicio Meteorológico Nacional (SMN) official network, and from a rain gauge located in Colonia Caroya, with data available as from 1929, that were selected by extension criteria, temporal integrity and geographical proximity. The non-parametric Mann-Kendall test (M-K) was used to identify the nature of the change, and the Theil-Sen (T-S) to evaluate intensity. These determine a significant linear trend (P < 0.05) in the region for total amount (PP) of rainfall, number of events (DPP), proportion of warm semester rainfall with respect to annual value, intensity (INT and PPmax) and variability, both annually and for the warm semester, when most of the annual rainfall occurs. However, adjusting these rainfall series to a simple sine function reveals a periodic behavior that explains a greater proportion of the observed variability with respect to the linear model, showing a negative phase of PP, DPP and PPmax during the period 1930–1950, which subsequently becomes positive between 1970 and 2000. Rainfall variability presents great dispersion in the region so neither the sine function nor the third-degree polynomial were appropriate to represent its long-term behavior. The linear trend loses statistical significance when the periodic behavior is incorporated into the analysis, leaving open the question about the influence of global warming on changes in the hydrological cycle and particularly on the rainfall regime in the central territory of Argentina. [ABSTRACT FROM AUTHOR]

Highlights • New Pb-Pb ages with paleomagnetic results of Precambrian dykes from Dharwar Craton. • Paleomagnetic data revealed a stable reverse magnetic polarity at ∼2207 Ma. • Paleomagnetic results on 2252 Ma N-S dyke swarm yields a normal polarity. • Rapid drift rate along with a clockwise rotation of ∼30° between 2216 and 2207 Ma. Abstract We report new TE-TIMS precise Pb-Pb baddeleyite ages together with consistent paleomagnetic results on two Paleoproterozoic mafic dyke swarms, intruding Archean basement rocks in the northern and southern region of Cuddapah basin, Eastern Dharwar Craton. The distinct E-W to NW-SE striking dykes, which gives a weighted mean age of 2206.8 ± 2.1 Ma, confirms the presence of a radiating mafic dyke swarm with a fanning angle of 50°, convergence towards a focal point in the north-western part of Dharwar Craton. The North-South trending parallel dyke swarm yields an emplacement age of 2252.4 ± 1.2 Ma, making this the second oldest well dated dyke swarm of the Dharwar craton after the ∼2367 Ma event. Paleomagnetic results from the ∼2207 Ma radiating dykes have revealed a stable reverse polarity remanence directions with a mean direction of D = 26°, I = 65° (α 95 = 6°) and corresponding paleopole situated at 57°N and 113°E (dp = 8°, dm = 10°). Reliable baddeleyite age determinations at paleomagnetic sampling sites, positive baked contact test and a secular variation correlation test proves that the reporting remanence directions are primary. We have also reviewed the published paleomagnetic results from similar N-S dykes, which are yielding a normal polarity with a paleopole position at 16°S and 299°E (dp = 13°, dm = 15°). We consider this result as a key paleopole for the ∼2252 Ma dyke swarm. We have attempted to reconstruct an updated APWP for Dharwar craton utilizing six well constrained key paleopoles from 2.37 Ga to 1.89 Ga periods. The path suggests that the Dharwar craton has relatively rapid drift rate (28.77 cm/yr) from 46° to 53° paleolatitude along with a clockwise rotation (30°) during the short time span of ∼9 Ma (2216 Ma–2207 Ma), when a magnetic polarity reversal occurred. However, we speculate that major tectonic activity or plate tectonic processes were responsible for rapid drift of Dharwar craton at that time. The identification of coeval mafic magmatic episodes (∼2.2 Ga) from several other Archean cratons over the globe firmly supports the popular hypothesis of supercraton breakup at ∼2.2 Ga time. [ABSTRACT FROM AUTHOR]

Highlights • Middle-scale experiment was conducted in HUGS to investigate piezomagnetic effects. • Negative anomalies of approximately −2.1 nT were observed over a seven-month period. • The result constitutes evidence for piezomagnetic effect in middle-scale condition. Abstract To investigate possible piezomagnetic effects associated with imposed tectonic stresses, repeated measurements of total magnetic field in an array (∼120 km × 90 km in extent) were performed across the Hutubi underground gas storage (HUGS). Forty repeated measurement stations were established in the HUGS and its surrounding areas. Proton precession magnetometers with a sensitivity of 0.15 nT @ 1 Hz were used to measure the total magnetic field at each repeated station. We conducted two field measurement surveys, one with a minimum pressure during April 2017 and the other with a maximum pressure during November 2017 in the HUGS. The local magnetic field (LMF) of the HUGS and its surrounding areas was obtained after data processing, which included diurnal variation reduction (DVR), secular variation reduction (SVR) and LMF acquisition. The average of the mean standard deviations (σ) of the geomagnetic DVR values were 0.038 nT and 0.045 nT for the first and second field surveys respectively. The amplitude of the LMF varied between about 10 nT and 90 nT in the local area. Negative anomalies with a maximum amplitude of −2.1 nT was observed over a seven-month period (April 2017–November 2017) during the loading process with a constantly increasing pressure in the HUGS. The wide range of negative LMF anomalies shows a negative correlation with the increasing pressure in the HUGS. This result constitutes powerful field observational evidence for the existence of piezomagnetic effects under middle-scale experimental conditions. [ABSTRACT FROM AUTHOR]

Highlights • The geomagnetic field for the past 400 years was re-examined. • Pacific low of the non-dipole field and secular variation do not persist. • The hemispherical asymmetry is caused by the movement of the drifting field. Abstract It is widely believed that in the Pacific region the geomagnetic secular variation is low and that the non-dipole field is weak. This is also known as hemispherical asymmetry between the Pacific and the Atlantic hemispheres, representing the low secular variation in the Pacific hemisphere. With this characteristic feature, a question has been left unsolved whether it is a permanent feature or a transient one. Reexamination of the geomagnetic field for the past 400 years indicates that in the early 17th century the non-dipole field was not weak, nor the secular variation was low in the Pacific region, implying that the hemispherical asymmetry is not a permanent phenomenon. This paper examines the secular variation for the past 400 years from viewpoint of drifting and standing field model. It has been found that the low secular variation in the Pacific region is mainly caused by the drift of the drifting field. At the Earth’s surface the drifting field is dominated by spherical harmonics Y 2 2 (θ , ϕ) with two pairs of ridges and troughs with their axes aligned in the meridional direction. The two pairs are not equal; one pair is stronger than the other. Movements of the different magnitude of the pairs are considered to have caused the hemispherical asymmetry of the secular variation for recent years, giving rise to the non-weak field in the Pacific region in the early 17th century. [ABSTRACT FROM AUTHOR]

Using 12-year-long series of data (2001-2012) from geomagnetic observatories and repeat stations in Austria and its neighboring countries, a regional spatial-temporal (ST) model is developed based on the polynomial expansion consisting of latitude, longitude, and time of the geomagnetic field components and total magnetic field F. Additionally, we have used three different global models (CHAOS-5, POMME-9, and EMM2015), which are built on spherical harmonics up to a maximum degree Lmax and give the core field and crustal field separately. The normal field provided by the ST model and its "model bias", which comprise the residuals of the differences between measured and predicted values, are calculated and the respective maps are shown. The residuals are considered an estimate of the local crustal field. In the case of global models, we have applied for each of these three methods to calculate the "model bias": residuals of the differences between observed values and predicted values of the model, residuals of the differences between observed values and core field values of the model, and the average bias for the period 2001-2012. The normal field of the region of Austria provided by each global model is also calculated. Generally, the regional and global models yield relatively similar crustal fields for the Austrian region, especially when the first method is used. The normal fields calculated by them are in good agreement with each other. Each of the global models directly provides the crustal field, and they are compared with the aeromagnetic data provided by aeromagnetic surveys over the Austrian region. The ST model is in better agreement with aeromagnetic data. We have also analyzed the secular variation over the region, which is calculated from the rate of change of normal field given by the ST and global models. [ABSTRACT FROM AUTHOR]

Measurements obtained at ground‐based observatories are crucial to understanding the geomagnetic field and its secular variation (SV). However, current data processing methods rely on piecemeal closed‐source codes or are performed on an ad hoc basis, hampering efforts to reproduce data sets underlying published results. We present MagPySV, an open‐source Python package designed to provide a consistent and automated means of generating high‐resolution SV data sets from hourly means distributed by the Edinburgh World Data Centre. It applies corrections for documented baseline changes, and optionally, data may be excluded using the ap index, which removes effects from documented high solar activity periods such as geomagnetic storms. Robust statistics are used to identify and remove outliers. Developing existing denoising methods, we use principal component analysis of the covariance matrix of residuals between observed SV and that predicted by a global field model to remove a proxy for external field contamination from observations. This method creates a single covariance matrix for all observatories of interest combined and applies the denoising to all locations simultaneously, resulting in cleaner time series of the internally generated SV. In our case studies, we present cleaned data in two geographic regions: monthly first differences are used to investigate geomagnetic jerk morphology in Europe, an area previously well‐studied at lower resolution, and annual differences are investigated for northern high latitude regions, which are often neglected due to their high noise content. MagPySV may be run on the command line or within an interactive Jupyter notebook; two notebooks reproducing the case studies are supplied. Key Points: MagPySV is an open‐source Python package for creating reproducible high‐resolution time series of internal secular variationImplemented denoising method uses principal component analysis to characterize external contamination in different geographic regionsDenoised data from MagPySV and their application to geomagnetic jerks presented in case studies for Europe and high northern latitudes [ABSTRACT FROM AUTHOR]

Paleomagnetic secular variations (PSV) give us information on the mechanisms of the geodynamo and can also be used for stratigraphic correlation on a regional scale. In this article we present a high-resolution paleomagnetic and rock magnetic study of two cores, LCTF1 and LCTF2, collected at Carmen Lake (Tierra del Fuego, Argentina). An analysis of rock magnetic data suggests that the remanence signal is carried by Titanomagnetite grains in stable pseudo single domain (PSD) state. Notwithstanding the special mechanism of sedimentary deposition, the sequence is characterised by good paleomagnetic properties and can be used to reconstruct a continuous stratigraphic record that provides high-resolution declination, inclination and relative paleointensity curves for the period 1000–4000 cal years BP. The constructed PSV curves are in very good agreement with the available records of Southern Argentina, implying very promising results in the construction of curve patterns for the region. A comparison of the records of southern Argentina with the most recent models available demonstrates that there is a noticeable lack of agreement, which is interpreted as the critical need to add more data from the southern hemisphere in the construction of the geomagnetic field models. [ABSTRACT FROM AUTHOR]

Coupled modeling of biogeochemical cycles leads to improved constraints on elemental fluxes and helps to identify mechanisms that drive secular variations in isotopic records. Here, we develop a coupled strontium-sulfur isotope mass-balance model to examine the Early Cretaceous marine sulfate (δ 34 S sw ) and strontium ( 87 Sr/ 86 Sr) isotope records. We also present an expanded marine barite S isotope record and pyrite S isotope record for the Early Cretaceous with updated age models in line with the Geologic Time Scale 2012. Collectively constraining the primary input fluxes that drive both Sr and S cycles – hydrothermal and weathering – helps to identify which parameters primarily control coherent, oppositional, and divergent isotopic perturbations. With the primary input fluxes constrained in the coupled model, the importance of additional factors responsible for secular changes in δ 34 S sw can be examined. Model results indicate that emplacement of the Ontong Java-Manihiki-Hikurangi Plateau, along with elevated mid-ocean ridge spreading rates, increased continental arc volcanism, and eruption of additional large igneous provinces, all contributed to the Early Cretaceous ~5‰ negative δ 34 S sw shift and maintained low δ 34 S sw over long time scales. The coupled strontium-sulfur modeling was able to demonstrate that a complex interplay of changes in multiple S cycle parameters – not simply total weathering and hydrothermal fluxes – was required to produce this major shift and new alternate state within the S isotope record. In addition, coupled modeling highlights the differences in chemical behavior (volatility) between S and Sr. Review of S cycle modeling and examination of a variety of initial conditions indicates a distinctly different range of δ 34 S values for the total weathering flux, weathered evaporite-pyrite ratios, the pyrite burial fraction, and the global integrated fractionation factor for pyrite burial compared to those used in previous S cycle models. Coupling the Sr cycle to other biogeochemical cycles that also reflect a balance between hydrothermal and riverine inputs offers a powerful tool for interpreting marine isotopic records. [ABSTRACT FROM AUTHOR]

Speleothems are potentially excellent archives of the Earth's magnetic field, capable of recording its past variations. Their characteristics, such as the continuity of the record, the possibility to be easily dated, the almost instantaneous remanence acquisition and the high time-resolution make them potentially unique high-quality Paleosecular Variation (PSV) recorders. Nevertheless, speleothems are commonly characterized by low magnetic intensities, which often limits their resolution. Here we present a paleomagnetic study performed on two cores from a flowstone from the Rio Martino Cave (Western Alps, Italy). U/Th dating indicates that the flowstone's deposition covers almost the entire Holocene, spanning the period ca. 0.5–9.0 ka, while an estimation of its mean growth rate is around 1 mm per 15 years. The flowstone is composed of columnar calcite, characterized by a highly magnetic detrital content from meta-ophiolites in the cave's catchment. This favorable geological context results in an intense magnetic signal that permits the preparation and measurement of thin (∼3 mm depth equivalent) samples, each representing around 45 yr. The Characteristic Remanent Magnetization (ChRM), isolated after systematic stepwise Alternating Field demagnetization, is well defined, with Maximum Angular Deviation (MAD) generally lower than 10°. Paleomagnetic directional data allow the reconstruction of the PSV path during the Holocene for the area. Comparison of the new data with archeomagnetic data from Italian archeological and volcanic records and using the predictions of the SHA.DIF.14k and pfm9k.1a global geomagnetic field models shows that the Rio Martino flowstone represents an excellent recorder of the Earth's magnetic field during the last 9,000 years. Our high resolution paleomagnetic record, anchored by a high-quality chronology, provides promising data both for the detection of short term geomagnetic field variations and for complementing existing regional PSV curves for the prehistoric period, for which well-dated data are still scarce. [ABSTRACT FROM AUTHOR]

We investigate slow magnetic Rossby waves in convection-driven dynamos in rotating spherical shells. Quasi-geostrophic waves riding on a mean zonal flow may account for some of the geomagnetic westward drifts and have the potential to allow the toroidal field strength within the planetary fluid core to be estimated. We extend the work of Hori et al. (2015) to include a wider range of models, and perform a detailed analysis of the results. We find that a predicted dispersion relation matches well with the longitudinal drifts observed in our strong-field dynamos. We discuss the validity of our linear theory, since we also find that the nonlinear Lorentz terms influence the observed waveforms. These wave motions are excited by convective instability, which determines the preferred azimuthal wavenumbers. Studies of linear rotating magnetoconvection have suggested that slow magnetic Rossby modes emerge in the magnetostrophic regime, in which the Lorentz and Coriolis forces are in balance in the vorticity equation. We confirm this to be predominant balance for the slow waves we have detected in nonlinear dynamo systems. We also show that a completely different wave regime emerges if the magnetic field is not present. Finally we report the corresponding radial magnetic field variations observed at the surface of the shell in our simulations and discuss the detectability of these waves in the geomagnetic secular variation. [ABSTRACT FROM AUTHOR]