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Abstract For the past 20 years, state of the art geomagnetic core field models have relied heavily on magnetic measurements made from space-based instrumentation. These models have revealed rapid global magnetic field variations on sub-decadal timescales originating in Earth’s core. With the end of the CHAMP mission in 2010 and the launch of Swarm in late 2013, there has been a 3-year gap in high-quality satellite measurements of the geomagnetic field. Geomagnetic field models have therefore relied on ground observatory data to fill in this gap period. However, ground observatories are unable to provide a truly global picture of the core field and its temporal changes. Many satellites in operation carry vector fluxgate “platform” magnetometers for attitude control, which can offer an alternative to relying on ground observatory measurements during the gap period. However, these instruments need to be carefully calibrated in order to provide meaningful information on Earth’s core field. Some previous studies attempted to calibrate such instruments with a priori geomagnetic field models. This approach has several disadvantages: (1) errors in the model will introduce errors in the calibration parameters, and (2) relying on an a priori model may not be feasible in the post-Swarm era. In this paper, we develop a novel approach to build a time-dependent geomagnetic field model from platform magnetometer data, by co-estimating their calibration parameters with the internal field parameters. This method does not require an a priori geomagnetic field model, but does require a dataset of previously calibrated data. We use CHAMP, Swarm, and ground observatory measurements to supply this dataset, and incorporate platform magnetic measurements from DMSP and Cryosat-2 during the gap years. We find that the calibration parameters of DMSP and Cryosat-2 can be reliably estimated, and these missions provide meaningful information on rapid core field variations during the gap period.

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]

A regional harmonic spline geomagnetic main field model, Southern Africa Core Field Model (SACFM-3), is derived from Swarm satellite and ground-based data for the southern African region, in the eastern part of the South Atlantic Anomaly (SAA) where the field intensity continues to decrease. Using SACFM-3 and the global CHAOS-6-×9 model, a detailed study was conducted to shed light on the high spatial and temporal geomagnetic field variations over Southern Africa between 2014 and 2019. The results show a steady decrease of the radial component Z in almost the entire region. In 2019, its rate of decrease in the western part of the region has reached high values, 76 nT/year and 78 nT/year at Tsumeb and Keetmanshoop magnetic observatories, respectively. For some areas in the western part of the region the radial component Z and field intensity F have decreased in strength, from 1.0 to 1.3% and from 0.9 to 1.2%, respectively, between the epochs 2014.5 and 2019.5. There is a noticeable decrease of the field intensity from the south-western coast of South Africa expanding towards the north and eastern regions. The results show that the SAA area is continuing to grow in the region. Abrupt changes in the linear secular variation in 2016 and 2017 are confirmed in the region using ground-based data, and the X component shows an abrupt change in the secular variation in 2018 at four magnetic observatories (Hermanus, Hartebeesthoek, Tsumeb and Keetmanshoop) that needs further investigation. The regional model SACFM-3 reflects to some extent these fast core field variations in the Z component at Hermanus, Hartebeesthoek and Keetmanshoop observatories. [ABSTRACT FROM AUTHOR]

This paper develops a composite absolute paleointensity record for Holocene paleomagnetic secular variation (PSV) from central North America. Twelve full‐vector (inclination, declination, paleointensity) PSV records were assessed in order to build the composite record. Nine of the paleointensity records come from sediment paleomagnetic studies and are considered relative in intensity. Three of the paleointensity records come from absolute paleointensity measurements of archeological materials and lava flows. This paper develops a new method to normalize the sediment relative paleointensity records to the absolute intensity records. The final composite paleointensity record describes intensity variability over a region of Central North America delineated by 35°–48.6°N and 240.4–291.4°E (∼14° × 50°). This composite record shows a distinctive long‐duration (∼104 year) oscillation and a series of millennial‐scale intensity oscillations that are consistent over our study region. Plain Language Summary: This study develops a composite record of geomagnetic field intensity for the central North America for the last 8,000 years. This record is important for considering the total geomagnetic field variability I this region and its dynamo source(s). Key Points: We develop a new composite absolute paleointensity record for Holocene Central North AmericaWe develop a new renormalization technique to combine absolute paleointensity records and sediment relative paleointensity recordsThe final composite absolute paleointensity record has a distinctive long‐term trend and millennial‐scale variability [ABSTRACT FROM AUTHOR]

Using hourly summer rain data recorded at 51 automatic meteorological stations during 1965—2018, the secular variation features of the long-duration rainfall（the raining duration is longer than 6 hours, LDR） in three regions in North China were analyzed by methods of Boostrap Procedure, Mann-Kendall（MK） test, moving t-test, wavelet analysis, and cross wavelet analysis. The possible influence of atmospheric circulation on rainfall factors was described. The results show that the frequently occurred region of long-duration rainfall is the East plain of North China（EP） with the characteristics of larger precipitation, longer duration and greater intensity. The ratio of the frequency of LDR over the total rainfall, and the ratio of the amount of LDR over the total rainfall showed a decreasing trend, while the amount ratio of the hourly peak intensity over the rainfall event increasing in EP, which may lead to more concentrated rainfall. The amount ratio of the hourly peak intensity over the rainfall event increased in north mountain of North China（NM）. In the last part named as central south of North China（CS）, the trend of rainfall factors has little significance. The rainfall factors of EP were significantly driven by cycles of quasi-8 a, and the duration and peak intensity had an abrupt change of period between 1988 and 1989, respectively. Western Pacific subtropical high ridge position had positive driving force on such quasi-8 a period of rainfall in EP. The rainfall factors of NM have quasi-4 a and CS have quasi-6 a respectively, and ENSO mainly has negative driving force on these two periods of rainfall. The influence of atmospheric circulation on long-duration rainfall was regionalized. Western Pacific subtropical high ridge position had significant connections with the duration in EP, while the variation in amount and duration in NM was closely correlated with the NAO anomalies which were mainly influenced in CS by Asian Meridional Circulation. [ABSTRACT FROM AUTHOR]

Abstract Several regional secular variation curves of the geomagnetic field have been proposed for Mexico over the last millennia. Despite a fairly large number of archeomagnetic data, these curves remain imprecise because of an uneven quality and geographic distribution, with a lack of data in Northern Mexico. Nine pottery kilns were sampled in Casas de Fuego, an archeological site in the Chihuahua state. These kilns belong to the Casas Grandes culture and were in use between 1250 and 1450 CE. Rock magnetic experiments indicate that the main magnetic phase is a Ti‐poor titanomagnetite in the SD range. Mean characteristic directions per kiln were estimated by alternating field and/or thermal demagnetization, and archeointensities with the Thellier‐Thellier protocol, after correction for anisotropy and cooling effects. The nine directions and eight intensities agree with data from USA and Mexico but do not support the peak in inclination modeled by the Mexican secular variation curve around 1200–1300 CE. For the last millennium, the Western North American curve is consistent with the SHAWQ2k global model, and better reflects the secular variation in Northern Mexico. For intensity, neither the SHAWQ2k model nor the regional intensity curve can depict the rapid secular variation that likely occurs circa 1500 CE. The Casas de Fuego results are the first full vector determinations obtained in Northern Mexico.

Recent paleomagnetic studies have shown that important short‐lived intensity fluctuations occurred during the first millennium BCE. However, the knowledge of the spatial and temporal extension of these features is still limited by the scarce availability of robust data. In this study we focus on the study of the intensity decrease that took place in Central Asia during the second half of the 1st millennium BCE after the high intensities that characterized the Levantine Iron Age Anomaly. Since previous archeointensities available for this period and region were obtained without accomplishing modern standards of quality, we present here new archeointensities that are derived from classical Thellier and Thellier experiments, including partial thermoremanent magnetization (pTRM) checks, thermoremanent magnetization (TRM) anisotropy and cooling rate corrections at the specimen level. The new 51 archeointensities, together with previous archeointensities, have been used to present a new local paleosecular variation curve for Central Asia. The results confirm the existence of an important geomagnetic field intensity decrease in South Uzbekistan from the 4th century BCE to the end of the 1st century BCE associated with rates of changes up to −15 μT/century. A critical analysis of the archeointensity global database indicates that this feature was present at continental scale, from Western Europe to Central Asia. However, this trend is not identified in other regions such as Japan or Mexico. Finally, the comparison with the dipole moment derived from recent global geomagnetic field reconstructions suggests a strong influence of non‐dipolar sources upon this continental intensity feature. Plain Language Summary: The Earth's magnetic field (also called the geomagnetic field) is continuously changing over time and space. Since the 19th century, the behavior of the geomagnetic field is known through direct observations performed in geomagnetic observatories and satellites. However, the only way to recover past geomagnetic field in ancient times is through paleomagnetic and archeomagnetic studies. In particular, archeomagnetism is the study of heated archeological materials, which include ferromagnetic particles acting as a compass and recording the geomagnetic field direction and strength at a certain time. When the archeological material cools down the ferromagnetic particles save the direction and intensity, like a photograph of the ancient geomagnetic field. Nowadays, there are very few archeomagnetic studies dealing with archeological materials from Central Asia. In this work, we present the variation of the geomagnetic field intensity between 600 BCE–600 CE in Uzbekistan deduced from a complete archeomagnetic investigation of 70 pottery fragments. The results confirm a rapid variation of geomagnetic field strength between 400 BCE and 100 BCE–1CE. To determine the spatial expression of this feature, we compared the obtained trend for Central Asia with the geomagnetic field intensity evolution in other regions of the world. Key Points: We present 51 new thermoremanent magnetization‐anisotropy corrected archeointensities for South UzbekistanAn important decrease in intensity is observed between 400 BCE and 100 BCE–1 CE, probably associated with non‐dipolar sourcesSimilar intensity V‐shaped trends are observed at continental scale [ABSTRACT FROM AUTHOR]

Several regional secular variation curves of the geomagnetic field have been proposed for Mexico over the last millennia. Despite a fairly large number of archeomagnetic data, these curves remain imprecise because of an uneven quality and geographic distribution, with a lack of data in Northern Mexico. Nine pottery kilns were sampled in Casas de Fuego, an archeological site in the Chihuahua state. These kilns belong to the Casas Grandes culture and were in use between 1250 and 1450 CE. Rock magnetic experiments indicate that the main magnetic phase is a Ti‐poor titanomagnetite in the SD range. Mean characteristic directions per kiln were estimated by alternating field and/or thermal demagnetization, and archeointensities with the Thellier‐Thellier protocol, after correction for anisotropy and cooling effects. The nine directions and eight intensities agree with data from USA and Mexico but do not support the peak in inclination modeled by the Mexican secular variation curve around 1200–1300 CE. For the last millennium, the Western North American curve is consistent with the SHAWQ2k global model, and better reflects the secular variation in Northern Mexico. For intensity, neither the SHAWQ2k model nor the regional intensity curve can depict the rapid secular variation that likely occurs circa 1500 CE. The Casas de Fuego results are the first full vector determinations obtained in Northern Mexico. Plain Language Summary: Archeological baked clays are the best material to reconstitute the past secular variation of the geomagnetic field because they acquired a generally stable thermoremanent magnetization parallel and proportional to the field at the time of their baking. Archeomagnetic data are used to build regional curves and global models of the secular variation. Several curves have been proposed for Mexico over the past few years, but they remain imprecise because of an uneven quality and geographic distribution of data. Data are especially lacking in Northern Mexico and for this reason, we studied nine pottery kilns from Casas de Fuego, an archeological site in Chihuahua state. These kilns belong to the Casas Grandes culture and were in use between 1250 and 1450 CE. The nine new directions and eight new intensities constitute the first full vector determinations obtained in Northern Mexico. For direction, our new data agree with the Western North American secular variation curve but not with the Mexican curve. Casas de Fuego and other regional data show a rapid secular variation circa 1500 CE that is not depicted by current regional and global geomagnetic models for the intensity curve. Key Points: Archeomagnetic study at Casas de Fuego archeological site, Chihuahua stateDirectional and intensity results from nine kilns, in use between 1250 and 1450 CEFirst full vector determinations of the geomagnetic field in Northern Mexico [ABSTRACT FROM AUTHOR]

Following the observed pattern of a new geomagnetic jerk every 3–4 years, certain predictions suggested that a new event should occur around 2020 after the one observed around 2017.5. In this work, we explore this scenario by analysing the secular variation of the East geomagnetic field component in both ground and satellite geomagnetic data. At ground, we use the available data from 2015 to 2021 in 10 observatories worldwide distributed. This analysis shows the occurrence of the mentioned jerk in mid-2017 at observatories located in the Pacific region, but also reveals a new jerk between mid-2019 and early 2020 with a clear global character. Swarm satellite data also corroborate these findings by means of the secular variation estimated using virtual observatories at 440 km altitude. In addition, a general view using the most recent CHAOS geomagnetic model confirms the global character of the 2020-jerk with V-shaped secular variation changes in meridional sectors covering the Eastern Pacific, America, Asia and the Indian Ocean; and Λ-shapes in Europe, Africa and Western Pacific. The radial geomagnetic field at the core–mantle boundary is investigated as the origin of the new jerk. Results show that the global-average secular acceleration of the radial field exhibits a new pulse at mid-2018, establishing the starting epoch of the 2020-jerk. Highlights: Observatory and Swarm data reveal a new geomagnetic jerk around 2019–2020. A very recent geomagnetic field model indicates the global character of the new jerk. A new pulse at the CMB around mid-2018 seems to be the starting point of this jerk. [ABSTRACT FROM AUTHOR]

We present the first Italian reference secular variation curves for both direction and intensity that cover the last 3000 years. Strict selection criteria were applied to guarantee the collection of the most reliable data within a 1000 km radius of Viterbo, including archaeomagnetic records from archaeological material and volcanic rocks. The new curves are calculated using Bayesian statistics and they depict interesting geomagnetic field features, such as high‐intensity periods. These are the first curves for Italy, including both directional and intensity data, and they can be used as reference curves for archaeomagnetic dating, not only in Italy but also in nearby countries. [ABSTRACT FROM AUTHOR]

We use 20 years of continuous magnetic field measurements from the Ørsted, CHAMP and Swarm satellite missions, supplemented by calibrated platform magnetometer data from the CryoSat-2 satellite, to study time variations of the Earth's core field at satellite altitude and at the core–mantle boundary (CMB). From the satellite data we derive composite time series of the core field secular variation (SV) with 4-month cadence, at 300 globally distributed Geomagnetic Virtual Observatories (GVO). A previous gap in the GVO series between 2010 and 2014 is successfully filled using CryoSat-2, and sub-decadal variations are identified during this period. Tests showed that similar sub-decadal SV patterns were obtained from the CryoSat-2 data regardless of whether IGRF-13 or CHAOS-6x9 was used in their calibration. Cryosat-2 radial field SV series at non-polar latitudes have a mean standard deviation level compared to smoothing spline fits of 3.5 nT/yr compared to 1.8 nT/yr for CHAMP and 0.9 nT/yr for Swarm. GVO radial SV series display regional fluctuations with 5–10 years duration and amplitudes reaching 20 nT/yr, most notably at low latitudes over Indonesia (2014), over South America and the South Atlantic (2007, 2011 and 2014), and over the central Pacific (2017). Applying the Subtractive Optimally Localized Averages (SOLA) method, we also map the radial SV at the CMB as a collection of locally averaged SV estimates. We demonstrate that using 2-year windows of CryoSat-2 data, it is possible to reliably estimate the SV and its time derivative, the secular acceleration (SA), at the CMB, with a spatial resolution, corresponding to spherical harmonic degree 10. Along the CMB geographic equator, we find strong SA features with amplitude ± 2.5 μ T / yr 2 under Indonesia from 2011–2014, under central America from 2015 to 2019, and sequences of SA with alternating sign under the Atlantic during 2004–2019. We find that platform magnetometer data from CryoSat-2 make a valuable contribution to the emerging picture of sub-decadal core field variations. Using 1-year windows of data from the Swarm satellites, we show that it is possible to study SA changes at low latitudes on timescales down to 1 year, with spatial resolution corresponding to spherical harmonic degree 10. We find strong positive and negative SA features appearing side-by-side in the Pacific in 2017, and thereafter drift westward. [ABSTRACT FROM AUTHOR]

Abstract In this work, we present 16 directional and 27 intensity high‐quality values from Iberia. Moreover, we have updated the Iberian archeomagnetic catalogue published more than 10 years ago with a considerable increase in the database. This has led to a notable improvement of both temporal and spatial data distribution. A full vector paleosecular variation curve from 1000 BC to 1900 AD has been developed using high‐quality data within a radius of 900 km from Madrid. A hierarchical bootstrap method has been followed for the computation of the curves. The most remarkable feature of the new curves is a notable intensity maximum of about 80 μT around 600 BC, which has not been previously reported for the Iberian Peninsula. We have also analyzed the evolution of the paleofield in Europe for the last three thousand years and conclude that the high maximum intensity values observed around 600 BC in the Iberian Peninsula could respond to the same feature as the Levantine Iron Age Anomaly, after travelling westward through Europe.

We present geomagnetic main field and secular variation time series, at 300 equal-area distributed locations and at 490 km altitude, derived from magnetic field measurements collected by the three Swarm satellites. These Geomagnetic Virtual Observatory (GVO) series provide a convenient means to globally monitor and analyze long-term variations of the geomagnetic field from low-Earth orbit. The series are obtained by robust fits of local Cartesian potential field models to along-track and East–West sums and differences of Swarm satellite data collected within a radius of 700 km of the GVO locations during either 1-monthly or 4-monthly time windows. We describe two GVO data products: (1) 'Observed Field' GVO time series, where all observed sources contribute to the estimated values, without any data selection or correction, and (2) 'Core Field' GVO time series, where additional data selection is carried out, then de-noising schemes and epoch-by-epoch spherical harmonic analysis are applied to reduce contamination by magnetospheric and ionospheric signals. Secular variation series are provided as annual differences of the Core Field GVOs. We present examples of the resulting Swarm GVO series, assessing their quality through comparisons with ground observatories and geomagnetic field models. In benchmark comparisons with six high-quality mid-to-low latitude ground observatories we find the secular variation of the Core Field GVO field intensities, calculated using annual differences, agrees to an rms of 1.8 nT/yr and 1.2 nT/yr for the 1-monthly and 4-monthly versions, respectively. Regular sampling in space and time, and the availability of data error estimates, makes the GVO series well suited for users wishing to perform data assimilation studies of core dynamics, or to study long-period magnetospheric and ionospheric signals and their induced counterparts. The Swarm GVO time series will be regularly updated, approximately every four months, allowing ready access to the latest secular variation data from the Swarm satellites. [ABSTRACT FROM AUTHOR]

The three candidate models submitted by the British Geological Survey for the 13th generation International Geomagnetic Reference Field are described. These DGRF and IGRF models are derived from vector and scalar magnetic field data from the European Space Agency Swarm satellites and ground observatories, covering the period 2013.9 to 2019.7. The internal field model has time dependence for degrees 1 to 15, represented by order 6 B-splines with knots at six monthly intervals. We also solve for a degree 1 external field time dependence describing annual and semi-annual signals with additional dependence on a bespoke Vector Magnetic Disturbance index. Satellite data are weighted by spatial density, along-track standard deviations, and a larger-scale noise estimator defined in terms of a measure of Local Area Vector Activity at the geographically closest magnetic observatories to the sampled datum. Forecasting of the magnetic field secular variation for 2020–2025 is by advection of the main field using steady core surface flows with steady acceleration applied. We also investigate the performance of the previous generation of candidate secular variation models, for IGRF-12, analysing the agreement of the candidates between 2015 and 2020 with the retrospective IGRF-13. We find that there is no clear distinction between the performance of mathematically and physically extrapolated forecasts in the period 2015–2020. We confirm that the methodology for the BGS IGRF-12 predictions performed well, despite observed secular accelerations that are highlighted by our analysis, and thus justify the methodology used for our IGRF-13 SV candidate. [ABSTRACT FROM AUTHOR]

The South Atlantic Anomaly (SAA) is a region at Earth's surface where the intensity of the magnetic field is particularly low. Accurate characterization of the SAA is important for both fundamental understanding of core dynamics and the geodynamo as well as societal issues such as the erosion of instruments at surface observatories and onboard spacecrafts. Here, we propose new measures to better characterize the SAA area and center, accounting for surface intensity changes outside the SAA region and shape anisotropy. Applying our characterization to a geomagnetic field model covering the historical era, we find that the SAA area and center are more time dependent, including episodes of steady area, eastward drift and rapid southward drift. We interpret these special events in terms of the secular variation of relevant large-scale geomagnetic flux patches on the core–mantle boundary. Our characterization may be used as a constraint on Earth-like numerical dynamo models. [ABSTRACT FROM AUTHOR]

Models of the geomagnetic field rely on magnetic data of high spatial and temporal resolution to give an accurate picture of the Earth's internal magnetic field and its time-dependence. The magnetic data from low-Earth orbit satellites of dedicated magnetic survey missions such as CHAMP and Swarm play a key role in the construction of such models. Unfortunately, there are no magnetic data available from such satellites after the end of the CHAMP mission in 2010 and before the launch of the Swarm mission in late 2013. This limits our ability to recover signals on timescales of 3 years and less during this gap period. The magnetic data from platform magnetometers carried by satellites for navigational purposes may help address this data gap provided that they are carefully calibrated. Earlier studies have demonstrated that platform magnetometer data can be calibrated using a fixed geomagnetic field model as reference. However, this approach can lead to biased calibration parameters. An alternative approach has been developed in the form of a co-estimation scheme which consists of simultaneously estimating both the calibration parameters and a model of the internal part of the geomagnetic field. Here, we go further and develop a scheme, based on the CHAOS field modeling framework, that involves co-estimation of both internal and external geomagnetic field models along with calibration parameters of platform magnetometer data. Using our implementation, we are able to derive a geomagnetic field model spanning 2008 to 2018 with satellite magnetic data from CHAMP, Swarm, secular variation data from ground observatories, and platform magnetometer data from CryoSat-2 and the GRACE satellite pair. Through a number of experiments, we explore correlations between the estimates of the geomagnetic field and the calibration parameters, and suggest how these may be avoided. We find evidence that platform magnetometer data provide additional information on the secular acceleration, especially in the Pacific during the gap between CHAMP and Swarm. This study adds to the evidence that it is beneficial to use platform magnetometer data in geomagnetic field modeling. [ABSTRACT FROM AUTHOR]

An approximation is developed that lends itself to accurate description of the physics of fluid motions and motional induction on short time scales (e.g. decades), appropriate for planetary cores and in the geophysically relevant limit of very rapid rotation. Adopting a representation of the flow to be columnar (horizontal motions are invariant along the rotation axis), our characterization of the equations leads to the approximation we call plesio-geostrophy, which arises from dedicated forms of integration along the rotation axis of the equations of motion and of motional induction. Neglecting magnetic diffusion, our self-consistent equations collapse all three-dimensional quantities into two-dimensional scalars in an exact manner. For the isothermal magnetic case, a series of fifteen partial differential equations is developed that fully characterizes the evolution of the system. In the case of no forcing and absent viscous damping, we solve for the normal modes of the system, called inertial modes. A comparison with a subset of the known three-dimensional modes that are of the least complexity along the rotation axis shows that the approximation accurately captures the eigenfunctions and associated eigenfrequencies. [ABSTRACT FROM AUTHOR]

As posted by the Working Group V of the International Association of Geomagnetism and Aeronomy (IAGA), the 13th generation of the International Geomagnetic Reference Field (IGRF) has been released at the end of 2019. Following IAGA recommendations, in this work we present a candidate model for the IGRF-13, for which we have used the available Swarm satellite and geomagnetic observatory ground data for the last year. In order to provide the IGRF-13 candidate, we have extrapolated the Gauss coefficients of the main field and its secular variation to January 1st, 2020. In addition, we have generated a Definitive Geomagnetic Reference Field model for 2015.0 using the same modelling approach, but focussed on a 1-year time window of data centred on 2015.0. To jointly model both satellite and ground data, we have followed the classical protocols and data filters applied in geomagnetic field modelling. Novelty arrives from the application of bootstrap analysis to solve issues related to the inhomogeneity of the spatial and temporal data distributions. This new approach allows the estimation of not only the Gauss coefficients, but also their uncertainties. [ABSTRACT FROM AUTHOR]

We describe a new, original approach to the modelling of the Earth's magnetic field. The overall objective of this study is to reliably render fast variations of the core field and its secular variation. This method combines a sequential modelling approach, a Kalman filter, and a correlation-based modelling step. Sources that most significantly contribute to the field measured at the surface of the Earth are modelled. Their separation is based on strong prior information on their spatial and temporal behaviours. We obtain a time series of model distributions which display behaviours similar to those of recent models based on more classic approaches, particularly at large temporal and spatial scales. Interesting new features and periodicities are visible in our models at smaller time and spatial scales. An important aspect of our method is to yield reliable error bars for all model parameters. These errors, however, are only as reliable as the description of the different sources and the prior information used are realistic. Finally, we used a slightly different version of our method to produce candidate models for the thirteenth edition of the International Geomagnetic Reference Field. [ABSTRACT FROM AUTHOR]

Earth's magnetic field is presently characterized by a large and growing anomaly in the South Atlantic Ocean. The question of whether this region of Earth's surface is preferentially subject to enhanced geomagnetic variability on geological timescales has major implications for core dynamics, core-mantle interaction, and the possibility of an imminent magnetic polarity reversal. Here we present paleomagnetic data from Saint Helena, a volcanic island ideally suited for testing the hypothesis that geomagnetic field behavior is anomalous in the South Atlantic on timescales of millions of years. Our results, supported by positive baked contact and reversal tests, produce a mean direction approximating that expected from a geocentric axial dipole for the interval 8 to 11 million years ago, but with very large associated directional dispersion. These findings indicate that, on geological timescales, geomagnetic secular variation is persistently enhanced in the vicinity of Saint Helena. This, in turn, supports the South Atlantic as a locus of unusual geomagnetic behavior arising from core-mantle interaction, while also appearing to reduce the likelihood that the present-day regional anomaly is a precursor to a global polarity reversal. [ABSTRACT FROM AUTHOR]

We simulated the impact of long‐term changes in the geomagnetic field on the spatial pattern of the Weddell Sea Anomaly (WSA). The Weddell Sea Anomaly, belonging to the region near the tip of the Antarctic Peninsula, Falklands, and surrounding seas, is the best‐known example of ionospheric reversed diurnal cycle. This paper investigates whether and to what extent long‐term changes in the geomagnetic field influence spatial morphology of the WSA. The major concept centers on a normalized density difference index, serving as a measure of difference between dayside and nightside ionospheric density. Through combined analysis of in situ ionospheric electron density data from past mission CHAMP, ongoing ESA mission Swarm, and simulations based on the NCAR TIEGCM model, we examine changes in the spatial pattern of the WSA‐like anomalies, which could be linked to long‐term changes in the geomagnetic field. A series of simulations provides theoretical analysis of changes in the morphology of the WSA on time scales longer than several decades. Numerical analysis shows that from the time when the WSA was discovered (around 1960) till present, maximum of the WSA has migrated by approximately 7° in longitude towards the Pacific Ocean, showing clear westward drift, consistent with temporal evolution of the geomagnetic dipole component. Key Points: Numerical reconstruction of the WSA is derived with the TIEGCM modelTwo LEO satellite missions are used for determining and tracking maximum of the WSAThe WSA exhibits westward migration towards the Pacific Ocean [ABSTRACT FROM AUTHOR]

For the past 20 years, state of the art geomagnetic core field models have relied heavily on magnetic measurements made from space-based instrumentation. These models have revealed rapid global magnetic field variations on sub-decadal timescales originating in Earth's core. With the end of the CHAMP mission in 2010 and the launch of Swarm in late 2013, there has been a 3-year gap in high-quality satellite measurements of the geomagnetic field. Geomagnetic field models have therefore relied on ground observatory data to fill in this gap period. However, ground observatories are unable to provide a truly global picture of the core field and its temporal changes. Many satellites in operation carry vector fluxgate "platform" magnetometers for attitude control, which can offer an alternative to relying on ground observatory measurements during the gap period. However, these instruments need to be carefully calibrated in order to provide meaningful information on Earth's core field. Some previous studies attempted to calibrate such instruments with a priori geomagnetic field models. This approach has several disadvantages: (1) errors in the model will introduce errors in the calibration parameters, and (2) relying on an a priori model may not be feasible in the post-Swarm era. In this paper, we develop a novel approach to build a time-dependent geomagnetic field model from platform magnetometer data, by co-estimating their calibration parameters with the internal field parameters. This method does not require an a priori geomagnetic field model, but does require a dataset of previously calibrated data. We use CHAMP, Swarm, and ground observatory measurements to supply this dataset, and incorporate platform magnetic measurements from DMSP and Cryosat-2 during the gap years. We find that the calibration parameters of DMSP and Cryosat-2 can be reliably estimated, and these missions provide meaningful information on rapid core field variations during the gap period. [ABSTRACT FROM AUTHOR]

The progressive southward reoccupation of territories of the Iberian Peninsula by the Christian kingdom against the Muslims from the eighth century AD onwards is a well-known process. However, there are few well-dated sites of this period, especially in the northern plateau of Spain. Here we report the full vector archaeomagnetic dating of two hearths from the archaeological site of La Pudia I (Caleruega, Castile-and-León, Spain). Both hearths were archaeomagnetically investigated in order to date their last use linked to the abandonment of the site. The archaeomagnetic direction was analysed through thermal (TH) and stepwise alternating field (AF) demagnetization of the natural remanent magnetization (NRM). Pseudo-single domain slightly substituted magnetite was identified as the main magnetic carrier. Thellier–Coe type absolute archaeointensity determinations were carried out on 48 samples from both hearths. The mean directions obtained were independently analysed both at sample and at specimen levels yielding very similar results but statistically distinguishable at 95% confidence level. The archaeomagnetic dating was carried out by comparing the mean directions and archaeointensity values of both hearths with the SHA.DIF.14k geomagnetic field model. The results obtained are in agreement with the archaeological context, suggesting that the abandonment of the archaeological site took place between the end of ninth century and the first half of the eleventh century AD. These results provide one of the first evidences of independently well-dated sites of the Christian conquest in the Iberian northern plateau at the onset of the Early–High Middle Ages. [ABSTRACT FROM AUTHOR]

The strength and structure of the Earth's magnetic field is gradually changing. During the next 50 years the dipole moment is predicted to decrease by ∼3.5%, with the South Atlantic Anomaly expanding, deepening, and continuing to move westward, while the magnetic dip poles move northwestward. We used simulations with the Thermosphere‐Ionosphere‐Electrodynamics General Circulation Model to study how predicted changes in the magnetic field will affect the climate of the thermosphere‐ionosphere system from 2015 to 2065. The global mean neutral density in the thermosphere is expected to increase slightly, by up to 1% on average or up to 2% during geomagnetically disturbed conditions (Kp≥4). This is due to an increase in Joule heating power, mainly in the Southern Hemisphere. Global mean changes in total electron content (TEC) range from −3% to +4%, depending on season and UT. However, regional changes can be much larger, up to about ±35% in the region of ∼45°S to 45°N and 110°W to 0°W during daytime. Changes in the vertical E→×B→ drift are the most important driver of changes in TEC, although other plasma transport processes also play a role. A reduction in the low‐latitude upward E→×B→ drift weakens the equatorial ionization anomaly in the longitude sector of ∼105–60°W, manifesting itself as a local increase in electron density over Jicamarca (12.0°S, 76.9°W). The predicted increase in neutral density associated with main magnetic field changes is very small compared to observed trends and other trend drivers, but the predicted changes in TEC could make a significant contribution to observationally detectable trends. Key Points: Predicted magnetic field changes from 2015 to 2065 cause a very minor (up to 1–2%) increase in global mean thermosphere neutral densityEffects on total electron content are larger, up to ±35% in the American sector (∼45°S to 45°N, 110°W to 0°W)Reduced vertical E→×B→ drifts at Jicamarca and surrounding longitudes (∼105–60°W) cause a weakening of the equatorial ionization anomaly [ABSTRACT FROM AUTHOR]

Study of the paleosecular variation of the Earth's Magnetic Field is an almost unique way to estimate the critical conditions at the core-mantle boundary. The continuous and quasi-periodic changes in the magnitude and directions (declination and inclination) could be detected from direct instrumental measurements or through the remanent magnetization of volcanic and sedimentary rocks. Burned archaeological artifacts are another reliable source of geomagnetic information. Besides the great efforts in Central and North America to retrieve the fine characteristics of the geomagnetic field during the last centuries, no homogeneous data distribution has still been achieved in Mexico and the surrounding areas. To improve the analysis of the geomagnetic full vector in Mexico, we compiled and updated available data for the last 200 years exclusively based on full geomagnetic vectors obtained from direct historical measurements, magnetic repeat stations, geomagnetic observatories and maritime exploratory campaigns. A relocation error procedure allowed the selection of 658 reliable data, discarding those belonging to the sites located in the most northern part of the analyzed area. The spectral analysis through the second derivatives reveals relatively fast fluctuations in both intensity and directions between 1890 and 1960. Moreover, a marked difference is observed between the Mexican record and global geomagnetic models between 1840 and 1940. • Updated available data for the last 200 years exclusively based on full geomagnetic vectors. • A relocation error procedure allowed the selection of 658 reliable data. • Homogeneously distribution across Mexico and the southern United States. • Marked differences with previously obtained records and main geomagnetic global models. • Extreme variation episodes between 1840 and 1940, some of which were observed in Europe. [ABSTRACT FROM AUTHOR]

The Earth’s ionosphere presents long-term trends that have been of interest since a pioneering study in 1989 suggesting that greenhouse gases increasing due to anthropogenic activity will produce not only a troposphere global warming, but a cooling in the upper atmosphere as well. Since then, long-term changes in the upper atmosphere, and particularly in the ionosphere, have become a significant topic in global change studies with many results already published. There are also other ionospheric long-term change forcings of natural origin, such as the Earth’s magnetic field secular variation with very special characteristics at equatorial and low latitudes. The ionosphere, as a part of the space weather environment, plays a crucial role to the point that it could certainly be said that space weather cannot be understood without reference to it. In this work, theoretical and experimental results on equatorial and low-latitude ionospheric trends linked to the geomagnetic field secular variation are reviewed and analyzed. Controversies and gaps in existing knowledge are identified together with important areas for future study. These trends, although weak when compared to other ionospheric variations, are steady and may become significant in the future and important even now for long-term space weather forecasts.

Abstract The process of magnetic field stretching transfers kinetic energy to magnetic energy and thereby maintains dynamos against ohmic dissipation. Stretching at depth may play an important role in shaping the field morphology and in the dynamo action. Here, we analyze snapshots from self-consistent 3D numerical dynamos to unravel the nature of field-flow interactions that induces stretching secular variation of the radial magnetic field at mid-depth of the shell. We search for roots of intense flux patches identified at the outer boundary. The deep radial field structures exhibit a position shift with respect to the locations of the outer boundary patches, consistent with a mixed effect of tangent cylinder rim and plume-like dynamics. A global stretching/advection rms ratio is ∼ 1.5–3 times larger than that of poloidal/toroidal flows. In addition, local stretching is often more effective than advection, in particular at regions of significant field-aligned flow. On average at roots of high-latitude flux patches, total stretching is 1.1 times larger than total advection despite the poloidal flow being only 0.37 of the toroidal flow. Radial stretching secular variation acts as an effective dynamo mechanism at regions where laterally varying radial flow shears toroidal field lines to generate a poloidal magnetic field. Stretching at depth exhibits similar parameter dependence as that of stretching at the outer boundary, with the strongest dependence being on the magnetic Prandtl number in both cases. Our results provide insights into the underlying deep dynamo mechanisms that sustain intense magnetic flux patches at the outer boundary.

Abstract CHAMP satellite and relatively densely spaced ground-based data measured over southern Africa between 2005 and 2010 are combined in a new regional, harmonic spline based, core field model. This new SACFM-2 model is compared to the regional SARM model, which is based only on satellite data, and the global CHAOS-6 model. The results agree well in the vertical (Z) component, with somewhat larger differences in the horizontal components. The Z component and total intensity F are used to investigate the evolution of the South Atlantic Anomaly in this region. The computed maps of main field of the Z component and total intensity F show a steady decrease in the field over the years during the study period, indicating the evolution of the South Atlantic Anomaly over southern Africa and suggesting an increase in the area of this feature.

We analyze the auroral boundary corrected geomagnetic latitude provided by the Auroral Boundary Index (ABI) database to estimate long-term changes of core origin in the area enclosed by this boundary during 1983–2016. We design a four-step filtering process to minimize the solar contribution to the auroral boundary temporal variation for the northern and southern hemispheres. This process includes filtering geomagnetic and solar activity effects, removal of high-frequency signal, and additional removal of a ~20–30-year dominant solar periodicity. Comparison of our results with the secular change of auroral plus polar cap areas obtained using a simple model of the magnetosphere and a geomagnetic core field model reveals a decent agreement, with area increase/decrease in the southern/northern hemisphere respectively for both observations and model. This encouraging agreement provides observational evidence for the surprising recent decrease of the auroral zone area.

Abstract—Paleo- and rock magnetic, sedimentological, and isotope geochemical study is carried out for the carbonate member of Late Neoproterozoic Nichatka Formation (Siberian Platform, western slope of the Aldan Shield) enclosed within glacial deposits corresponding to the hypothetical event of "Snowball Earth" global glaciation. Based on the sedimentological, rock magnetic, and geochemical indications it is established that sediments composing this member have varve-type seasonal stratification and, according to our estimates, have been accumulated for at most 13 thousand years. Obtaining the detailed paleomagnetic data for the Precambrian varves allowed us to reveal a linear trend in the distribution of the virtual geomagnetic poles and to link it with the peculiarity of secular variation of the geomagnetic field during the time span of the Nichatka Formation. The paleomagnetic record in the periglacial sediments of the Nichatka Formation testifies to their deposition close to the equator which might be considered as supporting the Snowball Earth hypothesis. However, the absence of annual temperature fluctuations within the equatorial belt makes the formation of seasonal deposits at low latitudes barely possible and completely excludes such a possibility in the conditions close to total glaciation. The contradiction between paleoclimatic and paleomagnetic data is not explained in the context of the actualistic model of the geomagnetic field. The peculiarities of the paleomagnetic record in the Nichatka Formation, similar to the record of the field during the reversal, suggest that the geomagnetic field in the Neoproterozoic could be determined by substantial contribution of the low-latitude non-axial-dipole component. This peculiarity of the Neoproterozoic geomagnetic field can explain the entire set of the worldwide paleomagnetic data implying low latitude glaciations in the Neoproterozoic. [ABSTRACT FROM AUTHOR]

Chronologies of lake-sediment records covering the last centuries to millennia are usually based on both short-lived radionuclides and radiocarbon dating. However, beyond the range of short-lived radionuclides, age model accuracy often suffers from large radiocarbon uncertainties. For high-altitude records, this issue is even more prominent as terrestrial plant fragments for radiocarbon dating are often lacking due to the sparse vegetation in such environments. In this study, we evaluate the potential of the geomagnetic field secular variations as a complementary tool to establish more robust age–depth relationships. Our palaeomagnetic study, applied to five high-altitude lakes from the western European Alps, first shows that recent unconsolidated sediments can carry stable remanent magnetization. The analysis of the magnetic parameters indicates that low-coercivity pseudo-single domain magnetite grains carry the natural magnetization. Nevertheless, the quality of palaeomagnetic secular variation records varies from one lake to another. This quality can be illustrated through the calculation of the declination/inclination maximum angular variations and their comparison to the expected value. Compared with available models, the declination variations are usually too large and the inclination too high. We discuss the validity of palaeosecular variation (PSV) of the Earth's magnetic field regarding rock magnetism, magnetization processes and possible deformation during coring. From a magnetic point of view, the quality of data is variable, but the characteristic remanent magnetization direction is consistent at site level between neighbouring lakes and with the reference curve, suggesting that geomagnetic field secular variations are approximately recorded. Finally, we attempt to correlate the declination/inclination variations of the characteristic remanent magnetization measured in the five records to the reference geomagnetic model to provide additional chronological markers for age–depth modelling. These stratigraphic chrono-markers appear in systematic agreement with our previous chronological data and enable a reduction of dating uncertainties up to 30% when including these chrono-markers in the age–depth modelling. This agreement supports the interpretation that PSV may have been recorded more or less accurately depending on the studied lake. Therefore, coupled with a comprehensive understanding through other analysis (sedimentology, dating, geochemistry), PSV can be used to improve the age models in the more favourable cases. [ABSTRACT FROM AUTHOR]

We normalize data from 43 Chinese observatories and select data from ten Chinese observatories with most continuous records to assess the secular variations (SVs) and geomagnetic jerks by calculating the deviations between annual observed and CHAOS-6 model monthly means. The variations in the north, east, and vertical eigendirections are studied by using the covariance matrix of the residuals, and we find that the vertical direction is strongly affected by magnetospheric ring currents. To obtain noise-free data, we rely on the covariance matrix of the residuals to remove the noise contributions from the largest eigenvalue or vectors owing to ring currents. Finally, we compare the data from the ten Chinese observatories to seven European observatories. Clearly, the covariance matrix method can simulate the SVs of Dst, the jerk of the northward component in 2014 and that of the eastward component in 2003.5 in China are highly agree with that of Vertically downward component in Europe, compare to CHAOS-6, covariance matrix method can show more details of SVs. [ABSTRACT FROM AUTHOR]

Geomagnetic jerks are the shortest temporal variations of the magnetic field generated in the Earth's core. The physical mechanism producing such abrupt changes as well as their spatio-temporal characteristics are not well understood. In order to explore geomagnetic jerks generation and their characteristics, we use a set of synthetic core flow models to solve the radial magnetic induction equation. We analyze changes of trend in the secular variation time series using a cubic polynomial fit, by invoking a new formalism of jerk amplitude per unit duration time. A new visualization scheme allows interpreting jerk amplitudes and occurrences in space and time. We find that a mild time-dependence of flow amplitude, while keeping a fixed pattern, reproduces geomagnetic jerk amplitudes. The polynomial fits were compared with two line-segments fits at ten sampled magnetic observatories about historical jerk occurrences. The differences between the misfits in the two approaches are small, which may question the definition of geomagnetic jerks as sharp "V-shape". The local time series in our models exhibit secular acceleration changes of sign that reproduce some main observed characteristics of geomagnetic jerks: (i) a range of amplitudes that encompass those observed in geomagnetic jerks, (ii) non-simultaneous occurrence, (iii) non-global occurrence, (iv) spatial variability of amplitudes and (v) strongest amplitudes in the radial component. • We explore the origin of geomagnetic jerks by forward solving the radial magnetic induction equation using a set of synthetic core flows. • Geomagnetic jerk amplitudes are well reproduced by a mild time-dependence of flow amplitude, while keeping a fixed pattern. • Our models recover geomagnetic jerk characteristics such as non-simultaneous behaviour and strongest jerks in the radial component. [ABSTRACT FROM AUTHOR]

Abstract Changes of the Earth's magnetic field have been observed with varying spatial and temporal resolution since the middle ages by means of compasses, and later by refined mechanical inclination and intensity instruments, until the modern use of quantum mechanical devices and satellite observations. Ancient records, on the other hand, rely on the natural magnetization of archaeological artifacts and other materials, such as rocks and sediments, being therefore indirect, less accurate and sparser. The combination of such heterogeneous records into a single, self-consistent, global model of the geomagnetic field is very challenging: the highly uneven data coverage, both in space and time, requires a careful handling of data uncertainties and error correlations. Previous models dealt with this problem using separated treatments of instrumental data and indirect records, respectively, as well as a different handling of the dipole field component, with respect to non-dipole terms. Here we present a global geomagnetic field model based, for the first time, on the simultaneous inversion of historical, archaeomagnetic, and volcanic records, using a Bayesian approach with minimal-committing time regularization that minimizes the energy of secular variation. A detailed assessment of data uncertainty and error correlation is used to minimize artifacts generated by the appearance of an overwhelming number of incomplete, mostly declination-only records, associated with shipboard measurements in the early colonial period. Our model yields lower dipole energies, which better match modern values, as well as higher non-dipole energies and a stronger secular variation of the dipole moment. Some model artifacts associated with extremely heterogeneous distributions of records in time and space are not completely eliminable and might be caused by incorrect a-priori uncertainty assessments. Alternatively, they might represent an intrinsic limit that can be overcome only by adding new records at critical locations and times. Highlights • Geomagnetic field model based on historical, archaeomagnetic and volcanic records • Self-consistent reconstruction of dipole and non-dipole energies • Critical evaluation of model uncertainty estimates [ABSTRACT FROM AUTHOR]

Highlights • Seven main distinctive zones through the entire core were identified. • A characteristic magnetization was retrieved from each sediment sample. • Comparison of inclination log obtained against the predicted by the global model. • An alternative sedimentation-rate independent age–depth model is proposed. • A correlation between magnetic mineralogy and paleoenvironments was stablished. Abstract We report detailed rock-magnetic and paleomagnetic results from a 27-m-long sedimentary core collected at depocenter of Lake Chapala, western Trans-Mexican Volcanic Belt. Rock-magnetic parameters point to PSD grain-size titanomagnetites as the dominant magnetic mineralogy. Observed variations, in both concentration and grain-size dependent magnetic parameters, reflect changes in the sediment supply through the studied depth, but mainly during the last half century. Stable characteristic remanent magnetization, representative of secular variation of the geomagnetic field, was retrieved after alternating field demagnetization of the natural remanent magnetization (NRM). The comparison of the obtained magnetic inclination and declination logs against the corresponding curves predicted by the Global Geomagnetic Field Model SHA.DIF.14k enables to establish an alternative sedimentation-rate independent age-depth model for the whole core, in reasonably good agreement with the one calculated using conventional radiocarbon dating. This approach works not only for Lake Chapala, but also for Lake Chalco. The correlation established between rock-magnetic parameters and paleoenvironments, based on diatom identification of representative samples within specific zones, can be considered as a novel proxy methodology. This approach opens a new perspective for new, more detailed rock-magnetic, paleomagnetic and paleoenvironmental investigations for better paleoclimatic reconstructions. [ABSTRACT FROM AUTHOR]

The convective state of the top of Earth's outer core is still under debate. Conflicting evidence from seismology and geomagnetism provides arguments for and against a thick stably stratified layer below the core-mantle boundary. Mineral physics and cooling scenarios of the core favor a stratified layer. However, a non-zero secular variation of the total geomagnetic energy on the core-mantle boundary is evidence for the presence of radial motions extending to the top of the core. We compare the secular variation of the total geomagnetic energy with the secular variation of the geomagnetic dipole intensity and tilt. We demonstrate that both the level of cancellations of the sources and sinks of the dipole intensity secular variation, as well as the level of cancellations of the sources and sinks of the dipole tilt secular variation, are either larger than or comparable to the level of cancellations of the sources and sinks of the total geomagnetic energy secular variation on the core-mantle boundary, indicating that the latter is numerically significant hence upwelling/downwelling reach the top of the core. Radial motions below the core-mantle boundary are either evidence for no stratified layer or to its penetration by various dynamical mechanisms, most notably lateral heterogeneity of core-mantle boundary heat flux.

This study carries out a statistical analysis of high-resolution PSV records for the last ~70 ka from three different regions of the Earth. We consider directional and intensity variability in each region on time scales of 103-105 years in order to evaluate long-term PSV. We then compare those results with more traditional long-term PSV statistical studies averaged over ~106 years. Three replicate PSV records from one region (subtropical North Atlantic Ocean) were averaged at overlapping 3 and 9 ka intervals. Variability in both scalar inclination and declination variability and vector angular dispersion are significant and coherent among the three records. The vector dispersion is relatively low for most of the time but contains two relatively narrow intervals (~30–42 and 60–65 ka) of high dispersion. (Vector dispersion in all records was calculated after removing directions with true excursional VGPs, VGPs < 45° N). We have carried out a comparable statistical analysis on two other PSV records from other parts of the Earth (Chile margin; Philippines/Indonesia). The results for these three regions are comparable in their overall style of variability. The scalar directional variability from the Philippines/Indonesia is quite different in detail from the other two regions, as might be expected, but the scalar directional variability between the Western Hemisphere regions is remarkably consistent considering their distance from one another. This may be associated with them being on the same longitude swath and having some coherent dynamo activity occurring along that path. Three magnetic field excursions occur in the study interval. All three excursions are associated with the two highest vector dispersion intervals. Paleointensity records from the three regions were subjected to the same statistical analysis as the directions. These records are all coherent in their pattern of variability. The similarity in paleointensity variability on a global scale is expected even though the detailed scalar directional variability is not coherent on a global scale. The pattern of intensity variability is strongly correlated with the pattern of vector dispersion and excursions on a global scale—high (low) intensity is associated with low (high plus excursions) vector dispersion. The fact that regional directional variability is always larger than “normal” during low intensity/excursional intervals, even though the effect of true excursional directions was removed, suggests that we need to reevaluate what field variability was like during low intensity/excursional intervals on a global scale and how/why it was different from today's field (last 104 years).

Variations of the geomagnetic field prior to direct observations are inferred from archeo- and paleomagnetic experiments. Seemingly unusual variations not seen in the present-day and historical field are of particular interest to constrain the full range of core dynamics. Recently, archeomagnetic intensity spikes, characterized by very high field values that appear to be associated with rapid secular variation rates, have been reported from several parts of the world. They were first noted in data from the Levant at around 900 BCE. A recent re-assessment of previous and new Levantine data, involving a rigorous quality assessment, interprets the observations as an extreme local geomagnetic high with at least two intensity spikes between the 11th and 8th centuries BCE. Subsequent reports of similar features from Asia, the Canary Islands and Texas raise the question of whether such features might be common occurrences, or whether they might even be part of a global magnetic field feature. Here we use spherical harmonic modeling to test two hypotheses: firstly, whether the Levantine and other potential spikes might be associated with higher dipole field intensity than shown by existing global field models around 1,000 BCE, and secondly, whether the observations from different parts of the world are compatible with a westward drifting intense flux patch. Our results suggest that the spikes originate from intense flux patches growing and decaying mostly in situ, combined with stronger and more variable dipole moment than shown by previous global field models. Axial dipole variations no more than 60% higher than observed in the present field, probably within the range of normal geodynamo behavior, seem sufficient to explain the observations.