Global Geomagnetic Field Research Articles

The CSES global geomagnetic field model (CGGM): an IGRF-type global geomagnetic field model based on data from the China Seismo-Electromagnetic Satellite

Using magnetic field data from the China Seismo-Electromagnetic Satellite (CSES) mission, we derive a global geomagnetic field model, which we call the CSES Global Geomagnetic Field Model (CGGM). This model describes the Earth’s magnetic main field and its linear temporal evolution over the time period between March 2018 and September 2019. As the CSES mission was not originally designed for main field modelling, we carefully assess the ability of the CSES orbits and data to provide relevant data for such a purpose. A number of issues are identified, and an appropriate modelling approach is found to mitigate these. The resulting CGGM model appears to be of high enough quality, and it is next used as a parent model to produce a main field model extrapolated to epoch 2020.0, which was eventually submitted on October 1, 2019 as one of the IGRF-13 2020 candidate models. This CGGM candidate model, the first ever produced by a Chinese-led team, is also the only one relying on a data set completely independent from that used by all other candidate models. A successful validation of this candidate model is performed by comparison with the final (now published) IGRF-13 2020 model and all other candidate models. Comparisons of the secular variation predicted by the CGGM parent model with the final IGRF-13 2020–2025 predictive secular variation also reveal a remarkable agreement. This shows that, despite their current limitations, CSES magnetic data can already be used to produce useful IGRF 2020 and 2020–2025 secular variation candidate models to contribute to the official IGRF-13 2020 and predictive secular variation models for the coming 2020–2025 time period. These very encouraging results show that additional efforts to improve the CSES magnetic data quality could make these data very useful for long-term monitoring of the main field and possibly other magnetic field sources, in complement to the data provided by missions such as the ESA Swarm mission.

Analyzing the geomagnetic axial dipole field moment over the historical period from new archeointensity results at Bukhara (Uzbekistan, Central Asia)

Since the mid-19th century, direct measurements of both intensity and direction of the Earth's magnetic field have been available, allowing an accurate determination of its spatio-temporal variations. Prior to this time, between ~1600 and 1840, only direct directional measurements are available. Therefore, the construction of global field models over this period requires either a specific treatment of the axial dipole field component or the use of archeomagnetic intensity data. In this study, we use a regional approach based on the construction of an archeointensity variation curve in Central Asia. We analyze baked clay brick fragments sampled in Bukhara (Uzbekistan), dated between the end of the 16th century and the beginning of the 19th century. This city is of particular interest for archeomagnetism due to the well-preserved old buildings accurately dated by documentary archives. A series of archeointensity results is obtained using the Triaxe experimental protocol, which shows a decreasing trend in intensity from ~1600 to ~1750, with intensities during the 18th century lower than expected from global geomagnetic field models. These new data appear consistent with other Triaxe data previously obtained in western Europe and western Russia, when transferred to Bukhara using the field geometry of the gufm1 model. Together, these data are used to recalibrate the axial dipole moment evolution provided by this model. The resulting evolution appears non-linear, with a clear relative minimum in the magnitude of the axial dipole during the late 18th century. We illustrate the fact that at present this evolution can neither be satisfactorily confirmed nor refuted by other datasets available in western Eurasia (as well as at a wider spatial scale), mainly due to the significant dispersion of the data. Our interpretation relies on the accuracy of the field geometry of the gufm1 model, which appears less reliable prior to ~1750. Nevertheless, the minimum proposed in the 18th century seems to be a true feature of axial dipole behavior.

Application of multiple dating techniques to the Holocene sediments of Angrenjin Co in the southern Tibetan Plateau

The production of high-resolution and accurate age models for lake sediments in the Tibetan Plateau (TP) is a prerequisite for subsequent paleoclimatic studies. However, Holocene lake sedimentary records with high-quality age controls from the TP are extremely scarce. Here we present the results of a comparative study of the results of the application of multiple dating techniques (14C, 210Pb, 137Cs) and a paleomagnetic secular variation (PSV) study of the sediments of Angrenjin Co in the southern central TP, during the last 3600 years. The PSV data are verified by comparison with a stacked PSV record from East Asia and with a global geomagnetic field model. The results of 14C-dating, validated by independent 210Pb, 137Cs and PSV dating, together with Bacon age modeling, indicate that the reservoir has an age range of ~2287 ± 58–6434 ± 165 years; in addition, the median offset ages estimated by the Bacon age-depth model range from ~1330 ± 428 to 3102 ± 513 years. This cross-dating approach effectively reduces dating uncertainties for lake sediments in the TP, and it can accommodate a variable reservoir effect. Comparison of the paleomagnetic inclination record from Angrenjin Co with previously published records from Tangra Yumco, Taro Co and Nam Co indicates an overall high degree of similarity, suggesting that the paleomagnetic data from different lakes in the TP are reproducible. In addition, the similar inclination patterns across this large region indicate a common geomagnetic origin.

Correlation based snapshot models of the archeomagnetic field

SUMMARY For the time stationary global geomagnetic field, a new modelling concept is presented. A Bayesian non-parametric approach provides realistic location dependent uncertainty estimates. Modelling related variabilities are dealt with systematically by making little subjective a priori assumptions. Rather than parametrizing the model by Gauss coefficients, a functional analytic approach is applied. The geomagnetic potential is assumed a Gaussian process to describe a distribution over functions. A priori correlations are given by an explicit kernel function with non-informative dipole contribution. A refined modelling strategy is proposed that accommodates non-linearities of archeomagnetic observables: First, a rough field estimate is obtained considering only sites that provide full field vector records. Subsequently, this estimate supports the linearization that incorporates the remaining incomplete records. The comparison of results for the archeomagnetic field over the past 1000 yr is in general agreement with previous models while improved model uncertainty estimates are provided.

Archaeomagnetic investigations in Bolgar (Tatarstan)

The objective of this study is to provide a well-dated point for a future palaeosecular variation (PSV) reference curve for western Russia. For this purpose archaeomagnetic and magnetic property analyses were carried out on a pottery kiln unearthed at the UNESCO World Heritage site of ancient Bolgar, having a rather precise age dating. The archaeological context provided an age between 1340 and 1360 C.E. The characteristic remanence vector was determined through alternating field demagnetisation and Thellier-Thellier palaeointensity experiments. Some innovations were introduced regarding palaeointensity. The check testing the equality of blocking and unblocking temperature was redefined. This allowed waiving the commonly used additional zero-field cooling steps during the Thellier-Thellier experiment. Another innovation concerns the calculation of archaeointensity at structure level. A Bayesian approach was introduced for averaging individual specimen archaeointensities using a prior probability distribution of unknown uncertainties. Next, an additional prior probability distribution was used to correct for cooling rate effects. This resulted in a lower uncertainty compared to common practice and in eluding time consuming cooling rate experiments. The complex magnetic mineralogy consists of maghaemite, multi-domain haematite and Al-substituted haematite. Some samples contained also some non-stoichiometric magnetite. The magnetic mineralogy was determined through hysteresis loops, backfield and remanence decay curves, measurements of the frequency dependence of magnetic susceptibility and through low temperature magnetisation curves. Accompanying high-temperature thermomagnetic analyses revealed an excellent thermo-chemical stability of the studied specimens. Directions obtained from alternating field demagnetisation and those extracted from archaeointensity experiments are congruent and have low uncertainties. The obtained archaeomagnetic results are fairly in agreement with global geomagnetic field models and contemporary PSV data of the wider area. The geomagnetic field vector obtained for ancient Bolgar is of high quality, deserving thus its inclusion in a future PSV reference curve for European Russia.

Two archaeomagnetic intensity maxima and rapid directional variation rates during the Early Iron Age observed at Iberian coordinates. Implications on the evolution of the Levantine Iron Age Anomaly

Variations of geomagnetic field in the Iberian Peninsula prior to Late Iron Age times are poorly constrained. Here we report 14 directional and 10 palaeointensity results from an archaeomagnetic study carried out on 17 combustion structures recovered from six archaeological sites in eastern Spain. The studied materials have been dated by archaeological evidences and supported by radiocarbon dates (8th-5th centuries BC). Rock magnetic experiments indicate that the characteristic remanent magnetization is carried by a low coercivity magnetic phase with Curie temperatures of 500-575°C, most likely titanomagnetite/maghemite with low titanium content. Archaeointensity determinations were carried out by using the classical Thellier-Thellier experiment including pTRM-checks and magnetic anisotropy corrections. A new full vector Iberian Paleosecular Variation Curve for the Iron Age is presented. High fluctuation rates on both directions and intensities are observed during the Early Iron times that seems to be related with the Levantine Iron Age Anomaly (LIAA), the most prominent anomaly of the geomagnetic field of the last three millennia. Two intensity maxima were observed at Iberian coordinates, the oldest around 750 BC (associated with easterly declinations of around 23°) and the second 275 yrs later (475 BC) with northerly directions. The related virtual axial dipole moment was up to 14⋅1022 Am2 for the oldest materials (750 BC) and reaching 16⋅1022 Am2 for the materials corresponding to the end of the Early Iron Age.In order to investigate the origin of the unusually high fluctuations of the palaeofield we have developed a new global geomagnetic field reconstruction, the SHAWQ-IronAge model, which is based on a critical revision of the global archeomagnetic and volcanic dataset. The new model provides an improved description of the evolution of the LIAA, which is related to a normal flux patch at the core-mantle boundary (CMB) below Arabian Peninsula clearly observed at around 950 BC. This flux patch expanded towards the north-west, while decreasing in intensity, reaching Iberia at around 750 BC. Around 600-500 BC, it underwent a revival below the European continent after that it seems to vanish in situ.

Current Status and Main Scientific Results of In-flight CSES Mission

The CSES (China Seismo-Electromagnetic Satellite) is the electromagnetism satellite of China's Zhangheng mission which is planned to launch a series of microsatellites within next 10 years in order to monitor the electromagnetic environment, gravitational field. The CSES 01 probe (also called ZH-1) was launched successfully on 2 February 2018, from the Jiuquan Satellite Launch Centre (China) and is expected to operate for 5 years in orbit. The second probe CSES 02 is going to be launched in 2022. The scientific objectives of CSES are to detect the electromagnetic field and waves, plasma and particles, for studying the seismic-associated disturbances. To meet the requirements of scientific objective, the satellite is designed to be in a sun-synchronous orbit with a high inclination of 97.4° at an altitude around 507 km. CSES carries nine scientific payloads including Search-coil magnetometer, Electric Field Detector, High precision Magnetometer, GNSS occultation Receiver, Plasma Analyzer, Langmuir Probe, two Energetic Particle Detectors (including an Italian one), and Tri-Band Transmitter. Up to now, CSES has been operating in orbit for 2 years with stable and reliable performance. By using all kinds of data acquired by CSES, we have undertaken a series of scientific researches in the field of global geomagnetic field re-building, the ionospheric variation environment, waves, and particle precipitations under disturbed space weather and earthquake activities, the Lithosphere-Atmosphere-Ionosphere coupling mechanism research and so on.

Features and Source Current of Long‐Period Induced Geoelectric Field During Magnetic Storms: A Case Study

AbstractWe present a case study on the long‐period ( s) induced geoelectric field disturbance during magnetic storms. A set of continuous 33‐day measurements of the geoelectric field, geomagnetic field, geomagnetic indices, and interplanetary magnetic field are analyzed. In the studied 33 days, two magnetic storms occurred after 10 geomagnetic quiet days. To exclude the effects from the different underground electrical structures, geomagnetic and geoelectric field measurements from the same ground observatory enrolled in Chinese Meridian Project are studied. Besides, Space Weather Modeling Framework is adopted to calculate the global geomagnetic field disturbances and the contribution from different current systems. The wavelet power spectra analysis reveals that the long‐period geoelectric field disturbance appears only during magnetic storms. Especially, stronger magnetic storm generates weaker geoelectric disturbances at the same observatory. The opposite‐direction eastward geomagnetic field disturbance, which is generated mainly by the field‐aligned current at different magnetic local time, significantly changes the magnitude and direction of the induced geoelectric field via the underground impedance tensor. Therefore, both the effects caused by ring current and field‐aligned current should be considered in analyzing the ground induced geoelectric field. Excluding the effect from the geospace current source, the more accurate electrical conductivity in the upper mantle would be obtained in future.

Modeling of regional magnetic field applying spherical functions: practical aspect

Observation of geomagnetic field, measurement of its components values and establishment on their base models of geomagnetic field as well as geomagnetic mapping are the main trend of geomagnetic studies. Analytical model of geomagnetic field allows calculating the value of any component of geomagnetic field at any point of near-earth space and on the Earth. A new method has been proposed for construction of geomagnetic potential regional model. In accordance with Gauss’ fundamental studies, classical concept of geomagnetic potential became its record as endless series of Legeandre functions. A program of writing the series of geomagnetic potentials according to their spherical or ellipsoidal functions is used for most cases for modeling of global (normal) geomagnetic field with the length of a series equal to 9—13 harmonics. However in case when the sphere is not complete and only its part (a segment of the sphere or its trapezium) is taken into account the spherical Legeandre functions lose their orthogonality. In this connection for elaboration of regional field models different modifications of spherical harmonic analysis with application of spherical Legeandre functions even grade and real order are used. Such functions form an orthogonal by weight system of functions on arbitrary spherical trapezium but do not have recurrent correlation therefore we are to use for their calculation an arrangement as hypergeometric series. The area of determination of such functions is spherical segment. Working formulae have been obtained for constructing the model mentioned above. As front-end data for constructing a model of regional magnetic field the values of its components obtained at geomagnetic observatories have been used. It has also been suggested to make calculations of regional model of geomagnetic potential by a method proposed within the limits of a procedure deletion-computing-restoration. Initially a systematic product of components is found using a global model of geomagnetic field. Then abnormal values of components are calculated. A model of regional abnormal geomagnetic field is computed using basic functions. A parameter of Tikhonov’s regularizing is introduced to stabilize the solution.

First in-orbit results of the vector magnetic field measurement of the High Precision Magnetometer onboard the China Seismo-Electromagnetic Satellite

The High Precision Magnetometer (HPM) is one of the main payloads onboard the China Seismo-Electromagnetic Satellite (CSES). The HPM consists of two Fluxgate Magnetometers (FGM) and the Coupled Dark State Magnetometer (CDSM), and measures the magnetic field from DC to 15 Hz. The FGMs measure the vector components of the magnetic field; while the CDSM detects the magnitude of the magnetic field with higher accuracy, which can be used to calibrate the linear parameters of the FGM. In this paper, brief descriptions of measurement principles and performances of the HPM, ground, and in-orbit calibration results of the FGMs are presented, including the thermal drift and magnetic interferences from the satellite. The HPM in-orbit vector data calibration includes two steps: sensor non-linearity corrections based on on-ground calibration and fluxgate linear parameter calibration based on the CDSM measurements. The calibration results show a reasonably good stability of the linear parameters over time. The difference between the field magnitude calculated from the calibrated FGM components and the magnitude directly measured by the CDSM is just 0.5 nT (1σ) when the linear parameters are fitted separately for the day- and the night-side. Satellite disturbances have been analyzed including soft and hard remanence as well as magnetization of the magnetic torquer, radiation from the Tri-Band Beacon, and interferences from the rotation of the solar wing. A comparison shows consistency between the HPM and SWARM magnetic field data. Observation examples are introduced in the paper, which show that HPM data can be used to survey the global geomagnetic field and monitor the magnetic field disturbances in the ionosphere.

Last 3000 years of geomagnetic field intensity from India: New reference palaeointensity data from two east coast archaeological sites and archaeomagnetic dating insights

A comprehensive rock magnetic and absolute archaeointensity (AI) investigation was carried out on baked bricks from the east coast of India with an aim to obtain a set of well-defined palaeointensity (PI) data. The new PI data from India comprise a significant first step to improve the regional intensity reference curve, which can be used as reliable dating tool mainly for archaeomagnetic material. Standard Thellier-Coe experiments were applied on 12 independent brick samples collected from a Buddhist monument at Adurru in the Godavari delta and a Medieval Fort at Kolleti Kota site situated on an island in the Kolleru Lake. The samples thus collected represent two different archaeological periods, one around the 3rd century BCE and the other between the 4th and 7th centuries CE respectively. The data obtained from thermal demagnetization of Saturation Isothermal Remanent Magnetization (SIRM), temperature dependent magnetic susceptibility (χ–T), Isothermal Remanent Magnetization (IRM) acquisition curves, and hysteresis measurements showed that the primary carriers of Natural Remanent Magnetization (NRM) are fine-grained Pseudo-Single Domain (PSD) (Stable Single Domain (SSD) + Superparamagnetic (SP)) titanomagnetite with various titanium contents. This implies that the samples had significant stable Thermoremanent Magnetization (TRM), and, therefore, good candidates for PI analysis. Strict selection and quality-control criteria for detecting magnetomineralogical alteration and intensity data corrected for anisotropy and the cooling rate effects on TRM acquisition, offer high quality PI determinations with respect to the previous available data that missed such corrections.A total 33 specimen level PI determinations that correspond to linear NRM–TRM plots, were made using the Thellier GUI software. The corrected data yielded mean PI values of 44.6 ± 1.4 μT for the Buddhist Monument, and 36.6 ± 2.9 μT for the Fort site. By comparison and evaluation of new PI data with Global Geomagnetic Field Model predictions, the sampled bricks were archaeomagnetically dated using the Matlab tool developed by Pavón-Carrasco et al. (2011). Both ARCH3K.1 and SHA.DIF.14 K global models were used to evaluate the age estimates. The SHA.DIF.14 K model provided the most accepted age estimates that varied between 305 and 182 BCE for the early historic Buddhist Structure at Adurru, and 516–889 CE for the Medieval Fort at Kolleti Kota. These new reference archaeointensity data highlight the potential of archaeomagnetic dating to improve the archaeologically determined time periods of bricks, and also show the need for new high quality reference data to improve the geomagnetic field models from India.

Late-Quaternary secular variation data from Mexican volcanoes

We present 32 new paleomagnetic directions and 21 absolute paleomagnetic intensities (PI) from 33 volcanoes, sampled at 66 sites and covering the last 46 ka. Of these, 29 were radiocarbon-dated, 3 by thermo-luminescence, and one is of historical age (AD 1793). Rock magnetic experiments show that the dominant minerals are magnetite and titanomagnetite of low to intermediate titanium content, and of pseudo single domain size. Paleodirections were determined using stepwise demagnetization protocols, and PI by the IZZI or Thellier-Coe protocols, and strict selection criteria were applied to ensure the reliability of the data. Previously published data were evaluated to fulfill similar quality criteria as our data, resulting in: 65 directional data point; 18 paleointensity entries from lavas; 59 archeointensities are found to be acceptable. All accepted data come from a region within an 860 km radius around Mexico City. Paleosecular variation (PSV) curves of the past geomagnetic full-vector were established by using the bootstrap-resampling algorithm combined with cubic P-Splines for smoothing and interpolation. Because of the uneven time distribution of the data, this was done for two successive periods, between AD 2000 – 1600 BC and 1600 BC - 44000 BC. The full-vector PSV curves are unique for the American continents because of their temporal extension end because they include both, directions and absolute PI. The recent PSV curves, the last 3600 years, are of high resolutions which certainly have implication for both regional and global geomagnetic field models. Fast PSV change centered around AD 700 and AD 900, separated by a period of much slower PSV were observed, and rapid PI changes were found around 400 BC and AD 1000. In the period between 1600 BC and 44000 BC, PSV curves are well defined only within some periods and otherwise only define general PSV trends. Several intensity peaks with values up to 65 μT (VADM 15⋅1022 Am2) were observed around 250 BC, 12000 BC, 15000 BC and 27000 BC, and intensity lows around AD 700, 1200 BC, 24000-17000 BC, and 34000 BC. Abnormal directions were found around 26000 BC, with negative inclinations down to about -30°, which may be related to the Mono Lake or Rockall event. A comparison with other PSV curves is complicated by the large distance to Europe and the Hawaiian islands, with longitude differences of 99° and 57°, respectively. Lake sediment data from southern Texas show more differences than agreements, suggesting that either of the data may be affected by systematic errors.

A Bayesian iterative geomagnetic model with universal data input: Self-consistent spherical harmonic evolution for the geomagnetic field over the last 4000 years

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.

Rock-magnetic and paleomagnetic study on a 27-m-long core from Lake Chapala, western Mexico: Paleoenvironmental implications for the last 10 ka

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.

Emergence and evolution of the South Atlantic Anomaly revealed by the new paleomagnetic reconstruction SHAWQ2k

The South Atlantic Anomaly (SAA) is one of the most outstanding features of the present geomagnetic field. Thus, a good knowledge of the SAA is clue for a better understanding of the dynamical behavior of the geomagnetic field. To achieve this goal, paleomagnetic data are crucial since they provide the unique way to investigate past changes in the Earth's magnetic field. Here, we present a new global geomagnetic field reconstruction, the SHAWQ2k model, which is based on a critical revision of the global archeomagnetic and volcanic dataset. The new model provides an improved description of the geomagnetic field during the last 2 millennia, and yields surprising outcomes about the emergence and development of the SAA. It shows that the reversed flux patch observed at the core-mantle boundary and linked to the SAA, emerged in the Southern Hemisphere from at least 950 AD. This patch moved westward from the equator to southern latitudes, being clearly isolated after 1400 AD. In addition, since 1550 AD a second reversed flux patch moving northeastward is observed in the North Atlantic. The new data now available for the Southern Hemisphere coming from Africa and South America together with the use of an appropriated weighting scheme in the modeling process have improved our understanding of past geomagnetic field behavior and showed new evolutionary aspects of the SAA.

Extending Global Continuous Geomagnetic Field Reconstructions on Timescales Beyond Human Civilization

AbstractStudy of the late Quaternary geomagnetic field contributes significantly to understanding the origin of millennial‐scale paleomagnetic secular variations, the structure of geomagnetic excursions, and the long‐term shielding by the geomagnetic field. A compilation of paleomagnetic sediment records and archeomagnetic and lava flow data covering the past 100 ka enables reconstruction of the global geomagnetic field on such long‐term scales. We use regularized inversion to build the first global, time‐dependent, geomagnetic field model spanning the past 100 ka, named GGF100k (Global Geomagnetic Field over the past 100 ka). Spatial parametrization of the model is in spherical harmonics and time variations with cubic splines. The model is heavily constrained by more than 100 continuous sediment records covering extended periods of time, which strongly prevail over the limited number of discrete snapshots provided by archeomagnetic and volcanic data. Following an assessment of temporal resolution in each sediment's magnetic record, we have introduced smoothing kernels into the forward modeling when assessing data misfit. This accommodates the smoothing inherent in the remanence acquisition in individual sediment paleomagnetic records, facilitating a closer fit to both high‐ and low‐resolution records in regions where some sediments have variable temporal resolutions. The model has similar spatial resolution but less temporal complexity than current Holocene geomagnetic field models. Using the new reconstruction, we discuss dipole moment variations, the time‐averaged field, and paleomagnetic secular variation activity. The new GGF100k model fills the gap in the geomagnetic power spectrum in the frequency range 100–1,000 Ma−1.

Refining Holocene geochronologies using palaeomagnetic records

Abstract The aperiodic nature of geomagnetic field variations, both in intensity and direction, can aid in dating archaeological artefacts, volcanic rocks, and sediment records that carry a palaeomagnetic signal. The success of palaeomagnetic dating relies upon our knowledge of past field variations at specific locations. Regional archaeo- and palaeomagnetic reference curves and predictions from global geomagnetic field models provide our best description of field variations through the Holocene. State-of-the-art palaeomagnetic laboratory practices and accurate independent age controls are prerequisites for deriving reliable reference curves and models from archaeological, volcanic, and sedimentary palaeomagnetic data. In this review paper we give an overview of these prerequisites and the available reference curves and models, discuss techniques for palaeomagnetic dating, and outline its limitations. In particular, palaeomagnetic dating on its own cannot give unique results, but rather serves to refine or confirm ages obtained by other methods. Owing to the non-uniform character of magnetic field variations in different regions, care is required when choosing a palaeomagnetic dating curve, so that the distance between the dating curve and the record to be dated is not too large. Accurate reporting and incorporation of new, independently dated archaeo- and palaeomagnetic results into databases will help to improve reference curves and global models for all regions on Earth.

Possible correlation between the vertical component of lithospheric magnetic field and continental seismicity

Recent magnetic satellite missions facilitate new birth of large-scale geomagnetic field models and their applications to tectonics. Here, we directly compare the global geomagnetic field models NGDC-720 with the tectonics and seismicity in Mainland China and surroundings. It is found that the tectonics and seismicity in this area show remarkable correlation with the vertical component of lithospheric magnetic field (Bz) calculated at an altitude of 200 km. Previous thought was that earthquakes are more likely to occur in zero Bz belts or in obvious anomaly gradient belts. On the contrary, we find that more than half (53.2%) of the earthquakes occurred in areas with Bz of − 5 to − 3 nT or in areas with a relatively small horizontal gradient of Bz in the same time interval with the satellite data. The percentage seismic energy in these areas (− 5 nT < Bz < − 3 nT) is even as high as 94.6%. We explain this unexpected result with a two equivalent source dipole model, arguing that the viscosity difference caused by the temperature gradient within the lithosphere likely accounts for the correlation between magnetic anomalies and seismicity.

Insights on the geomagnetic secular variation in the Eastern Mediterranean: First directional data from Cyprus

The first directional archaeomagnetic results from Cyprus are presented, based on the study of seven baked clay structures excavated at the copper smelting site of Agia Varvara-Almyras, situated some 20 km south of Nicosia. This ancient copper working site represents a unique example of complete primary production of the copper metal in Cyprus. In total five furnaces, one ore roasting pit and an oven were sampled in situ for an archaeomagnetic investigation. All the studied structures are well dated, based on pottery finds and radiocarbon dates, with ages ranging from 600 BCE to 50 BCE. Several rock magnetic experiments were performed to determine the main magnetic carrier and to investigate the thermal stability of the studied material. The direction of the Characteristic Remanent Magnetization (ChRM) for each structure has been determined by demagnetization of 7–15 specimens in alternating magnetic fields up to 100–120 mT. Mean directions were calculated assuming a Fisherian distribution and are well defined for five out of the seven structures studied. The new directions are compared with the directional data and the secular variation curves available from the surrounding area, and with the predictions of global geomagnetic field models. They are an important contribution to improving our knowledge of the geomagnetic field variations in the Eastern Mediterranean and offer new insights about fast geomagnetic directional changes during the first millennium BCE that were not previously identified by the available geomagnetic field models’ predictions in this area.

Comparison of global geomagnetic field models and evaluation using marine datasets in the north-eastern Atlantic Ocean and western Mediterranean Sea

Many models describing the Earth’s magnetic field exist and are maintained by international research organizations. However, no clear general overview of the cross-compared quality of these models is available. An evaluation of the internal component and of the anomaly field as estimated by a selection of nine geomagnetic models with global coverage is presented here. Anomaly field models are also compared with an external, independently processed marine dataset extracted from Shom’s (the French Hydrographic and Oceanographic Service) geophysical database. The comparative study shows that internal geomagnetic fields computed outside the provisional domain, up to degrees 12 or 13 of spherical harmonics (SH), can be considered as equivalent. Relative amplitude differences with the World Magnetic Model 2015 (WMM2015) and the 12th generation of the International Geomagnetic Reference Field (IGRF-12) taken as references are lower than 10 nT. For an extent reduced to the north-eastern Atlantic Ocean and western Mediterranean Sea, anomaly field models up to SH degree 720 and the 3rd version of the Earth Magnetic Anomaly Grid (EMAG2v3) at the equivalent 0.5° resolution are the closest models to the filtered marine data. However, they are significant spatial discrepancies and standard deviations of differences between models and data are about 65 nT. The 3-arc-minute World Digital Magnetic Anomaly Map (WDMAM) best reflects the unfiltered marine data, but with significant standard deviations of about 70 nT. Results demonstrate that any model can be used with insignificant errors to compute raw anomalies from total field measurements by removing the internal component of the Earth’s magnetic field. In addition, they suggest that the priority should be given to WDMAM for levelling and evaluating marine datasets and for computing enhanced high-resolution regional magnetic maps.