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Polarity reversals and excursions are the most significant geomagnetic field changes generated in the liquid outer core of the Earth, therefore studying them helps understand geodynamo processes. This study examines the Matuyama-Brunhes (MB) reversal using a new reconstruction of the global geomagnetic field based on paleomagnetic data, termed GGFMB (Global Geomagnetic Field Model for the MB reversal). GGFMB covers 900-700 ka, including late Matuyama and early Brunhes. This allows us to also investigate the Kamikatsura excursion (ca. 888 ka). The model is based on 38 high-quality paleomagnetic sediment records with age control mostly independent of the magnetic signal. GGFMB suggests that the MB reversal began about ∼799 ka, when non-dipole field components increased and the axial dipole component decreased. The transitional fields first appeared on Earth’s surface in the high-latitude southern hemisphere and equatorial regions. The minimum dipole strength was reached around 780 ka and the axial dipole changed sign. After ∼10 kyr, the field stabilized in the normal polarity of the early Brunhes. The MB reversal lasted ∼29 kyr (from 799 to 770 ka) and had slower rate of dipole decay than recovery as well as lower dipole moment for several millennia before than after the reversal. According to GGFMB, the dipole moment during the Kamikatsura excursion was approximately half that of the current field and it was a regional excursion observed only over eastern Asia and North America. Our sediment data collection is heavily biased towards the northern hemisphere, thus more southern hemisphere records are needed to demonstrate GGFMB’s robustness in this region. Key Points A global geomagnetic field model for 900--700 ka is presented, including theMatuyama-Brunhes (MB) reversal and Kamikatsura excursion. The mid-point of the MB reversal is at 780 ka, with an overall duration ofnearly 30 kyr. The dipole field decays from moderate dipole moment in the late Matuyama and recovers quickly to higher values in the early Brunhes. Plain Language Summary The Earth’s magnetic field originates deep inside the planet and extends far into space, and it has undergone significant direction and intensity changes throughout geological history. The most extreme magnetic field changes are reversals, when the field changes its polarity and global field intensity reaches a minimum. We reconstruct the global geomagnetic field evolution over the time interval 900 000 to 700 000 years ago. The information about the magnetic field variations comes from the paleomagnetic signal recovered from sediment drill cores from locations all over the globe, and with good constraints on the ages of the material. Our model includes the Matuyama-Brunhes (MB) field reversal, which occurred ∼780 000 years ago. It provides a global view of this drastic field change, indicating, for example, that the recovery of the global field intensity was much faster than its decay and the average global field strength was lower for several millennia before than after the polarity change. Our model suggests that the reversal started at ∼799 ka, the actual polarity change of the axial dipole field occurred at ∼780 ka, and a stable normal polarity was reached at ∼770 ka. The whole process of the reversal therefore took ∼29 000 years.

The Space Weather (SWE) Service Network of the European Space Agency’s (ESA’s) Space Safety Programme provides timely and reliable space weather information to end users allowing the mitigation and prevention of the impact of hazards from space on the communication and navigation systems, power grids, and aviation, etc. The SWE Service Network consists of 5 Expert Service Centres (ESCs), Solar Weather, Space Radiation, Ionospheric Weather, Geomagnetic Conditions, and Heliospheric Weather, which are distributed and established across Europe providing coverage of space weather phenomena and impacts on ground and space-based infrastructure. Note that a free registration at the ESA's SWE portal is necessary to get access to the protected applications or products.Here, we give an overview of 18 products that currently are provided by GFZ in the Ionospheric Weather ESC (I-ESC) and Geomagnetic Conditions ESC (G-ESC). These products are derived from the Low Earth Orbiting (LEO) satellite and ground-based observations. In I-ESC, the following Swarm mission products are provided: Rate Of the change of TEC (ROT), Total Electron Content (TEC), in-situ electron density (Ne), Ionospheric Bubble Index (IBI), Rate Of the change of TEC Index (ROTI). And in G-ESC: the location and intensity level of the Polar Electrojet (PEJ), Field-Aligned Currents (FACs), and Vector Magnetic Field (MAG) components. The following global geomagnetic indices are provided in G-ESC, nowcast Kp (three-hourly), Hp60 (one-hourly and open-ended), Hp30 (half-hourly and open-ended) indices, most recent definitive Kp index, Kp, Ap, Hp60, ap60, Hp30, ap30 indices on tabular form, Kp and Ap (since 1932), Hp60, ap60, Hp30 and ap30 (since 1995) indices archive.

Sedimentary records of the Earth’s magnetic field often contain unwanted climatic overprints, distorting the reconstruction of paleomagnetic field intensity variations. In case of field reconstructions based on the cosmogenic isotope 10Be, whose production is modulated by the solar activity and the dipolar component of the geomagnetic field, environmental overprints arise from climatic modulations of the source distribution, transport, and sediment scavenging efficiencies. Although the lithological dependence of the scavenging efficiency is supposed to be removed by normalizing 10Be with the stable isotope 9Be, this normalization can introduce its own environmental effects, caused by changes in source, distribution and transport of two isotopes. These processes lead to inter-sites differences observed between 10Be/9Be and corresponding relative paleointensity records, limiting use for global magnetic field models constructions. Principal component analysis (PCA) and independent component analysis (ICA) of four 10Be/9Be records from West Pacific and North Atlantic Ocean sites, characterized by different environmental settings, allowed us to extract the common pattern controlled by the evolution of the dipole field. These observations are made on records covering the last 380 ka, including seven geomagnetic excursions.While the first component of cosmogenic 10Be records clearly reflects geomagnetic dipole changes, it seems that the second and third components are dominated by 100- and 23-ka periodic oscillations respectively, corresponding to Earth’s orbital forcing. PCA and ICA methods are shown to be a powerful tool for disentangling and assessing different components of cosmogenic beryllium records. The geomagnetic component can serve to better understand the long-term geomagnetic field evolution, thus improving our knowledge of driving mechanisms sustaining the geodynamo.

So-called platform magnetometers, mounted on a variety of non-dedicated satellites in Low-Earth orbit, are promising instruments to increase the spatiotemporal coverage of space-based measurements of the Earth’s magnetic field. However, these instruments often need to be calibrated to ensure a scientific accuracy and usability of the data they collect. To do this, it is important to gather information about the satellite to correct artificial disturbances caused by other payload systems as well as other influencing properties. In the past, we demonstrated that a Machine Learning-based calibration achieves competitive results. By using machine learning techniques, the magnetometer signal can be adapted to account for artificial disturbances and the proposed non-linear regression method can automatically identify relevant features and their crosstalk, enabling the use of a wider range of inputs. This reduces the analytical work required for the calibration of platform magnetometers, resulting in faster, more precise, and easily accessible magnetic datasets from non-dedicated missions. The calibrated datasets are made publicly available.In this work, we propose an extension for the known approach by incorporating the physical Biot-Savart formula into a neural network, which results in a physics-informed neural network. This improves the modeling and correction of the impact of current-induced artificial magnetic fields on the satellite and its magnetic measurements. In addition, the Average Magnetic field and Polar current System (AMPS) model is combined with the CHAOS-7 model, improving the reference model of the calibration, especially for the polar regions. This extended approach is applied to the GOCE and GRACE-FO satellite missions and their respective measurements. In the future, the underlying software shall be published and applied to a wider variety of satellites to improve the accuracy of their platform magnetometer measurements. By making this tool publicly available, we hope to enable other satellite operators to calibrate their instruments, improve the quality of their data, and make additional data available to the scientific community.

Modern geomagnetic field models are constructed from satellite and observatory data, while models on the millenial timescale are constructed from indirect records of thermoremanent and sedimentary origin. An intermediate period, spanning the last four centuries, is covered by historical survey data and ship-logs, which is strongly dominated by geomagnetic declination information. We apply a sequentialized, Gaussian process based modeling technique to this dataset and propose a new field model for this era. In order to investigate the information gained from declination records from ship-logs, we seperate the dataset and construct a second model, where unpaired declination records are removed. Instead of uncovering a more detailed field, the availability of more records leads to better resolution of the global field structure. The availability of more records helps notably to constrain global field properties like the dipole moment. It also allows to resovle some detailed field structures more accurately. Based on the model constructed from the full dataset, we perfom an analysis of the South Atlantic Anomaly and regions of low field intensity in general. We extend a recent analysis of center of mass movement and area evolution of the South Atlantic Anomaly further back in time and confirm the findings of its non-monotonous growth.

Magnetic field reversals are irregular events in Earth’s history when the geomagnetic field changes its polarity. Reversals are recorded by spot and continuous remanent magnetization data collected from lava flows and marine sediments, respectively. The latest field reversal, the Matuyama-Brunhes reversal (MBR), is better covered by paleomagnetic data than prior field reversals, hence providing an opportunity to understand the physical mechanisms. Despite the quantity of data, a full understanding of the MBR is still lacking. The evolution of the MBR in time and space is explored in this work by compiling a global set of paleomagnetic data, both from sediments and volcanic rocks, which encompass the period 900–700 ka. After careful evaluation of data and dating quality, regional and global stacks of virtual axial dipole moment (VADM), virtual geomagnetic pole (VGP), and paleosecular variation index (Pi) are constructed from the sediment records using bootstrap resampling. Individual VADMs and VGPs calculated from lavas are compared to these stacks. Four phases of full-vector field instability are observed in these stacks over the period 800–770 ka. The first three phases, observed at 800–785 ka, reflect a rapid weakening of the field coupled with low VGP latitude, after which the field returned to the reverse polarity of the Matuyama chron. The fourth phase, lasting from 780 to 770 ka, is when the field reversal process completed, such that the field entered the Brunhes normal polarity state. These findings point to a complex reversal process lasting ∼30 kyr, with the reversal ending at ∼770 ka.

On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2021. The hours reading and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. The COVID pandemic continued to impose additional stresses on the review process, as many reviewers had to juggle increased family commitments, hours of online meetings, remote work and instruction, lack of physical access to library resources, and other hardships, to maintain the quality and timeliness of their reviews. We received 4,832 submissions in 2021 and 5,232 reviewers contributed to their evaluation by providing 8,874 reviews in total. Although we witnessed an increase in the number of submissions, the average number of days to complete a review increased only slightly! That says a lot about the diligence of our reviewers. We deeply appreciate their contributions in these challenging times.

The geomagnetic field prevents energetic particles, such as galactic cosmic rays, from directly interacting with the Earth's atmosphere. The geomagnetic field is not static but constantly changing, and over the last 100,000 years several geomagnetic excursions occurred. During geomagnetic field excursions, the field strength is significantly decreased and the field morphology is controlled by non-dipole components, and more cosmic ray particles can access the Earth's atmosphere. Paleomagnetic field models provide a global view of the long-term geomagnetic field evolution, however, with individual spatial and temporal resolution. Here, we reconstruct the geomagnetic shielding effect over the last 100,000 years by calculating the geomagnetic field cutoff rigidity using four global paleomagnetic field models, i.e., GGF100k, GGFSS70, LSMOD.2, and CALS10k.2. We find that the non-dipole components of the geomagnetic field are not negligible for estimating the long-term geomagnetic shielding effect, in particular during excursions. Our results indicate that cosmic ray flux, impact area, and cosmic ray radiation intensity increase strongly during the excursions. Our results provide the possibility to accurately estimate the cosmogenic isotope production rate and cosmic radiation dose rate covering the last 100,000 years.

The recent Kalmag and archaeomagnetic ArchKalmag14K models together represent the global geomagnetic field model evolution over the past 14000 years and resolve temporal scales of the order of a month over the last 122 years. They are obtained through the sequential assimilation of archeomagnetic and volcanic data, and survey, observatory and satellite data, respectively. Both these models provide full posterior information about the core field, and in the case of Kalmag also about other magnetic sources such as the lithospheric or some tidal fields. These models are made accessible online through different physical and statistical quantities associated with them. In this presentation, we will detail our modeling strategy, the type of results we are getting with it, and how the community can access and use our models by an online interface at https://ionocovar.agnld.uni-potsdam.de/Kalmag/ and https://ionocovar.agnld.uni-potsdam.de/Kalmag/Archeo/.

The Earth's magnetic field varies on a range of spatial and temporal scales. Recent models, spanning the past 100 ka, greatly improved our knowledge of the long-term changes of the geomagnetic field, and geomagnetic excursions - events associated with strong directional deviations and low field intensities. However, these models are limited by the spatial and temporal data distribution, and magnetic and age uncertainties of underlying data. Variations in the production of cosmogenic radionuclides, such as 10Be from ice cores and sediments, provide an independent proxy of paleointensity variations for a range of timescales. This study demonstrates the potential of a joint inversion of paleomagnetic data with cosmogenic nuclide production rates to reconstruct the geomagnetic field evolution over the past 70 ka. Here, we present a global compilation of 10Be records, converted to magnetic field intensity, and compare them to paleomagnetic sediment records. General trends of the virtual axial dipole moment (VADM) stacks derived from the two data sets agree well. Two models have been constructed: GGFSS70-10Be where the converted 10Be records are used similarly to the paleomagnetic records, and GGFSS70-10Be-DIP where the converted 10Be data have been considered to be a function of the dipole only (the first three Gauss coefficients). These models are compared to the previously published GGFSS70 model based on paleomagnetic data only. Geomagnetic excursions are reconstructed in the multi-proxy models: the most pronounced Laschamps event (41 ka) and a few other excursional signatures at the times of the Norwegian-Greenland Sea and Mono Lake/Auckland excursions. Model predictions and global field maps show that the cosmogenic isotope records have regional effects on the reconstructed variations (Greenland, South/Southern Ocean, east-equatorial Pacific), especially those with high resolution (Greenland ice cores). The 10Be records not only provide an independent source of information on the geomagnetic field, thus confirming the reliability of the paleomagnetic records, they also improve the global geomagnetic field reconstructions over long timescales.

We present pymagglobal, a simple to use python interface for global geomagnetic field models. Pymagglobal was developed to provide easy access to global, spherical harmonics based magnetic main field models over historical and paleomagnetic times. The software readily handles cubic-spline based geomagnetic field models stored in the same file format as gufm1 or the CALSxk model series out of the box. Models in other file formats can be incorporated with minimal effort using the python backend. The python interface can, e.g., give model curves for any location, time series of dipole moment or spherical harmonic coefficients or grids and maps of magnetic field components. Pymagglobal can be installed by a single command and comes with a command line interface and a GUI, that allows easy extraction and visualization of information from the models. Additionally, the python backend can be used to access the models, for example to generate synthetic data or refer to them in your own analysis. Emphasis is put on documentation and accessibility. The package is available via a git repository and a custom website at https://git.gfz-potsdam.de/sec23/korte/pymagglobal.

The geomagnetic field is created by the motion of molten iron inside the liquid outer core of the Earth. It has undergone a number of drastic changes over geological time, the most notable of which are field reversals, in which the magnetic north and south poles change locations. Because sediments can retain information about past magnetic field directional and intensity changes through depositional or post-depositional remanent magnetization acquisition mechanisms, oceanic sediment cores can be used to track reversals. Lavas also document reversals by spot readings of thermal remanent magnetization. The Matuyama-Brunhes reversal (MBR) was the last time the Earth’s poles reversed, and it is documented by the largest number of sediment records compared to earlier reversals. Investigating the MBR globally therefore can help to understand the physical processes that occur in the Earth’s core. A few global spherical harmonic (SH) models have previously been proposed for the MBR. However, the number, distribution, and timescale reliability of the input data are limitations of these models, the last one of which has been published more than 10 years ago. In this study, we take advantage of new sediments and lava data for the MBR that have been published since these models were developed, and often have better age control and higher temporal resolution than data used in previous SH models. Smoothing splines were used to examine the temporal resolution of all sediment records in our new global compilation, and the results show a median smoothing time of 350 years (±200 years). We present a new global SH geomagnetic field model for the MBR, constructed from 67 sediment cores and 93 lava sites that span the last 900-700 ka and have a reasonable geographical distribution. In addition, we investigate the robustness of model features by deriving models from sub-sets of data, e.g., using only well-dated, high-resolution sediment data that are consistent with surrounding records (if any exist). The model’s featured will be discussed, including (1) field morphology at the CMB and at the Earth’s surface; (2) axial dipole (AD) and non-axial dipole (NAD) power at the CMB; and (3) magnetic energy of AD and NAD fields at the Earth’s surface. Furthermore, we will compare the current models to the previous MBR models.

Today’s geomagnetic field can prevent energetic particles, including solar energetic particles and galactic cosmic rays, from directly hitting the Earth’s atmosphere. However, when the geomagnetic field strength is significantly decreased during geomagnetic field excursions or reversals, the geomagnetic field shielding effect becomes less prominent. Geomagnetic cutoff rigidity, as a quantitative estimation of the shielding effect, can be calculated using trajectory tracing or theoretical equations. We use a recent high-resolution continuous geomagnetic field model (LSMOD.2) to study the geomagnetic cutoff rigidity during the Laschamps excursion. Global grids of the geomagnetic cutoff rigidities are presented, in particular for the excursion midpoint when the geomagnetic field is weak and not dipole-dominated anymore at Earth’s surface. We compare the cutoff rigidity calculation results between a trajectory tracing program and theoretical equations and we find that the influence of the non-dipole component of the geomagnetic field cannot be ignored during the excursion. Our results indicate that the exposure of Earth’s atmosphere to energetic particles of cosmic and solar origin is high and nearly independent of latitude in the middle of the Laschamps excursion. Our results will be useful for future studies associated with cosmic radiation dose rate and cosmogenic isotope production rate during the Laschamps excursion.

The Earth’s magnetic field, or geomagnetic field, forms the magnetosphere around Earth, which shields our habitat from cosmic radiation and solar wind. It is generated by dynamic processes in Earth’s fluid outer core and changes constantly. These changes are slow on human timescales, but can be drastic on geological scales: over Earth’s history, the geomagnetic field has changed its polarity multiple times. While the occurrence of such events is firmly established, the underlying processes in Earth’s core and potential consequences for our habitat are not well understood.

The Gravity field and steady-state Ocean Circulation Explorer (GOCE) is part of ESA’s Earth Explorer Program. The satellite carries magnetometers that control the activity of magnetorquers for navigation of the satellite, but are not dedicated as science instruments. However, intrinsic steady states of the instruments can be corrected by alignment and calibration, and artificial perturbations, e.g. from currents, can be removed by their characterisation correlated to housekeeping data. The leftover field then shows the natural evolution and variability of the Earth’s magnetic field. This article describes the pre-processing of input data as well as calibration and characterisation steps performed on GOCE magnetic data, using a high-precision magnetic field model as reference. For geomagnetic quiet times, the standard deviation of the residual is below 13 nT with a median residual of (11.7, 9.6, 10.4) nT for the three magnetic field components (x, y, z). For validation of the calibration and characterisation performance, we selected a geomagnetic storm event in March 2013. GOCE magnetic field data show good agreement with results from a ground magnetic observation network. The GOCE mission overlaps with the dedicated magnetic field satellite mission CHAMP for a short time at the beginning of 2010, but does not overlap with the Swarm mission or any other mission flying at low altitude and carrying high-precision magnetometers. We expect calibrated GOCE magnetic field data to be useful for lithospheric modelling and filling the gap between the dedicated geomagnetic missions CHAMP and Swarm. Graphic Abstract

Paleomagnetic records from sediments, archeological artifacts, and lava flows provide the foundation for studying geomagnetic field changes over 0–100 ka. Late Quaternary time-varying spherical harmonic models for 0–100 ka produce a global view used to evaluate new data records, study the paleomagnetic secular variation on centennial to multimillennial timescales, and investigate extreme regional or global events such as the Laschamp geomagnetic excursion. Recent modeling results (GGF100k and LSMOD.2) are compared to previous studies based on regional or global stacks and averages of relative geomagnetic paleointensity variations. Time-averaged field structure is similar on Holocene, 100 ky, and million-year timescales. Paleosecular variation activity varies greatly over 0–100 ka, with large changes in field strength and significant morphological changes that are especially evident when field strength is low. GGF100k exhibits a factor of 4 variation in geomagnetic axial dipole moment, and higher-resolution models suggest that much larger changes are likely during global excursions. There is some suggestion of recurrent field states resembling the present-day South Atlantic Anomaly, but these are not linked to initiation or evolution of excursions. Several properties used to characterize numerical dynamo simulations as “Earth-like” are evaluated and, in future, improved models may yet reveal systematic changes linked to the onset of geomagnetic excursions. Modeling results are useful in applications ranging from ground truth and data assimilation in geodynamo simulations to providing geochronological constraints and modeling the influence of geomagnetic variations on cosmogenic isotope production rates.

International audience; Archaeomagnetic data are fundamental for our understanding of the evolution of Earth's magnetic field on centennial to millennial timescales. From the earliest studies of the Thelliers, Aitken, Nagata and others in the 1950s and 1960s, archaeomagnetic data have been vital for extending our knowledge of the field to times prior to observational measurements. Today, many thousands of archaeomagnetic data allow us to explore the geomagnetic field in more detail than ever before. Both regional time series of archaeomagnetic data and the inclusion of archaeomagnetic data in time-varying global spherical harmonic field models have revealed a range of newly discovered field behaviour. More sophisticated approaches to developing regional curves and global models have allowed us to resolve the field in certain regions more robustly and with greater resolution than previously possible. In this review we give an overview of the widely used global archaeomagnetic database GEOMAGIA50, discuss the methods used to obtain archaeomagnetic data, their challenges, and explore progress over the past twenty years in developing regional secular variation curves and global spherical harmonic models of the archaeomagnetic field. We end the review by covering what we see as the “grand challenges” in archaeomagnetism, including which regions of the world should be focussed on with regards to data acquisition.

In a previous study, a new snapshot modeling concept for the archeomagnetic field was introduced (Mauerberger et al., 2020, https://doi.org/10.1093/gji/ggaa336). By assuming a Gaussian process for the geomagnetic potential, a correlation���based algorithm was presented, which incorporates a closed���form spatial correlation function. This work extends the suggested modeling strategy to the temporal domain. A space���time correlation kernel is constructed from the tensor product of the closed���form spatial correlation kernel with a squared exponential kernel in time. Dating uncertainties are incorporated into the modeling concept using a noisy input Gaussian process. All but one modeling hyperparameters are marginalized, to reduce their influence on the outcome and to translate their variability to the posterior variance. The resulting distribution incorporates uncertainties related to dating, measurement and modeling process. Results from application to archeomagnetic data show less variation in the dipole than comparable models, but are in general agreement with previous findings., Plain Language Summary: Global reconstructions of the past geomagnetic field are useful tools to study the geodynamo process that generates the Earth's magnetic field in the outer core. Data���based field reconstructions are traditionally represented by a fixed number of coefficients in space and time. In a previous study, a new modeling concept for individual epochs of the magnetic field was introduced, which is better adapted to inhomogeneous data distributions as found in archeomagnetic data, and which provides more realistic uncertainty estimates. This new modeling concept effectively has one coefficient per data point. Here, the new method is expanded to also consider the time evolution and build continuous models of the past geomagnetic field. Uncertainties in archeomagnetic input data and in their ages are taken into account and contribute to estimating reasonable uncertainties for the resulting model. The application of the new method to archeomagnetic data over the past 1,200 years gives general agreement with previous findings with less variation in the dipole field contribution than seen in comparable models., Key Points: Extension of a previous study on spatial correlation based archeomagnetic modeling to the temporal domain. Dating uncertainties are incorporated by the noisy input Gaussian process formalism. Results show general agreement with comparable models with less variation in the dipole field contribution., Deutsche Forschungsgemeinschaft (DFG) http://dx.doi.org/10.13039/501100001659

The Global Geodetic Observing System (GGOS) of the International Association of Geodesy (IAG) provides the geodetic infrastructure needed to monitor the Earth system.. The understanding of forced temporal variations of celestial pole motion (CPM) could bring us significantly closer to meeting the GGOS goals (i.e. 1 mm accuracy and 0.1 mm/year stability on global scales in terms of the ITRF defining parameters). Besides astronomical forcing, CPM excitation depends on the processes in the fluid core and the core-mantle boundary. The same processes are responsible for the variations of the geomagnetic field (GMF). This study investigates the interconnection between the celestial pole offset (CPO) and effective geophysical processes that contribute to the Earth's rotational variation. We use the CPO time series obtained from very long baseline interferometry (VLBI) observations together with the latest GMF data such as geomagnetic jerk and magnetic dipole moment, and a state-of-the-art geomagnetic field model to explore the correlation between CPM and GMF. Our results confirm the findings of previous studies, revealing that substantial free core nutation (FCN) disturbance occurred at the epochs close to the GMJ events. The results also reveal some common features in the FCN and GMF variation, which show the potential to improve knowledge regarding the GMF's contribution to the Earth's rotation.

We present the GFZ candidate field models for the $$13{\mathrm{th}}$$ 13 th Generation International Geomagnetic Reference Field (IGRF-13). These candidates were derived from the geomagnetic core field model, which is constrained by Swarm satellite and ground observatory data from November 2013 to August 2019. Data were selected from magnetically quiet periods, and the model parameters have been obtained using an iteratively reweighted inversion scheme approximating a robust modified Huber norm as a measure of misfit. The root mean square misfit of the model to Swarm and observatory data is in the order of 3–5 nT for mid and low latitudes, with a maximum of 44 nT for the satellite east component data at high latitudes. The time-varying core field is described by order 6 splines and spherical harmonic coefficients up to degree and order 20. We note that the temporal variation of the core field component of the model is strongly damped and shows a smooth secular variation that suits well for the IGRF, where secular variation is represented as constant over 5-year intervals. Further, the external field is parameterised by a slowly varying part and a more rapidly varying part controlled by magnetic activity and interplanetary magnetic field proxies. Additionally, the Euler angles of the magnetic field sensor orientation are co-estimated. A widely discussed feature of the geomagnetic field is the South Atlantic Anomaly, a zone of weak and decreasing field strength stretching from southern Africa over to South America. The IGRF and indicate that the anomaly has developed a second, less pronounced eastern minimum at Earth’s surface since 2007. We observe that while the strong western minimum continues to drift westwards, the less pronounced eastern minimum currently drifts eastward at Earth’s surface. This does not seem to be linked to any eastward motion at the core–mantle boundary, but rather to intensity changes of westward drifting flux patches contributing to the observed surface field. Also, we report a sudden change in the secular variation measured at two South Atlantic observatories around 2015.0, which occurred shortly after the well-known jerk of 2014.0.

On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2020. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. The COVID pandemic imposed additional stresses on the review process, as many reviewers had to juggle increased family commitments, hours of online meetings, remote work and instruction, lack of physical access to library resources, and other hardships to maintain the quality and timeliness of their reviews. Although we witnessed an increase in the number of submissions, the average number of days to complete a review increased by less than one day!! That says a lot about the diligence of our reviewers. We deeply appreciate their contributions in these challenging times. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Individuals in italics provided three or more reviews for Geophysical Research Letters during the year. In total, 5,177 referees contributed to 8,786 individual reviews. Thank you again. We look forward to the coming year of exciting advances in the field and communicating those advances to our community and to the broader public.

Reconstructions of the geomagnetic field on long timescales are important to understand the geodynamo processes in the Earth’s core. The geomagnetic field exhibits a range of variations that vary from normal, dipole–dominated secular variation to geomagnetic excursions and reversals. These transitional events are associated with significant directional deviations and very low intensities. Here we present a new, global geomagnetic field model spanning the period 70–15 ka (GGFSS70) that includes three excursions: Norwegian–Greenland Sea, Laschamps, and Mono Lake/Auckland. The model is built from nine globally distributed, high–resolution, well–dated, sedimentary paleomagnetic records. The GGFSS70 indicates that the axial–dipole component changed sign for about 300 years in the middle of the Laschamps excursion (41.25–40.93 ka). The energy comparison at the Earth’s surface reveals that the axial–dipole energy is always higher than the non-axial-dipole except over the Laschamps. In the other two excursions, the axial-dipole is reduced by about one order of magnitude for the Norwegian–Greenland Sea excursion and less for the Mono Lake/Auckland. At the core–mantle boundary, the large–scale non-axial-dipole power is comparable to the axial-dipole power, except over the excursions when the axial-dipole decreases, though less clearly for the Mono Lake/Auckland excursion. The axial dipole moment over the 15–70 ka varies from 0 to 8 ×1022 Am2, with an average and standard deviation of 5.1±1.5 ×1022Am2. The Laschamps excursion is associated with growth and poleward movement of reversed flux patches and reversed field in the tangent cylinder at the excursion midpoint, which is not the case for the other two excursions. Plain Language Summary Earth possesses a magnetic field that is constantly changing in time and space. Its source is located in the outer core and the mechanism by which the geomagnetic field is generated is known as the geodynamo. Studying the field on long timescales contributes to a better understanding of the source processes. The geomagnetic field can be approximated as a dipole, except during reversals and excursions. The past 70,000 years period contains three geomagnetic excursions, the Norwegian--Greenland Sea (65,000 years ago), Laschamps (41,000), and Mono Lake/Auckland (34,000). During the excursions, the intensity is globally very low and the poles may or may not change polarity. However, they always recover to the original polarity in contrast to the reversals, when the North Pole becomes South and the South becomes North. To model the long--term field variations, we rely on geological archives, sediments and rocks, that record and preserve the geomagnetic field. We selected the best sedimentary records in terms of magnetic components and good age control. The Laschamps excursion is the most pronounced event in this period, has the lowest dipole moment, and is observed globally. The other two events occur at regional scales and have only a moderate dipole moment decrease.

Earth, Planets and Space, 73 (1), ISSN:1343-8832, ISSN:1880-5981

Geomagnetism deals with all aspects of the Earth's magnetic field, also called the geomagnetic field, that surrounds our habitat in the form of the magnetosphere. We cannot sense the geomagnetic field in any way, and instrumental measurements as continuous recordings or dedicated surveys are necessary to study it. Each geomagnetic field observation contains contributions from several sources: the strongest one from the geodynamo process in Earth's outer core, from magnetized rocks in the lithosphere, and from a variety of electrical current systems flowing within the magnetosphere. The individual field contributions have different spatial and temporal characteristics. Although a clear source separation remains a challenge, all the contributions to the geomagnetic field can serve as valuable sources of information about our planet's interior, from the crust to the core, and about the condition of our space environment. This article provides an overview of all aspects of geomagnetism, with a focus on Earth's magnetic field sources inside the Earth and on its applications to understand Earth's interior.

On behalf of the journal, AGU, and the scientific community, the editors would like to sincerely thank those who reviewed the manuscripts for Geophysical Research Letters in 2019. The hours reading and commenting on manuscripts not only improve the manuscripts but also increase the scientific rigor of future research in the field. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. With the revival of the “major revisions” decisions, we appreciate the reviewers' efforts on multiple versions of some manuscripts. With the advent of AGU's data policy, many reviewers have helped immensely to evaluate the accessibility and availability of data associated with the papers they have reviewed, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. Many of those listed below went beyond and reviewed three or more manuscripts for our journal, and those are indicated in italics.

SUMMARY Observations of changes in the geomagnetic field provide unique information about processes in the outer core where the field is generated. Recent geomagnetic field reconstructions based on palaeomagnetic data show persistent westward drift at high northern latitudes at the core–mantle boundary (CMB) over the past 4000 yr, as well as intermittent occurrence of high-latitude weak or reverse flux patches. To further investigate these features, we analysed time-longitude plots of a processed version of the geomagnetic field model pfm9k.1a, filtered to remove quasi-stationary features of the field. Our results suggest that westward drift at both high northern and southern latitudes of the CMB have been a persistent feature of the field over the past 9000 yr. In the Northern Hemisphere we detect two distinct signals with drift rates of 0.09° and 0.25° yr−1 and dominant zonal wavenumbers of m = 2 and 1, respectively. Comparisons with other geomagnetic field models support these observations but also highlight the importance of sedimentary data that provide crucial information on high-latitude geomagnetic field variations. The two distinct drift signals detected in the Northern Hemisphere can largely be decomposed into two westward propagating waveforms. We show that constructive interference between these two waveforms accurately predicts both the location and timing of previously observed high-latitude weak/reverse flux patches over the past 3–4 millennia. In addition, we also show that the 1125-yr periodicity signal inferred from the waveform interference correlates positively with variations in the dipole tilt over the same time period. The two identified drift signals may partially be explained by the westward motion of high-latitude convection rolls. However, the dispersion relation might also imply that part of the drift signal could be caused by magnetic Rossby waves riding on the mean background flow.

A full vector paleomagnetic record, comprising directional data and relative paleointensity (rPI), was derived from 16 sediment cores recovered from the southeastern Black Sea. The obtained data were used to create a stack covering the time window between 68.9 and 14.5 ka. Age models are based on radiocarbon dating and correlations of warming/cooling cycles monitored by high‐resolution X‐ray fluorescence (XRF) elementary ratios as well as by ice‐rafted debris (IRD) in Black Sea sediments to the sequence of ‘Dansgaard‐Oeschger’ (D‐O) events defined from the Greenland ice core oxygen isotope stratigraphy. The reconstructed prominent lows in paleointensity at about 64.5 ka, 41.2 ka and 34.5 ka are coeval with the Norwegian–Greenland Sea, the Laschamps, and the Mono Lake excursions, respectively. For a further analysis, the stacked Black Sea paleomagnetic record was converted into one component being parallel to the direction expected from a geocentric axial dipole (GAD) and two components perpendicular to it (EW, inclined NS), representing definitely only non‐GAD components of the geomagnetic field. Discussions of the field configurations at the Black Sea site are focused on the three excursional events. The Norwegian‐Greenland Sea excursion was dominated by a decaying axial dipole and persisting weak non‐dipole field, with directional variations still within the range of normal secular variations. The Laschamps excursion comprises two full polarity transitions and a short stable interval of reversed polarity in between. The Mono Lake excursion was mostly dominated by a non‐dipole field, though with a less pronounced weakening of the axial dipole component.

In December 2019, the 13th revision of the International Geomagnetic Reference Field (IGRF) was released by the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group V-MOD. This revision comprises two new spherical harmonic main field models for epochs 2015.0 (DGRF-2015) and 2020.0 (IGRF-2020) and a model of the predicted secular variation for the interval 2020.0 to 2025.0 (SV-2020-2025). The models were produced from candidates submitted by fifteen international teams. These teams were led by the British Geological Survey (UK), China Earthquake Administration (China), Universidad Complutense de Madrid (Spain), University of Colorado Boulder (USA), Technical University of Denmark (Denmark), GFZ German Research Centre for Geosciences (Germany), Institut de physique du globe de Paris (France), Institut des Sciences de la Terre (France), Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation (Russia), Kyoto University (Japan), University of Leeds (UK), Max Planck Institute for Solar System Research (Germany), NASA Goddard Space Flight Center (USA), University of Potsdam (Germany), and Universit\'e de Strasbourg (France). The candidate models were evaluated individually and compared to all other candidates as well to the mean, median and a robust Huber-weighted model of all candidates. These analyses were used to identify, for example, the variation between the Gauss coefficients or the geographical regions where the candidate models strongly differed. The majority of candidates were sufficiently close that the differences can be explained primarily by individual modeling methodologies and data selection strategies. None of the candidates were so different as to warrant their exclusion from the final IGRF-13. The IAGA V-MOD task force thus voted for two approaches: the median of the Gauss coefficients of the candidates for the DGRF-2015 and IGRF-2020 models and the robust Huber-weighted model for the predictive SV-2020-2025. In this paper, we document the evaluation of the candidate models and provide details of the approach used to derive the final IGRF-13 products. We also perform a retrospective analysis of the IGRF-12 SV candidates over their performance period (2015-2020). Our findings suggest that forecasting secular variation can benefit from combining physics-based core modeling with satellite observations.

The Earth's core magnetic field model Mag.num was the parent model for the GFZ IGRF 13 candidate submission. The model is based on geomagnetic ground observatory and Swarm satellite observations. Epochs 2020.0 and beyond were not covered by the data available at the time of submission and our results were based on predictions. In this study, we investigate the effect of the more recent available data on our results of the 2020.0 epoch and the predicted secular variation by generating an updated Mag.num version. We especially focus on the spatial and temporal patterns of the local geomagnetic field minimum of the South Atlantic Anomaly (SAA). Recently, global geomagnetic field models have shown that an additional, although shallow, secondary minimum at Earth's surface has developed since around 2005. The location and significance of the secondary minimum and of the saddle point between the two minima are assessed also in view of the respective differences among the candidate models.

For the global time stationary geomagnetic core field, a new modeling concept for Holocene archeomagnetic data is presented. Major challenges consist of the uneven data distribution, missing vector field components and non-linear relations between observations and the geomagnetic potential. Instead of a truncated spherical harmonics approach, we propose a fully Bayesian, Gaussian process based model. Inherently, the Bayesian approach provides location dependent uncertainties.The geomagnetic potential is assumed to be a Gaussian process whose covariance structure is given by an explicit kernel function, including several hyperparameters. For this kind of non-parametric models, the full Bayesian posterior is numerically intractable. Instead, we propose an approximate computation using a Bayesian update system. In a first step, the full vector records are used to obtain, within Laplace approximation, a rough field estimate. This estimate serves as a point of linearization for the non-linear observations. The approximate posterior is then given by a Gaussian mixture. Marginals for all relevant parameters and the field itself can be computed. We are able to quantify the impact of data coverage on uncertainty reduction.

Global data compilations and the production of time-varying paleomagnetic field models over the past hundred thousand years provide insights into geomagnetic field evolution.

The geomagnetic field prevents energetic particles, such as galactic cosmic rays, from directly interacting with the Earth’s atmosphere. The geomagnetic field is not static but constantly changing, and over the last 100,000 years, several geomagnetic excursions occurred. During geomagnetic field excursions, the field strength is significantly decreased and the field morphology is strongly influenced by non-dipole components, and more cosmic ray particles can access the Earth’s atmosphere. Paleomagnetic field models provide a global view of the long-term geomagnetic field evolution, however, with individual spatial and temporal resolution and uncertainties. Here, we reconstruct the geomagnetic shielding effect over the last 100,000 years by calculating the geomagnetic cutoff rigidity using four global paleomagnetic field models, i.e., the GGF100k, GGFSS70, LSMOD.2, and CALS10k.2 model. We compare results for overlapping periods and find that the model selection is crucial to constrain the cutoff rigidity variation. However, all models indicate that the non-dipole components of the geomagnetic field are not negligible for estimating the long-term geomagnetic shielding effect. We provide a combined record of global cutoff rigidities using the best available model for individual time intervals. Our results provide the possibility to estimate the cosmogenic isotope production rate and cosmic radiation dose rate covering the last 100,000 years according to the best current knowledge about geomagnetic field evolution, and will be useful in further long-term solar activity and climate change reconstruction.

We report here on results of palaeo- and rock magnetic investigations of two sediment cores from the Upper Congo deep-sea fan. The sediments have a high organic content and contain a heterogeneous Fe-mineral assemblage with biogenic magnetite and detrital (Ti-)magnetite as the main magnetic carrier minerals. Pyrite, hematite, and Fe-oxyhydroxides were identified by comparing high-temperature magnetic susceptibility curves with those from Fe-minerals of known composition. According to AMS 14C dates, the 6.8 m-long profile spans the last 37 kyr. Sediments older than 20 ka are affected by reductive diagenesis that has led to a loss of the fine-grained magnetic mineral fraction. Sediments younger than 20 ka have stable magnetizations. Characteristic remanent magnetization records of inclination and declination were obtained for each core. There is a little agreement between these records, modelled curves, and other sediment records from Equatorial Africa, so no composite record could be established. The cores are not ideal relative palaeointensity recorders and estimates using different normalizers did not yield consistent signals from both cores. Normalization methods used for relative palaeointensity estimation were not developed for sediments that contain large amounts of ultra-fine-grained biogenic magnetite; therefore, the relative palaeointensity estimates should be considered with caution. However, in view of the incoherent picture given by the scarce available palaeointensity information from the region off South-West Africa, the GeoB6517-2 record may provide a tentative relative palaeointensity record for comparison, at least for the past 10 kyr.