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Copyright of Reviews of Geophysics & Planetary Physics is the property of Editorial Office of Reviews of Geophysics & Planetary Physics and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The South Atlantic Anomaly (SAA) is one of the most remarkable structural features of the Earth's magnetic field. The region is characterized by weak magnetic field intensity. Because of this weakening, particles within the inner radiation belt can mirror at lower altitudes, consequently increasing the local particle flux. Over centuries, the SAA has displayed notable secular variation (SV) characteristics, with a consistent decline in strength, expansion in area, and an overall westward drift. Both paleomagnetic and modern magnetic data indicate that SAA is currently undergoing rapid changes, with a rapid expansion in area and a rapid decreasing in intensity. These changes are coinciding with an increasing field asymmetry. Recent observations suggest that the evolution of SAA has become increasingly anisotropic, showing two magnetic intensity minima. This may indicate that the evolution of SAA has entered a new stage, which has aroused people's attention and discussion on the long-term evolution and future of SAA. The manifestation of the SAA on the Earth’s surface corresponds to the response of an inverse flux path at the core-mantle boundary (CMB) to the radial component of the geomagnetic field, positioned approximately beneath the South Atlantic Ocean. Consequently, studying the SV of SAA can understand the details of the Earth's internal dynamics. Moreover, these SV scenarios provide some speculative predictions for the SAA. The evolutionary details of SAA not only serve as a reference for future trends but also imposes additional constraints on Earth's core dynamic model, rendering it a compelling subject for scholarly inquiry. In this paper, we review the results of paleomagnetic and modern geomagnetic models concerning the past, present, and future evolution of the SAA, while also summarizing advancements in understanding the origin of this anomaly.

Earth's magnetic field is generated through the dynamo action in the liquid outer core. Using observational data from geomagnetic satellites and observatories, we can construct geomagnetic models that describe spatial and temporal variations of the core field. Due to the limited temporal and spatial resolution of observations and incomplete parameterization of the model, the inversion problem of geomagnetic field modeling is non-unique. Therefore, we need to incorporate regularization constraints based on prior information to alleviate the non-uniqueness of the inversion problem for geomagnetic modeling. For the core field, the non-uniqueness of modeling inversion manifests in the temporal variations of the field. Studies on the influence of regularization can help us build more reliable core field models, especially on how to suppress artificial secular variations through regularizations. In this study, we construct a series of models based on different regularization parameters within the framework of CHAOS modeling to explore the impact of regularization on the core field modeling. Our results indicate that it is necessary to use regularization constraints for the core field modeling based solely on satellite data. By comparing models with different regularization intensities of the third time derivative of the core field, we show that regularization of the third time derivative with appropriate intensity can effectively suppress artificial signals caused by overfitting of the model to observational data. However, too strong regularizations will reduce the temporal resolution of small-scale secular variations.

The secular variation of the geomagnetic field is the temporal variation that takes place in the field on time scales between few years to hundreds or thousands of years. On the other hand, impulses or ”geomagnetic jerks” indicate a rapid change in the slope of the secular variation. The origin of these phenomena is linked to the behavior of the geodynamo and the contribution of the outer mantle-core interaction.ROMP (acronym from spanish “ Red de Observatorios Magnéticos Permanentes”) is composed by the following observatories: ”Las Acacias ”(LAS) at Buenos Aires,” Trelew ”(TRW) at Chubut,” Pilar ”(PIL) at Córdoba and ”Orcadas”(ORC) at the Argentine sector in Antartic; the first two depend on the National University of La Plata and the latter on the National Meteorological Service. In the present work one analyzes the secular variation and the determination of jerks in the geomagnetic elements recorded at the ROMP stations, in historical times up to the present.

Fluctuations in the geomagnetic field occur over a broad range of timescales. Short‐period fluctuations are called secular variation, whereas excursions and reversals are viewed as anomalous transient events. An open question is whether distinct mechanisms are required to account for these different forms of variability. Clues are sought in trends b of the axial dipole moment from six time‐dependent geomagnetic field models. Variability in b has a well‐defined dependence on the time interval (or window) for the trend. The variance of b reveals a simple relationship to trends during excursions and reversals. This connection hints at a link between reversals, excursions and secular variation. Stochastic models exhibit a similar behavior in response to random fluctuations in dipole generation. We find that excursions, reversals and secular variation can be distinguished on the basis of trend durations rather than differences in the underlying physical process. While this analysis does not rule out distinct physical mechanisms, the paleomagnetic observations suggest that such distinctions are not required. Plain Language Summary: Geomagnetic field models capture details of excursions and reversals over the past 2 Myr. A comparison of these transient events with field behavior during times of stable polarity offers insights into the underlying physical mechanisms. The focus of comparison is the trend of the axial dipole moment over a prescribed duration. Six geomagnetic field models reveal consistent and predictable changes in the variance of dipole trends as a function of duration. Trends into reversals and excursions can also be recovered from the geomagnetic field models. A simple relationship between transient events and the trends during stable polarity hints at a common underlying mechanism. This connection is supported by a stochastic model for dipole fluctuations, which is capable of reproducing the observed changes in the trend variance with duration. The same model quantitatively reproduces the relationship between transient events and secular variation. Given that trends in the stochastic model are driven entirely by fluctuations in dipole generation, a similar interpretation may account for excursions, reversals and secular variation without requiring different physical mechanisms. Key Points: Statistics of dipole trends are computed for six paleomagnetic field modelsTrends during excursions and reversals are linked to variability during stable polarityA relationship between excursions, reversals and secular variation suggests a common underlying mechanism [ABSTRACT FROM AUTHOR]

Abstract High-resolution dating of volcanic rocks is the foundation for understanding the evolution of volcanoes and for estimating possible hazards. However, dating is often difficult for submarine volcanoes, where radiocarbon or other dating is frequently unavailable or imprecise. Here, we report paleointensity results from submarine basalts around Izu-Oshima Island, a typical island-arc volcano, and their bearing on age constraints. Basaltic lava samples were collected from submarine ridges located southeast of Izu-Oshima Island. Rock magnetic data indicate that the samples contain Ti-rich titanomagnetite with blocking temperatures of around 250–400 °C. The magnetic properties of the samples do not change significantly when heated in Ar or vacuum. We apply the Tsunakawa-Shaw method to estimate absolute paleointensity. Samples from different submarine ridges show distinct behavior. One ridge (SE1) shows moderate paleointensity of about 37 μT, while another ridge (SE3) records relatively strong magnetic fields of about 60 μT. Comparing those results with regional paleointensity data, we estimate the age of the SE1 ridge to be younger than 0.5 ka or around 1.4 ka. The other ridge (SE2) exhibits different paleointensity for two samples obtained from different localities, implying that the ridge consists of multiple eruptions. These results demonstrate that paleomagnetism can improve the dating of submarine volcanic rocks. Graphical Abstract

High-resolution dating of volcanic rocks is the foundation for understanding the evolution of volcanoes and for estimating possible hazards. However, dating is often difficult for submarine volcanoes, where radiocarbon or other dating is frequently unavailable or imprecise. Here, we report paleointensity results from submarine basalts around Izu-Oshima Island, a typical island-arc volcano, and their bearing on age constraints. Basaltic lava samples were collected from submarine ridges located southeast of Izu-Oshima Island. Rock magnetic data indicate that the samples contain Ti-rich titanomagnetite with blocking temperatures of around 250–400 °C. The magnetic properties of the samples do not change significantly when heated in Ar or vacuum. We apply the Tsunakawa-Shaw method to estimate absolute paleointensity. Samples from different submarine ridges show distinct behavior. One ridge (SE1) shows moderate paleointensity of about 37 μT, while another ridge (SE3) records relatively strong magnetic fields of about 60 μT. Comparing those results with regional paleointensity data, we estimate the age of the SE1 ridge to be younger than 0.5 ka or around 1.4 ka. The other ridge (SE2) exhibits different paleointensity for two samples obtained from different localities, implying that the ridge consists of multiple eruptions. These results demonstrate that paleomagnetism can improve the dating of submarine volcanic rocks. [ABSTRACT FROM AUTHOR]

Abstract Fluctuations in the geomagnetic field occur over a broad range of timescales. Short‐period fluctuations are called secular variation, whereas excursions and reversals are viewed as anomalous transient events. An open question is whether distinct mechanisms are required to account for these different forms of variability. Clues are sought in trends b of the axial dipole moment from six time‐dependent geomagnetic field models. Variability in b has a well‐defined dependence on the time interval (or window) for the trend. The variance of b reveals a simple relationship to trends during excursions and reversals. This connection hints at a link between reversals, excursions and secular variation. Stochastic models exhibit a similar behavior in response to random fluctuations in dipole generation. We find that excursions, reversals and secular variation can be distinguished on the basis of trend durations rather than differences in the underlying physical process. While this analysis does not rule out distinct physical mechanisms, the paleomagnetic observations suggest that such distinctions are not required.

The improvement of the archaeomagnetic dating method requires compiling new and older data of the Earth's magnetic field (EMF) variations for the last millennia. Combustion events from fumier sequences have been proposed as good directional EMF recorders. However, they are subjected to diverse taphonomical processes and how these affect the archaeomagnetic record has not yet been studied. In order to evaluate it, here we report the first archaeomagnetic and rock-magnetic results on samples from experimentally recreated fumiers since 2014 under controlled conditions. A facies description with unprecedent resolution was used to study the variation of magnetic properties in depth. Rock-magnetic analyses indicate a homogenous magnetic mineralogy dominated by pseudo-single domain magnetite as main carrier in all facies, with not very high and similar contribution of the finest (superparamagnetic) grains. The low values of anisotropy of magnetic susceptibility (AMS) indicate that the studied sample set is mainly isotropic. The directional behaviour in well-preserved burned facies (here described as G, LM and DGB), are jointly characterized by highly reversible thermomagnetic curves, high Koenigberger (Q n) ratio values and intense, univectorial and normal polarity orthogonal NRM demagnetization diagrams. On the contrary, specimens affected by mechanical alteration processes are less magnetic and show anomalous directional behaviours. The high thermomagnetic reversibility of ashes indicates that they reached ca. 600–700 °C, in line with the thermocouples' data. Temperatures of 460 °C were obtained for the DGB facies (subyacent black carbonaceous facies). Sampling of ashes located on the top of these combustion events should be avoided for archaeomagnetism. Being just beneath the last stabling episode they are the most prone to undergo mechanical alterations and do not preserve well the Earth's magnetic field direction. Despite their unlithified nature and the multiple taphonomic processes that fumier sequences may undergo, under certain quality criteria, they are valid geomagnetic field recorders providing both information of archaeological and geophysical interest. [ABSTRACT FROM AUTHOR]

Earth's magnetic field is a dynamic, changing phenomenon. The geomagnetic field consists of contributions from several sources, of which the main field originating in Earth's core makes up the bulk. On regional and local scales at Earth's surface, the lithospheric field can make a substantial contribution to the overall field and therefore needs to be considered in field models. A locally derived regional core field model, named HMOREG, has been shown to give accurate predictions of the southern African region. In this study, a new regional field model called the South African Regional Core and Crust model (SARCC) is introduced. This is the first time that a local lithospheric model, estimated by employing the revised spherical cap harmonic analysis modelling method, has been combined with the core component of CHAOS-6, a global field model. It is compared here with the existing regional field model as well as with global core field models. The SARCC model shows small-scale variations that are not present in the other three models. Including a lithospheric magnetic field component likely contributed to the better performance of the SARCC model when compared to other global and local field models. The SARCC model showed a 33% reduction in error compared to surface observations obtained from field surveys and INTERMAGNET stations in the Y component, and HMOREG showed a 7% reduction in error compared to the global field models. The new model can easily be updated with global geomagnetic models that incorporate the most recent, state-ofthe- art core and magnetospheric field models. Significance: Earth's magnetic field is an integral part of many current navigational methods in use. Updates of geomagnetic field models are required to ensure the accuracy of maps, navigation, and positioning information. The SARCC regional geomagnetic field model introduced here was compared with global geomagnetic field models, and the inclusion of a lithospheric magnetic field component likely contributed to the better performance of the SARCC model. This regional model of southern Africa could easily be updated on a regular basis, and used for high-resolution information on the Earth's magnetic field for the wider scientific community. [ABSTRACT FROM AUTHOR]

Earth’s magnetic field is a dynamic, changing phenomenon. The geomagnetic field consists of contributions from several sources, of which the main field originating in Earth’s core makes up the bulk. On regional and local scales at Earth’s surface, the lithospheric field can make a substantial contribution to the overall field and therefore needs to be considered in field models. A locally derived regional core field model, named HMOREG, has been shown to give accurate predictions of the southern African region. In this study, a new regional field model called the South African Regional Core and Crust model (SARCC) is introduced. This is the first time that a local lithospheric model, estimated by employing the revised spherical cap harmonic analysis modelling method, has been combined with the core component of CHAOS-6, a global field model. It is compared here with the existing regional field model as well as with global core field models. The SARCC model shows small-scale variations that are not present in the other three models. Including a lithospheric magnetic field component likely contributed to the better performance of the SARCC model when compared to other global and local field models. The SARCC model showed a 33% reduction in error compared to surface observations obtained from field surveys and INTERMAGNET stations in the Y component, and HMOREG showed a 7% reduction in error compared to the global field models. The new model can easily be updated with global geomagnetic models that incorporate the most recent, state-of-the-art core and magnetospheric field models. Significance: Earth’s magnetic field is an integral part of many current navigational methods in use. Updates of geomagnetic field models are required to ensure the accuracy of maps, navigation, and positioning information. The SARCC regional geomagnetic field model introduced here was compared with global geomagnetic field models, and the inclusion of a lithospheric magnetic field component likely contributed to the better performance of the SARCC model. This regional model of southern Africa could easily be updated on a regular basis, and used for high-resolution information on the Earth’s magnetic field for the wider scientific community.

The agonic line, characterized by zero values of geomagnetic declination, has had a westward drift during the last centuries, crossing the location of the Real Observatorio de Madrid at the end of the year 2021. This fact, which was monitored by the Instituto Geográfico Nacional, moves us to study the evolution of the magnetic declination in this emblematic emplacement between the last two crosses of the agonic line. Our results point out that the current westward drift started around the year 1810 and, before this period, the agonic line moved from west to east, crossing the location of the Real Observatorio around 1650–1675.

The end-Triassic extinction (ETE) on land was synchronous with the initial lavas of the Central Atlantic Magmatic Province (CAMP) and occurred just after the brief 26 thousand year (kyr) reverse geomagnetic polarity Chron E23r that can be used for global correlation. Lava-by-lava paleomagnetic secular variation data, previously reported from Morocco and northeastern United States combined with our data for the North Mountain Basalt from the Fundy Basin of Canada show that the initial phase of CAMP volcanism occurred in only five directional groups or pulses each occupying less than a century. The first four directional groups occur during a ~40 kyr period based on available astrochronology and U-Pb geochronology. The coincidence of the initial major pulse of CAMP volcanism with the ETE points to short-lived volcanic winters albedo-induced by sulfate aerosols as a plausible key agent of the extinctions in the tropical continental realm, whereas looser correlations allow prolonged CO 2 emissions to contribute to more long-ranging effects in the marine realm via ocean acidification and longer-term warming., Competing Interests: Competing interests statement:The authors declare no competing interest.

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