Reuse of medieval bricks as important limitation for construction of geomagnetic secular variation curves based on archeomagnetic studies of brick buildings in Poland.

<p>The bricks can be one of the best material for archeomagnetic studies. Their backing technique (i.e. horizontal location in the furnace) allow to determine also the value of inclination of geomagnetic field.  However, reuse of older bricks for the construction of newer objects can limit the utility of this material in archeomagnetic studies. A set of the brick samples from 26 historical buildings in SE Poland was taken for archeomagnetic investigations. As a result of this study, the secular variations of palaeointensity and inclination of the geomagnetic field from 1200 to 1800 AD were defined for this part of Poland. The paleointensity of geomagnetic field  was determined using the IZZI-Thellier-Thellier protocol. The course of the new regional palaeosecular curves is approximately the same as so far obtained in other parts of Europe. Data obtained from four brick buildings, however, do not fit substantially to the reference European curves. The remarkable difference  is a rapid and deeper drop of inclination and significantly higher than expected values of  palaeointensity. These features indicate that bricks used for the construction of these buildings (dated on XVI – XVII centuries) were taken from older brick constructions, most probably from the Gothic time (XIII/XIV c.). We compared our data with the earlier data obtained from brick buildings in N Poland. The regional archeomagnetic curves calculated for these two regions of Poland are completely different in their segments as old as the first half of the 18<sup>th</sup> century. This fact could be explained by the reuse of medieval bricks during the construction of studied objects from N Poland (dated on the first half of the 18<sup>th</sup> century) and applied for the construction of reference curve or by later secondary heating of original bricks.</p><p>This research was supported by  the National Science Centre of Poland (project no: UMO-2016/23/B/ST10/0129).</p>

Geomagnetic secular variation forecast using the NASA GEMS ensemble Kalman filter: A candidate SV model for IGRF-13

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

Correction for Geomagnetic Secular Variation Effects: A New Method (Case Study of the Altai Geodynamic Network)

––The study focuses on detection of geomagnetic secular variation and the respective correction of tectonomagnetic data. A new technique is proposed for picking the secular variation component in the Earth’s main magnetic field recorded by precise measurements at 100 to 500 km sites on the surface. Long-period field variations presumably arise from fluid motions in the liquid core, at depths of 3000 km, whereas the sizes of observation networks are within 500 km. The sources of secular variation, irrespective of their configuration, are much deeper than those of tectonomagnetic anomalies located above the Curie surface depths of ~10 to 20 km. Therefore, the surfaces that represent the space distribution of secular variation must be smoother than the respective surfaces for tectonomagnetic anomalies. The problem is thus to separate the regional and local signals from the two types of sources located at different depths. The new method is tested using data of yearly geomagnetic measurements at more than 30 repeat stations of a ~120 km long geodynamic network in Gorny Altai spanning the period from 2004 through 2018. The secular variation pattern is reconstructed by quadratic interpolation. The precise data corrected for secular variation of the main field reveal previously hidden tectonomagnetic anomalies up to 12 nT. The 3 nT positive anomaly falls within the zone of surface deformation caused by the Mw = 7.3 Chuya earthquake of 27 September 2003.

Geomagnetic secular variation forecast using the NASA GEMS ensemble Kalman filter: A candidate SV model for IGRF-13

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

Geomagnetic secular variation forecast using the NASA GEMS ensemble Kalman filter: A candidate SV model for IGRF-13

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

Geomagnetic secular variation forecast using the NASA GEMS ensemble Kalman fllter: A candidate model for IGRF 2020

We have produced a 5 year mean secular variation (SV) of the geomagnetic field for the period 2020-2025. We use the NASA Geomagnetic Ensemble Modeling System (GEMS), which consists of the NASA Goddard geodynamo model and ensemble Kalman filter (EnKF) with 512 ensemble members. Geomagnetic field models are used as observations for the assimilation, including gufm1 (1590-1960), CM4 (1961-2000) and CM6 (2001-2019). The forecast involves a bias correction scheme that assumes that the model bias changes on timescales much longer than the forecast period, so that they can be removed by successive forecasts. The algorithm was validated on the time period 2010-2015 by comparing with the 2015 IGRF before being applied to the 2020-2025 time period. This forecast has been submitted as a candidate model for IGRF 2025.

Elevated paleomagnetic dispersion at Saint Helena suggests long-lived anomalous behavior in the South Atlantic

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

Uncertainty in hourly mean data from classical magnetometers

Hourly mean values obtained from analog magnetometers in what can be considered the “classical” period constitute the largest quantity of data we have on the evolution of the Earth’s magnetic field. They are used for a wide variety of applications such as estimating long-term solar–terrestrial interactions, the production of magnetic indices, or studying geomagnetic secular variation originated in the Earth’s core. However, these data do not have an associated uncertainty that would allow us to quantify the final uncertainty of the results of these models. Hence, our study tries to assess the degree of uncertainty that these data actually have. In this paper, using Ebro Observatory classical instrumentation, we work out these uncertainties by estimating the particular uncertainties of each significant variable involved in the measuring procedure. Although the study is implemented for Ebro, the method can be applied to any other observatory. We found that, in general, uncertainties vary from one magnetic component to another, depending on the nature of the instruments that were used. In each component, we identified the weakest points where the biggest part of the error resides. With our results we can state that total uncertainties ranged from 1 to 4 nT.

Unblocking temperature of secondary magnetic component to outline anomaly thermomagnetic maps of archaeological fireplaces from the southern region of Mexico City, Mexico

We report a magnetic study of two preceramic hearths located at the southern region of Mexico City. Thermal demagnetization was applied between 200 to 540 °C to define the remanent magnetization components. The maximum unblocking temperature of the secondary magnetic component reached due to the last heating was identified in most samples. They were used to develop thermomagnetic anomaly source maps. The direction of this component was used to place the blocks to their last cooling spot and configurate a real and primal magnetic anomaly map produced by the thermoremanent magnetization acquired. A protocol to relocate blocks frame is proposed using geomagnetic secular variation curves. The thermomagnetic mapping revealed the position and temperature (220 to 460 °C) of the last heat sources. The distribution of these sources allowed to model an extensional zone of benefit and probable uses of the hearths.