Definitive Geomagnetic Reference Field Research Articles

Bootstrapping Swarm and observatory data to generate candidates for the DGRF and IGRF-13

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

Special issue “International Geomagnetic Reference Field—the twelfth generation”

This special issue of Earth, Planets and Space, synthesizes the efforts made during the construction of the twelfth generation of the International Geomagnetic Reference Field (IGRF-12) that was released online in December 2014 (http://www.ngdc.noaa.gov/IAGA/vmod/ igrf.html). The IGRF-12 is a series of standard mathematical models describing the large scale internal part of the Earth’s magnetic field between epochs 1900.0 and 2015.0 with a forecast to epoch 2020.0. This activity has been maintained since 1968 by a working group of volunteer scientists from several international institutions but grew out from discussions started in the early 1960s (Barton, 1997). The IGRF task force operates under the auspices of the International Association of Geomagnetism and Aeronomy/Association Internationale de Geomagnetisme et d’Aeronomie (IAGA/AIGA), which is one of the International Union of Geodesy and Geophysics/Union Internationale de Geodesie et Geophysique (IUGG/UIGG), an “international organization dedicated to advancing, promoting, and communicating knowledge of the Earth system, its space environment, and the dynamical processes causing change” (http://www.iugg.org/). The twelfth generation of IGRF models extends and updates the previous one (the IGRF-11, Finlay et al. 2010). It provides a new Definitive Geomagnetic Reference Field model for epoch 2010.0. It proposes a provisional reference field model for epoch 2015.0 and a predictive part for epochs ranging from 2015.0 to 2020.0 (Thebault et al. 2015a). These models were derived from candidate models submitted by 10 teams. The teams were led by the British Geological Survey (UK), DTU Space (Denmark), ISTerre (France), IZMIRAN (Russia), NOAA/NGDC (USA), GFZ Potsdam (Germany), NASA/GSFC (USA), IPGP (France), LPG Nantes (France), and ETH Zurich (Switzerland). Modelers made use of the data measured at ground geomagnetic observatories and built their models using satellite

Candidate main-field models for the Definitive Geomagnetic Reference Field 1995.0 and 2000.0

Abstract Two geomagnetic main-field models for epochs 1995.0 and 2000.0 were proposed as candidate models for DGRF 1995 and DGRF 2000. A main-field model was derived for epoch 2000.0, using the high-quality data provided by the Ørsted satellite around this epoch. Since no high-quality satellite vector measurements of the magnetic field were acquired between 1980 and 1999, our approach was to extrapolate this 2000.0 accurate model back to 1995. To do this we produced a secular-variation model for the time-span 1995-2000, from ground measurements. The models obtained were incorporated into DGRF 1995 and DGRF 2000 as part of the 9th generation of the IGRF in 2003.

Reliability of geomagnetic paleointensity data: the effects of the NRM fraction and concave-up behavior on paleointensity determinations by the Thellier method

To test the reliability of the Thellier method for paleointensity determinations, we studied six historic lavas from Hawaii and two Gauss-age lava flows from Raiatea Island (French Polynesia). Our aim is to investigate the effects of the NRM fraction and concave-up behavior of NRM–thermal remanent magnetization (TRM) diagrams on paleointensity determinations. For the Hawaiian samples, the paleointensity results were investigated at both sample and site levels. For consistency and confidence in the paleointensity results, it is important to measure multiple samples from each cooling unit. The results from the Raiatea Island samples confirm that reliable paleointensities can be obtained from NRM–TRM diagrams with concave-up curvature, provided the data are accompanied by successful partial TRM (pTRM) checks and no significant chemical remanent magnetization (CRM) production. We conclude that reliable determinations of the paleofield strength require analyses of linear segments representing at least 40–50% of the total NRM. This new criterion has to be considered for future studies and for evaluating published paleointensities for calculating average geomagnetic field models. Using this condition together with other commonly employed selection criteria, the observed mean site paleointensities are typically within 10% of the Definitive Geomagnetic Reference Field (DGRF). Our new results for the Hawaii 1960 lava flow are in excellent agreement with the expected value, in contrast to significant discrepancies observed in some earlier studies. Overestimates of paleointensity determinations can arise from cooling-rate dependence of TRM acquisition, viscous remanent magnetization (VRM) at elevated temperatures, and TRM properties of multidomain (MD) particles. These outcomes are exaggerated at lower temperature ranges. Therefore, we suggest that, provided the pTRM checks are successful and there is no significant CRM production, it is better to increase the NRM fraction used in paleointensity analyses rather than to maximize correlation coefficients of line segments on the NRM–TRM diagrams. We introduce the factor, Q = N〈 q〉, to assess the quality of the weighted mean paleointensity, H w, for each cooling unit.

Geomagnetic effects on time-integrated cosmogenic nuclide production with emphasis on in situ 14C and 10Be

Production of cosmogenic nuclides (CNs) in geologic material is a function of the cosmic-ray flux at the Earth's surface, which in turn is a function of the intensity and orientation of the Earth's geomagnetic field. Temporal variations in the intensity of the geomagnetic field and the position of the geomagnetic dipole axis (i.e., polar wander) must be considered when calculating production rates that are integrated through time. We have developed a model, based in part on protocols set forth by Desilets and Zreda [Earth Planet. Sci. Lett. 206 (2003) 21–42], that accounts for these variations in an effort to systematically determine their impact on time-integrated production of short-lived (in situ 14C; t1/2=5.73 ka) and long-lived (in situ 10Be; t1/2=1.5 Ma) CNs. Our modeling results show that for samples exposed for the last 3 ka, integrated in situ 14C and 10Be production rates that account for temporal variations in the intensity of the Earth's geomagnetic field are up to ∼13% lower than modern rates at the same location [modern rates are referenced to the 1945.0 Definitive Geomagnetic Reference Field (DGRF)]. In contrast, intensity-corrected 10Be rates are up to ∼30% higher than modern for samples exposed for >25–30 ka. Intensity variations have little effect (<5%) on integrated CN production for samples exposed for the last 15–20 ka, regardless of site location or nuclide used.Our modeling results also show that the impact of polar wander on integrated CN production is secondary compared to intensity variations. Accounting for polar wander is critical, however, when determining modern production rates at midlatitudes (30–40°) because of the current offset between the geomagnetic and geographic poles. At sea level, integrated in situ 14C production rates that account for both intensity variations and polar wander range from 27% higher to 24% lower than modern rates at the same location, and integrated in situ 10Be rates range from 48% higher to 26% lower than modern. Differences between integrated and modern rates increase significantly at higher altitudes. Based on these results, we recommend correcting the modern production rate (referenced to the 1945.0 DGRF or another specific geomagnetic reference field) at site latitudes <60° for variations in the intensity of the Earth's geomagnetic field during exposure and for polar wander over the last 10 ka.

The 1995 Revision of the Joint US/UK Geomagnetic Field Models I. Secular Variation

We present the methods used to derive mathematical models of global secular variation of the main geomagnetic field for the period 1985 to 2000. These secular-variation models are used in the construction of the candidate US/UK models for the Definitive Geomagnetic Reference Field at 1990, the International Geomagnetic Reference Field for 1995 to 2000, and the World Magnetic Model for 1995 to 2000 (see paper II, Quinn et al., 1997). The main sources of data for the secular-variation models are geomagnetic observatories and repeat stations. Over the areas devoid of these data secular-variation information is extracted from aeromagnetic and satellite data. We describe how secular variation is predicted up to the year 2000 at the observatories and repeat stations, how the aeromagnetic and satellite data are used, and how all the data are combined to produce the required models.

Time‐varying fluid flow at the top of Earth's core derived from definitive geomagnetic reference field models

The effects of a time‐varying fluid velocity have been included in the estimation of frozen‐flux core surface flow. This has been done to improve the quality, performance, and accuracy of kinematic analyses of the geomagnetic secular variation. The derived flows consist of a reference velocity plus a steady acceleration and thus evolve linearly with time. The method features simultaneous solution for the initial radial geomagnetic field component at the core‐mantle boundary and the subadjacent, time‐varying fluid motion. The trade‐off between time‐averaged spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models for 1945–1980 is explored. At fixed flow complexity, simultaneous solution for both initial geomagnetic field and steadily accelerating flow gives substantial reductions in the square‐weighted residuals per degree of freedom when compared with previous solutions for either initial field and steady flow or steady flow alone. The apparent timescale for flow change varies nonmonotonically with flow complexity between about 23 and 41 years; the single maximum value is associated with unit negative slope of the log‐log trade‐off curve and is used to help identify a preferred solution. This solution has an rms speed of but 7.50 km yr−1 and an rms acceleration of 0.183 km yr−2, or 1.84×10−13 m s−2. The latter indicates extremely weak net forces upon the liquid outer core.

Simultaneous solution for core magnetic field and fluid flow beneath an electrically conducting mantle

The effects of laterally homogeneous mantle electrical conductivity have been included in steady, frozen‐flux core surface flow estimation along with refinements in method and weighting. The refined method allows simultaneous solution for both the initial radial geomagnetic field component at the core‐mantle boundary and the subadjacent fluid motion; it also features Gauss' method for solving the nonlinear inverse problem associated with steady motional induction. The trade‐off between spatial complexity of the derived flows and misfit to the weighted Definitive Geomagnetic Reference Field models is studied for various mantle conductivity profiles. For simple flow and a fixed initial geomagnetic condition a fairly high deep‐mantle conductivity performs better than either insulating or weakly conducting profiles; however, a thin, very high conductivity layer at the base of the mantle performs almost as well. Simultaneous solution for both initial geomagnetic field and fluid flow reduces the misfit per degree of freedom even more than does changing the mantle conductivity profile. Moreover, when both core field and flow are estimated, the performance of the solutions and the derived flows become insensitive to the conductivity profile.

A Comparative Analysis and Evaluation of the DGRF-85 and IGRF-90 Candidate Models.

Candidate models for the Definitive Geomagnetic Reference Field (DGRF) main field model at the 1985.0 epoch, as well as candidate models for the International Geomagnetic Reference Field (IGRF) main field model at the 1990.0 epoch along with the corresponding secular variation models at the 1992.5 epoch were evaluated against each other, and against data observations from both Project MAGNET aeromagnetic data and recently obtained Polar Orbiting Geomagnetic Survey (POGS) satellite data. Models for the 1985.0 epoch show some regional differences in model by model comparisons but, in general, are quite similar.

Analysis of Geomagnetic Secular Variation during 1980-1985 and 1985-1990, and Geomagnetic Models Proposed for the 1991 Revision of the International Geomagnetic Reference Field.

The secular variation of the main geomagnetic field during the periods 1980-1985 and 1985-1990 was analyzed in terms of spherical harmonics up to the eighth degree and order. Data from worldwide magnetic observatories and the Navy's Project MAGNET aerial surveys were used. The resulting pair of secular-variation models was used to update the Definitive Geomagnetic Reference Field (DGRF) model for 1980, resulting in new main-field models for 1985.0 and 1990.0. These, along with the secular-variation model for 1985-1990, were proposed for the 1991 revision of the International Geomagnetic Reference Field (IGRF).

US/UK Candidates for the Definitive Geomagnetic Reference Field Model DGRF-85 and the Predictive International Geomagnetic Reference Field Model IGRF-90.

Candidate models for the Definitive Geomagnetic Reference Field 1985 (DGRF-85) and the 1990 International Geomagnetic Reference Field (IGRF-90) have been produced jointly by the British Geological Survey and the United States Naval Oceanographic Office as part of their respective World Magnetic Modeling and Charting programs which are sponsored by the British Hydrographic Office and by the United States Defense Mapping Agency. These models are based on spherical harmonic expansions of the geomagnetic field to degree 10 (120 coefficients) for the main field and degree 8 (80 coefficients) for the secular variation. They are presented here along with a discussion of the data and modeling techniques that were used to produce them.

IGRF commercialization defeated, planetary studies probed by IAGA

Sessions of the International Association of Geomagnetism and Aeronomy (IAGA) at the XX International Union of Geodesy and Geophysics meeting in Vienna, Austria (August 11–24), lasted for nearly the full 2 weeks of the conference. Once again, the program offered a vast array of scientific topics ranging from the outer limits of the heliosphere to middle atmospheric studies to motions within the core of the Earth. This report tries to capture the flavor of the meeting by describing in detail a few of the sessions in a number of different disciplines.One of the most controversial occurrences during the meeting was the attempt to get IAGA to agree to commercialize the International Geomagnetic Reference Field (IGRF). Although the idea behind this commercial ization, which would generate funds to install and operate geomagnetic observatories in third world nations, was a worthy and excellent aim, scientists at the meeting raised many concerns about the details of commercialization. Eventually, the motion to commercialize was defeated. The next version of the IGRF, consisting of a Definitive Geomagnetic Reference Field (DGRF) for 1985, an IGRF for 1990, and a secular variation model for 1990–1995 was agreed to at the meeting. The coefficients for these models will be available soon.

Geomagnetic secular variation over Spain 1970–1988 by means of spherical cap harmonic analysis

A spherical cap harmonic analysis was applied to Spain and neighbouring areas, covering a region with a cap halfangle of 16°. The technique, due to Haines, has allowed us to obtain a model of secular variation (SV) for a time interval of 18 years reduced to the epoch 1987.5. The total number of selected data points was 1581 provided from ten observatories and 60 repeat stations. The SV model has a maximum spatial index of four and temporal degree of two. The coefficients were tested and the statistically non-significant terms removed. Another model covering a smaller region with greater density of data has been developed and both results compared. The average secular variations derived from the definitive geomagnetic reference field (DGRF) and the international geomagnetic reference field (IGRF) were compared with these models and maps plotting the calculated first field derivatives are presented. The integration of the final model will serve to update satellite data in order to produce a regional main field model.

Evaluations of candidate DGRF 1945, 1950, 1955 and 1960 models

The candidate models for the definitive geomagnetic reference field (DGRF) for the years 1945, 1950, 1955 and 1960—S45, 50, 55, 60, SFA45, 50, 55, 60, SFAS45, 50, 55, 60, and BGSMF50, 60—are evaluated using magnetic observatory data and observatory anomaly solutions given by the CSFC(9/80) and GSFC(12/83) models. The results indicate that the series of SFA models is the most successful series of models, and the BGSMF60 model is the best individual model, but the BGSMF50 model is less successful.

Assessment of models proposed for the 1985 revision of the international geomagnetic reference field

Geomagnetic measurements from land, marine and aerial surveys conducted in the years 1945–1964 were used to test the 14 models proposed as additions, for that period, to the series of definitive geomagnetic reference field (DGRF) models. Overall, NASA's ‘SFAS’ models and the BGS (British Geological Survey) models agree best with these data. Comparisons of the two proposed definitive main-field models for 1980.0, with each other and with the existing IGRF 1980 main-field model, show mostly close agreement, with the greatest absolute differences (several tens of nanotesla) occurring in the region of Antarctica. Comparison of the the three proposed forecast secular-variation models for 1985–1990 with estimates of recent rates of change at 148 magnetic observatories shows that the IZMIRAN (U.S.S.R.) and USGS models are in closest agreement with these data.

Cubic approximations of definitive geomagnetic reference field models

The definitive geomagnetic reference field (DGRF) models for 1965, 1970, and 1975 and the international geomagnetic reference field (IGRF) model for 1980 with linear interpolation between the four epochs is defined as the DGRF linear model. It has 480 spherical harmonic coefficients. This model contains a piecemeal linear representation of coefficients, and hence of the field itself, that results in discontinuities in the secular variation which may lead to difficulties if the model is used at different epochs to subtract values from observations to obtain anomaly fields. A new DGRF cubic model is defined by the coefficients of a cubic curve for the time dependence of each spherical harmonic coefficient. It exactly fits the DGRF linear model coefficients at 1965, 1970, 1975, and 1980 and has continuous first, second, and third derivatives throughout the interval 1965–1980. This DGRF cubic model also contains 480 coefficients which can replace the 480 coefficients of the DGRF linear model. Differences in field components computed using the two models for epochs between the key epochs of the DGRF linear model are considerable, amounting to over 60 nT in 1977.5 for the Z component. No significant differences were found in the comparison of the two models with observatory data. This comparison is difficult because of observatory anomaly biases which appear to be variable with time because of observatory measurement errors or inadequate models. Because the DGRF cubic model provides a model of continuous secular variation, it is recommended that it replace the DGRF linear model.