Generation International Geomagnetic Reference Field Research Articles

Non‐Geophysical Interhemispheric Asymmetries in Large Magnetic Field Residuals Between Swarm Observations and Earth Magnetic Field Models During Moderate to Quiet Geomagnetic Conditions

AbstractWe present a statistical study of large magnetic field vector residuals between Swarm observations and the 13th generation International Geomagnetic Reference Field (IGRF‐13) model under quiet to moderate geomagnetic conditions. Since the vast majority of data are taken during these conditions, statistics of these large residuals are important for satellite operation when using IGRF‐13 as reference, as well as for magnetosphere‐ionosphere‐thermosphere studies. Residuals with magnitude of vector differences between observations and model estimations larger than 300 nT under Dst > −50 nT are studied from 2014 to 2020. All large residuals appear in the high‐latitude auroral zone region peaking around 70° (−70°) magnetic latitude (MLAT) in the northern hemisphere (southern hemisphere) with also a secondary occurrence peak just below 80° (−80°) MLAT. However, the two hemispheres show clear asymmetries in the magnetic longitude and magnetic local time distribution where both hemispheres show high concentration of large residuals around the geographic poles. Since polar satellite's orbits give rise to highly biased number of observations around the geographic poles, it results in a decrease in occurrence rate with respect to the total number of measurements. We suggest that the asymmetries of large residuals in the two hemispheres around the geographic pole are not geophysical, but due to satellite orbit bias around the geographic poles and difference between IGRF and observations due to model error and geophysical processes not included in the model. Identifying these biases is helpful to better separate out geophysical electric current signatures from non‐geophysical ones.

Interhemispheric Asymmetries in Magnetosphere and Ionosphere Magnetic Field Residuals Between Swarm Observations and Earth Magnetic Field Models

AbstractWe present a statistical study of magnetic field vector residual between Swarm observations and two Earth magnetic field models: the 13th generation International Geomagnetic Reference Field (IGRF) model and the CHAOS‐7 model. Statistics of these residuals are important for estimating potential errors for satellite operations when using Earth magnetic models as a reference, as well as for magnetosphere‐ionosphere‐thermosphere studies examining energy input into the system. Magnetic field residuals are calculated as vector differences between observations and model estimation at Swarm satellite positions from 2014 to 2020. Magnetic field residuals for both models increase as geomagnetic activity level increases, and the largest magnitude of vector difference can be around 1800 nT with relatively small angle differences. The CHAOS‐7 model shows lower magnetic field residuals compared to IGRF‐13. North‐south hemispheric asymmetries are seen in magnetic field residuals for high Kp values larger than 6 with the southern hemisphere (SH) having more frequent occurrence of magnetic field residuals larger than 300 nT, especially during SH summer. Most large residual values appear in the high‐latitude region with SH seeing additional large residuals around the South Atlantic Anomaly region. Midnight and noon sectors show the strongest interhemispheric asymmetries. The northern hemisphere shows more frequent occurrence of large residuals above 75° magnetic latitude throughout all local times compared to the SH. Identifying asymmetries in large magnetic residuals under high geomagnetic activity levels is helpful for studying the difference in response to ionospheric disturbances in the two hemispheres.

The BGS candidate models for IGRF-13 with a retrospective analysis of IGRF-12 secular variation forecasts

The three candidate models submitted by the British Geological Survey for the 13th generation International Geomagnetic Reference Field are described. These DGRF and IGRF models are derived from vector and scalar magnetic field data from the European Space Agency Swarm satellites and ground observatories, covering the period 2013.9 to 2019.7. The internal field model has time dependence for degrees 1 to 15, represented by order 6 B-splines with knots at six monthly intervals. We also solve for a degree 1 external field time dependence describing annual and semi-annual signals with additional dependence on a bespoke Vector Magnetic Disturbance index. Satellite data are weighted by spatial density, along-track standard deviations, and a larger-scale noise estimator defined in terms of a measure of Local Area Vector Activity at the geographically closest magnetic observatories to the sampled datum. Forecasting of the magnetic field secular variation for 2020–2025 is by advection of the main field using steady core surface flows with steady acceleration applied. We also investigate the performance of the previous generation of candidate secular variation models, for IGRF-12, analysing the agreement of the candidates between 2015 and 2020 with the retrospective IGRF-13. We find that there is no clear distinction between the performance of mathematically and physically extrapolated forecasts in the period 2015–2020. We confirm that the methodology for the BGS IGRF-12 predictions performed well, despite observed secular accelerations that are highlighted by our analysis, and thus justify the methodology used for our IGRF-13 SV candidate.

NOAA/NCEI and University of Colorado candidate models for IGRF-13

The International Geomagnetic Reference Field (IGRF) is a set of parameters representing the large-scale internal part of Earth’s magnetic field. The 13th generation IGRF requested candidate models for a definitive main field for 2015.0, a provisional main field for 2020.0, and a predictive secular variation covering the period 2020.0–2025.0. The University of Colorado (CU) and the National Centers for Environmental Information (NCEI), part of the National Oceanic and Atmospheric Administration (NOAA), have produced these three candidate models for consideration in IGRF-13. In this paper, we present the methodology used to derive our candidate models. Our candidates were built primarily from Swarm satellite data, and also relied on geomagnetic indices derived from the ground observatory network. The ground observatories played a crucial role as independent data in validating our candidates. This paper also provides a retrospective assessment of the CU/NCEI candidate model to the previous IGRF (IGRF-12) and discusses the impact of differences between candidate and final IGRF models on global model errors.

NOAA/NGDC candidate models for the 12th generation International Geomagnetic Reference Field

The International Geomagnetic Reference Field (IGRF) is a model of the geomagnetic main field and its secular variation, produced every 5 years from candidate models proposed by a number of international research institutions. For this 12th generation IGRF, three candidate models were solicited: a main field model for the 2010.0 epoch, a main field model for the 2015.0 epoch, and the predicted secular variation for the five-year period 2015 to 2020. The National Geophysical Data Center (NGDC), part of the National Oceanic and Atmospheric Administration (NOAA), has produced three candidate models for consideration in IGRF-12. The 2010 main field candidate was produced from Challenging Minisatellite Payload (CHAMP) satellite data, while the 2015 main field and secular variation candidates were produced from Swarm and Ørsted satellite data. Careful data selection was performed to minimize the influence of magnetospheric and ionospheric fields. The secular variation predictions of our parent models, from which the candidate models were derived, have been validated against independent ground observatory data.

The BGS magnetic field candidate models for the 12th generation IGRF

We describe the candidate models submitted by the British Geological Survey for the 12th generation International Geomagnetic Reference Field. These models are extracted from a spherical harmonic ‘parent model’ derived from vector and scalar magnetic field data from satellite and observatory sources. These data cover the period 2009.0 to 2014.7 and include measurements from the recently launched European Space Agency (ESA) Swarm satellite constellation. The parent model’s internal field time dependence for degrees 1 to 13 is represented by order 6 B-splines with knots at yearly intervals. The parent model’s degree 1 external field time dependence is described by periodic functions for the annual and semi-annual signals and by dependence on the 20-min Vector Magnetic Disturbance index. Signals induced by these external fields are also parameterized. Satellite data are weighted by spatial density and by two different noise estimators: (a) by standard deviation along segments of the satellite track and (b) a larger-scale noise estimator defined in terms of a measure of vector activity at the geographically closest magnetic observatories to the sample point. Forecasting of the magnetic field secular variation beyond the span of data is by advection of the main field using core surface flows.

The BGS magnetic field candidate models for the 11th generation IGRF

We describe the British Geological Survey’s 11th generation International Geomagnetic Reference Field candidate models. These models are based on a ‘parent model’ consisting of a degree and order 60 spherical harmonic expansion of selected vector and scalar magnetic field data from satellite and observatory sources within the period 1999.0 to 2010.0. The parent model’s internal field time dependence for degrees 1 to 13 is represented by linear spline with knots 400 days apart. The parent model’s degree 1 external field time dependence is described by periodic functions for the annual and semi-annual signals, and by dependence on the 20-minute Vector Magnetic Disturbance index. Signals induced by these external fields are also parameterised. Satellite data are weighted according to two noise estimators. Firstly by standard deviation along segments of the satellite track and secondly a larger-scale noise estimator defined in terms of a vector activity measure at the geographically closest magnetic observatories to the sample point.

NOAA/NGDC candidate models for the 11th generation International Geomagnetic Reference Field and the concurrent release of the 6th generation Pomme magnetic model

The International Geomagnetic Reference Field (IGRF) is updated every five years based on candidate model submissions by research institutions worldwide. In the call for the 11th generation of IGRF, candidates were requested for the definitive main field in 2005, the predicted main field in 2010, and the predicted secular variation from 2010 to 2015. The NOAA/NGDC candidate models for IGRF-11 were produced from parent models parameterized in the same way as the 6th generation of our Pomme magnetic model. All models were based on CHAMP satellite measurements, while Orsted satellite measurements were used for model validation. The internal field in Pomme-6 is described by a 2nd degree Taylor time series of spherical harmonic expansion coefficients of a scalar magnetic potential. Magnetic fields of ionospheric origin are avoided by careful data selection. Instead of co-estimating magnetospheric fields, we subtract a magnetospheric field model estimated previously from a more extensive data set covering all local times. From comparison with Orsted measurements and general considerations of magnetic field predictability, we attribute a root mean square (RMS) uncertainty of 1.3 nT to our candidate model for the main field in 2005, 2.5 nT to the predicted main field in 2010 and 26 nT/a to the predicted secular variation from 2010 to 2015.

Spherical harmonic representation of the geomagnetic fields in the Nigeria sector of the Niger delta basin

A 0.5 x 0.5 representation of the geomagnetic elements and their rates of change have been predicted for the Nigeria sector of the Niger Delta basin using the spherical harmonic analysis. The geomagnetic elements computed are the total field (F), horizontal field (H), north component (X), east component (Y), vertical component (Z), declination (D), inclination (I) and their rates of change, D Z Y X H F ∆ ∆ ∆ ∆ ∆ ∆ , , , , , and I ∆ respectively. The geomagnetic elements were predicted using the latest IGRF-10 (10th Generation International Geomagnetic Reference Field) model which allows prediction of spot values of magnetic field. The spherical harmonic representation of the geomagnetic fields enables the regional field which has its origin from the outer core of the earth to be estimated. This approach of computing the regional fields is globally accepted and eliminates the problems that may arise when adjacent magnetic surveys are combined. It solves an additional problem of extreme interpolation of magnetic values due to lack of magnetic observatory in the Niger Delta. The highest total intensity for the region is 33431.0nT and the secular variation of the total field intensity is marked by decrease of 25.8nT/yr to 20.0nT/yr. The variations in geomagnetic values and the values computed for 2010.0 epochs reflect the current behavior of the geomagnetic field and the geodynamics of the earth’s interior under the region.

The BGS magnetic field candidate models for the 10th generation IGRF

Abstract In this paper we describe the derivation of the BGS candidate models for the 10th generation International Geomagnetic Reference Field. Our data set comprised quiet night-time data from the Èrsted and Champ satellites spanning 1999.2–2004.6 and observatory hourly means spanning 1999.0–2004.0. To improve the secular variation estimates for 2005.0–2010.0, predictions based on application of linear prediction filters to long series of observatory annual means were also used. These data were fitted by a spherical harmonic “parent” model with an internal field of maximum degree 36, a quadratic dependence on time up to degree 8, a linear dependence on time up to degree 12, an external field of maximum degree 2 with linear dependence on time, annual and semi-annual variations, and Dst dependence for degree 1 terms. Additionally for the external field, non-zonal degree 1 coefficients in the Geocentric Equatorial Inertial reference frame with annual variations and dependence on the Interplanetary Magnetic Field Y-component are included. The candidate models were then based, for the main field, on an extrapolation to 2005.0 of the truncated parent model, and for the secular variation, on its extrapolation to 2007.5. This latter set of coefficients was then used to generate a synthetic data set at the Earth’s surface and this set was augmented with long term linear predictions of observatory annual means, to produce the final candidate secular variation model at 2007.5.

NGDC/GFZ candidate models for the 10th generation International Geomagnetic Reference Field

Abstract Following the call for candidates for the 10th generation IGRF, we produced and submitted three main field and three secular variation candidate models. The candidates are derived from parent models which use a standard quadratic parameterisation in time of the internal Gauss coefficients. External magnetospheric fields are represented by combined parameterisations in Solar Magnetic (SM) and in Geocentric Solar Magnetospheric (GSM) coordinates. Apart from the daily and annual variations caused by these external fields, the model also accounts for induction by Earth rotation in a non-axial external field. The uncertainties of our candidates are estimated by comparing independent models from CHAMP and Èrsted data. The root mean square errors of our main field candidates, for the internal field to spherical harmonic degree 13, are estimated to be less than 8 nT at the Earth’s surface. Our secular variation candidates are estimated to have root mean square uncertainties of 12 nT per year. A hind-cast analysis of the geomagnetic field for earlier epochs shows that our secular acceleration estimates from post-2000 satellite data are inconsistent with pre-2000 acceleration in the field. This could confirm earlier reports of a jerk around 2000.0, with a genuine change in the secular acceleration.

The 10th generation international geomagnetic reference field

The International Association of Geomagnetism and Aeronomy (IAGA) on 12 December 2004 released the 10th generation International Geomagnetic Reference Field (IGRF)—the latest version of a standard mathematical description of the Earth's main magnetic field and used widely in studies of the Earth's deep interior, its crust, ionosphere and magnetosphere. The coefficients were finalised by a task force of IAGA. The IGRF is the product of a large collaborative effort between magnetic field modellers and the institutes involved in collecting and disseminating magnetic field data from satellites and observatories around the world.

10th Generation International Geomagnetic Reference Field

The International Association of Geomagnetism and Aeronomy (IAGA) released the 10th Generation International Geomagnetic Reference Field (IGRF) on 12 December 2004. This is the latest version of a standard mathematical description of the Earth's main magnetic field, and is used widely in studies of the Earth's deep interior, crust, ionosphere, and magnetosphere. The coefficients were finalized by a task force of IAGA, Division V, Working Group V‐MOD: Geomagnetic Field Modeling. The IGRF is the product of a large collaborative effort between magnetic field modelers and the institutes around the world involved in collecting and disseminating magnetic field data from satellites and observatories.

Ninth generation international geomagnetic reference field released

The coefficients for the new 9th Generation International Geomagnetic Reference Field (IGRF) were finalized at the XXIII General Assembly of the International Union of Geophysics and Geodesy (IUGG), held in Sapporo, Japan, in July 2003. The IGRF is widely used as a mathematical representation for the Earth's magnetic field in studies of the Earth's deep interior, crust, and ionosphere and magnetosphere. It is the product of a collaborative effort between magnetic field modelers and the institutes involved in collecting and disseminating magnetic field data from observatories and surveys around the world and from satellites.

The 9th Generation International Geomagnetic Reference Field

The 9th Generation International Geomagnetic Reference Field

International Geomagnetic Reference Field: The Sixth Generation.

In August 1991 the International Association of Geomagnetism and Aeronomy revised the International Geomagnetic Reference Field (IGRF) for the fifth time. This Sixth Generation IGRF now consists of 10 spherical harmonic models of the main geomagnetic field at five year intervals from 1945 through 1990 and a secular variation model extending the main field model of 1990 through 1995. For the interval 1945-1985 the models are definitive (DGRFs), in that it is unlikely that the underlying data sets will be significantly improved so there is no plan for future revision of the models. The present revision consists of adoption of a definitive main field model for 1985, DGRF 1985, a main field model for 1990 with secular variation coefficients for 1990-1995, IGRF 1990, and a provisional model, PGRF 1985, for 1985-1990 defined as the linear interpolation between DGRF 1985 and IGRF 1990. All main field models are of degree and order 10. The secular variation model is of degree and order 8. The models were derived from five candidate main field models at 1985, five at 1990, and three candidate secular variation models at epoch 1992.5. Weighted averages of the candidate models were specified to arrive at the final models. A brief description of the development of the IGRF is given, useful formulae are reviewed, and contour maps of the geomagnetic elements D, I, H, X, Y, Z, and F at 1990 are included. The nature of and philosophy behind the IGRF are discussed and a new procedure for deriving the IGRF is proposed.