Abstract
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.
Highlights
The International Geomagnetic Reference Field (IGRF) is a set of spherical harmonic models of the Earth’s main magnetic field updated every 5 years under the auspices of the International Association of Geomagnetism and Aeronomy (IAGA)
We confirm that the methodology for the British Geological Survey (BGS) IGRF-12 predictions performed well, despite observed secular accelerations that are highlighted by our analysis, and justify the methodology used for our IGRF-13 secular variation (SV) candidate
Retrospective analysis of IGRF‐12 secular variation candidates we examine the forecasts from the IGRF-12 SV candidate models (Thébault et al 2015b) to investigate whether specific strategies provided a better method for determining magnetic field change over five years
Summary
The International Geomagnetic Reference Field (IGRF) is a set of spherical harmonic models of the Earth’s main magnetic field updated every 5 years under the auspices of the International Association of Geomagnetism and Aeronomy (IAGA). Every year prior to the release of the generation, an open invitation is made for the submission of candidate models from the international community. These models are evaluated by a task force and the set of IGRF models are constructed from the candidates. Field modelling within BGS has matured since the release of the IGRF-12 candidate models of Hamilton et al (2015), Brown et al Earth, Planets and Space (2021) 73:42 with higher temporal and spatial resolution of the secular variation using order 6 B-splines, and the improvement of core flow advection using steady flow and steady flow acceleration to predict the secular variation. The underlying code base has been modernised and modularised, allowing massively parallel inverse solvers such as PETSc (Balay et al 1997) and SLEPc (Hernandez et al 2005) to be used (see Brown et al 2020)
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