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

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