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.