Global maps of ground magnetometer data

A statistical-based method combined with basis function expansion techniques is described in order to provide extensive maps of the ground level perturbation magnetic field from 40 degrees magnetic latitude to the north magnetic pole for all longitudes. The method combines historical data from the SuperMAG data base, Principal Component Analysis, and a spherical cap harmonic basis function expansion in order to fill in magnetic perturbation data where there are no magnetometers and produce the poloidal current potential. The maps have a regular grid with a 2 degrees latitude and 1 h longitude spacing. The statistical process uses SuperMAG data derived magnetic indices plus the solar zenith angle which orders the resulting spatial maps by geomagnetic activity indicators to enhance model agreement with the data. For quiet through to moderate magnetic activity intervals, the root-mean-square error between the input and the fitted data are 18 nT and 10 nT for the north-south and east-west components, respectively, which are of similar magnitude to the statistical uncertainty in the SuperMAG data set.