Spherical cap harmonic modelling of high latitude magnetic activity and equivalent sources with sparse observations

The new method of spherical cap harmonic analysis is used for modelling solely high latitude magnetic activity and equivalent sources. Data from 13 Canadian magnetic observatories, during the 6–8 February 1986 great storm, are used to model the perturbation fields and their equivalent internal and external currents. The mean hourly values are used to reduce the spatial aliasing of short period variations. Both the spatial and temporal variations of the 3 components of the field are modelled. Only those coefficients that are statistically significant are retained in the analysis. The harmonic degree is less for the internal sources than the external ones, and the analyses are anisotropic to optimize the modelling and computation. The standard deviation of fit is less than 10 nT during less active periods of the storm, and 20–50 nT (~ 15%) during peaks in the activity (500–1300 nT). Errors range from 5 to 25 nT when both the spatial and temporal variations are modelled during the first day of activity (<360 nT). During the peak of the storm the equivalent ionospheric currents are 0.9 A m⁻¹ and they extend into southern Canada. Large scale features of both regular and irregular magnetic activity and equivalent sources, can be well modelled with sparse magnetic observations using this technique.