Spherical cap harmonic analysis applied to regional field modelling for Italy

The technique of spherical cap harmonic analysis, due to HAINES (1985), has been applied to regional field modelling for the Italian area. The data sets used in the analyses include repeat station, magnetic survey, magnetic observatory, and Magsat observations. The spherical cap models derived used the residuals of these observations from the IGRF 1985 as input data. The models are thus essentially models of the crustal field. The truncation level was chosen to enable modelling of features with wavelengths greater than 330km approximately. Initial models were based on the ground data alone. The effect of supplementing the ground data set with Magsat observations was then investigated. The model proposed for adoption as a regional field model for Italy for 1985 is the IGRF 1985 combined with the spherical cap harmonic model derived using all the data sets. The performance of the spherical cap model is discussed in terms of the fit to the residual data, and in relation to previous regional field models for Italy.