Ionospheric mapping by regional spherical harmonic analysis: New developments

Abstract The method of Spherical Cap Harmonic Analysis ( SCHA ) /1/ has been applied /2/ to the critical frequency of the F 2 layer ( f 0 F 2 ) for mapping and modelling it over Europe. The model was based on longitudinal expansion in Fourier series, and fractional Legendre colatitudinal functions over a spherical cap including Europe. Here a new and simpler technique, previously developed for the regional modelling of the geomagnetic field /3/, is introduced and described. The basic improvement of the new method, called Adjusted Spherical Harmonic Analysis ( ASHA ), implies the use of conventional Spherical Harmonic ( SH ) functions after the colatitude interval is adjusted to that of a hemisphere. Examples are shown dealing with the application of ASHA to retrospective mapping and modelling of the monthly medians of f 0 F 2 over Europe.