Ionospheric mapping by regional spherical harmonic analysis: New developments

The method of Spherical Cap Harmonic Analysis (SCHA) /1/ has been applied /2/ to the critical frequency of the F₂ layer (f₀F₂) 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 f₀F₂ over Europe.