Regional spherical modeling of 2-D functions: the case of the critical frequency of the F 2 ionospheric layer

Abstract In this paper it is shown that Adjusted Spherical Harmonic Analysis (ASHA), previously used for modeling the three-dimensional (3-D) geomagnetic field in a restricted area can be adapted to model general bidimensional (2-D) spherical functions, f(γ, θ). As an example of application the case of the critical frequency of the F 2 ionospheric layer, f 0 F 2 is described. By assuming that, at a fixed epoch, the monthly median value of f 0 F 2 is a function only of the geographic longitude γ and colatitude θ, that is f 0 F 2 = f ( γ , θ ), ASHA has been applied to modeling and mapping this ionospheric parameter over Europe. Here, the FORTRAN-77 computer programs and subprograms are presented enabling the practical and easy use of the ASHA technique to obtain, as a final output, either a grid (2 × 2 degrees) of the computed monthly medians of f 0 F 2 in the European area and the calculated value of the parameter at one point, in the region of interest, as a function of time. The same codes can be adapted easily to be used for modeling any bidimensional function defined over a spherical portion of the Earth.