The Simplified Ionospheric Regional Model (SIRM) for HF Prediction: Basic Theory, Its Evolution and Applications

This paper is a final review of the Simplified Ionospheric Regional Model (SIRM) devel-
oped as a prototype in the early 1990s and improved in the following years. By means of
an algorithm based on the Fourier synthesis, the SIRM model in its prototype version pro-
vides predicted monthly median values of the main ionospheric characteristics such as: the
ordinary wave critical frequencies (foE, foF1, and foF2) of the E, F1, and F2 ionospheric
layers; the lowest virtual height (h’F) of the ordinary trace of the F region; the obliquity
factor for a distance of 3000 km (M(3000)F2). Instead, the improved version focuses only
on foF2 and M(3000)F2. The SIRM model has been largely employed in the framework
of different international research projects as the climatological reference to output foF2
and M(3000)F2 monthly median predictions, but in its SIRMUP version is used also as
a nowcasting model and as an intermediate step of complex procedures for a near real-
time three-dimensional representation of the ionospheric electron density. In this regard,
some results provided by both SIRM and SIRMUP for telecommunication applications are
shown. Moreover, the mathematical treatment concerning both the phase correction of the
Fourier synthesis and the fundamental steps carried out to define the SIRM algorithm in its
final version, never published so far, will be described in detail in dedicated Appendices.
Finally, for the first time the SIRM code is now downloadable for the benefit of users.