1 23 Surveys in Geophysics An International Review Journal Covering the Entire Field of Earth and Space Sciences ISSN 0169-3298 Surv Geophys DOI 10.1007/s10712-020-09600-w The Simplified Ionospheric Regional Model (SIRM) for HF Prediction: Basic Theory, Its Evolution and Applications M. Pietrella, M. Pezzopane, B. Zolesi, Lj. R. Cander & A. Pignalberi