## 1. Introduction

[2] Ionospheric models of the standard vertical incidence (VI) ionospheric characteristics oversimplify a number of the ionospheric phenomena of real significance for radio communications applications. Usually in these models, ionospheric parameters such as critical frequency of the ionospheric F2 layer foF2 and propagation factor M(3000)F2 represent monthly median values and vary as a function of geographic location, local time and solar activity. The Simplified Ionospheric Regional Model (SIRM) belongs to this group of ionospheric models and calculates the values of the key VI ionospheric characteristics such as foF2, M(3000)F2, virtual height of the ionospheric F2 layer h'F2, critical frequencies of the ionospheric F1 and E layers foF1 and foE that are used for prediction of operational parameters of HF telecommunication systems in a restricted area [*Zolesi et al.*, 1993, 1996]. The SIRM was developed under the COST (Cooperation in the field of Scientific and Technical Research) Action 238 PRIME (Prediction and Retrospective Ionospheric Modelling over Europe [*Bradley*, 1995]) and improved and tested under the COST Action 251 IITS (Improved Quality of Service in Ionospheric Telecommunication Systems Planning and Operation [*Hanbaba*, 1999]). For the purposes of this work, the improved version of SIRM is used [*Zolesi et al.*, 1999], based on the assumption that at constant local time there are no longitude changes of the ionospheric characteristics and that their diurnal and seasonal variations can be well represented by a Fourier expansion with a relative small number of numerical coefficients. The Fourier coefficients are coming from the analysis of the hourly monthly median values of the ionospheric characteristics measured at the midlatitudes stations over European region and collected under the COST Actions. The COST251 testing procedure consists in comparing measurements of all median hourly data available from a given set of ionospheric stations and the predicted values by different models. According to the results from this comparison, the overall root mean square (RMS) error from SIRM was slightly smaller than RMS error for ITU recommended model [*International Telecommunication Union-Radiocommunication* (*ITU-R*), 1994; *Levy et al.*, 1998]. This validation test proofs that SIRM performance is satisfactory for median ionospheric condition description in restricted area of midlatitudes.

[3] All the above indicate that the problem of having a realistic representation of the state of the ionosphere over Europe in real-time is still open. On the other hand, the SIRM provides an efficient and user-friendly software programme with a very simple mathematical formulation of the complex ionospheric media and reduced number of numerical coefficients involved. Therefore one could argue that SIRM, being one of the long term monthly median value models adopted by the scientific community, could be upgraded to a model that meets the need of nowcasting ionospheric conditions over Europe [*Zolesi et al.*, 2002]. In this paper, a real-time updating method of SIRM with autoscaled ionospheric parameters observed by four European digisondes currently capable to provide data in real-time mode has been studied. It is shown that the proposed method introduces a realistic nowcasting of the ionosphere over restricted Europe area, especially during strong geomagnetic storms that make the procedure even more relevant for HF propagation prediction. In the following sections, the methodology of SIRM real-time updating is described and then the simulation results are presented and discussed.