A short-term ionospheric forecasting empirical regional model (IFERM) to predict the critical frequency of the F2 layer during moderate, disturbed, and very disturbed geomagnetic conditions over the European area

Abstract. A short-term ionospheric forecasting empirical regional model (IFERM) has been developed to predict the state of the critical frequency of the F2 layer ( fo F2) under different geomagnetic conditions. IFERM is based on 13 short term ionospheric forecasting empirical local models (IFELM) developed to predict fo F2 at 13 ionospheric observatories scattered around the European area. The forecasting procedures were developed by taking into account, hourly measurements of fo F2, hourly quiet-time reference values of fo F2 ( fo F2 QT ), and the hourly time-weighted accumulation series derived from the geomagnetic planetary index ap, (ap(τ)), for each observatory. Under the assumption that the ionospheric disturbance index ln( fo F2/ fo F2 QT ) is correlated to the integrated geomagnetic disturbance index ap(τ), a set of statistically significant regression coefficients were established for each observatory, over 12 months, over 24 h, and under 3 different ranges of geomagnetic activity. This data was then used as input to compute short-term ionospheric forecasting of fo F2 at the 13 local stations under consideration. The empirical storm-time ionospheric correction model (STORM) was used to predict fo F2 in two different ways: scaling both the hourly median prediction provided by IRI (STORM_ fo F2 MED,IRI model), and the fo F2 QT values (STORM_ fo F2 QT model) from each local station. The comparison between the performance of STORM_ fo F2 MED,IRI , STORM_ fo F2 QT , IFELM, and the fo F2 QT values, was made on the basis of root mean square deviation (r.m.s.) for a large number of periods characterized by moderate, disturbed, and very disturbed geomagnetic activity. The results showed that the 13 IFELM perform much better than STORM_ fo F2,sub>MED,IRI and STORM_ fo F2 QT especially in the eastern part of the European area during the summer months (May, June, July, and August) and equinoctial months (March, April, September, and October) under disturbed and very disturbed geomagnetic conditions, respectively. The performance of IFELM is also very good in the western and central part of the Europe during the summer months under disturbed geomagnetic conditions. STORM_ fo F2 MED,IRI performs particularly well in central Europe during the equinoctial months under moderate geomagnetic conditions and during the summer months under very disturbed geomagnetic conditions. The forecasting maps generated by IFERM on the basis of the results provided by the 13 IFELM, show very large areas located at middle-high and high latitudes where the fo F2 predictions quite faithfully match the fo F2 measurements, and consequently IFERM can be used for generating short-term forecasting maps of fo F2 (up to 3 h ahead) over the European area.