Evaluation of IRI predicted characteristics of ionospheric F1 layer by ionosonde observations in Nicosia, Cyprus

This paper presents an investigation of F1 layer characteristics derived from manually scaled digital ionosonde measurements at the low-middle latitude European station in Nicosia, Cyprus (geographical coordinates: 35°N, 33°E, geomagnetic lat. 29.38°N, I = 51.7°) from low to high solar activity conditions (2009-2014) and their comparison with IRI-2012 predictions. It assesses the predictability of occurrence probability by employing all three options included in IRI-2012: IRI-95, Scotto-97 no L, and Scotto-97 with L. Results show that Scotto-97 no L option slightly overestimates the occurrence probability but predicts better than IRI-95 option, whereas IRI-95 option closely reflects the time range for F1 layer appearance. The Scotto-97 with L option slightly underestimates the occurrence probability of foF1 + ledge type. A seasonal variation marked with higher occurrence probability in the equinoctial months than in winter is observed, both at low and high solar activity. IRI predictions match reasonably well observed values of foF1 with maximum 0.3 MHz over/under estimation at low solar activity. At high solar activity, IRI slightly overestimates the values of foF1 throughout the year with a varying degree from 0.3 to 1.3 MHz, except for the months of August and September, when the values are underestimated by 0.3 MHz. A seasonal variation with higher foF1 values during equinoctial months than that of summer and winter is noticed in the observations at high solar activity in 2014. Long-term solar variation in the observed foF1 values were evident. Deviation of IRI-2012 predictions from observed foF1 values increases at high solar activity. Observed values of hmF1 demonstrate no distinct diurnal variation. However, these are lower in winter than those of summer and equinoctial months. No significant long-term solar variation in hmF1 was detected during the entire period under consideration. Throughout 2009-2014, with a few exceptions, IRI generally underestimates hmF1 during January-April but overestimates during September-December. Over/underestimation goes as high as 23 km.