Ionospheric F3 layer: Implications for the IRI model Original Research Article

Recent studies have revealed the existence of an additional layer in the topside equatorial ionosphere, which was named the F3 layer. Statistical studies using Fortaleza ionograms show that the F3 layer is present on 75% of the days during December solstices (local summer) of low solar activity period, becoming a very common feature on that location. During those events, the F peak density becomes smaller than its usual value and the peak height can be much higher than the usual hmax value. Vertical profiles obtained from ionograms, using true height inversion methods, show the existence of a very broad F layer peak, with a very slow height gradient. In this work we compare the observed F layer critical frequency and peak height with their IRI predictions (calculated using the URSI and CCIR coefficients). It was found that the IRI model overestimates the observed frequency by up to 3 MHz and underestimates the peak height by up to 100 km, when the F3 layer phenomena is present. The vertical distribution of the F region electron density calculated by IRI also differs significantly from the observations during periods of F3 layer occurrence. It is suggested that this phenomenon could account for some of the discrepancies between IRI predictions and observations, at least for daytime low-latitude locations.