## 4 Discussion  
Having precise in-time colocations between the Ebre and El Arenosillo ionosondes with COSMIC-2 RO EDPs allows us to combine these two different techniques of ionosphere probing in order to obtain a set of reference EDPs that cover an latitudinal range from 100 to 500-550 km. In such cases, the combined electron density profiles are constructed on the basis of only real observations without any artificial extrapolation. Both ionosonde and COSMIC-2 RO measurements provide already proven high-accuracy measurements within their areas of operation. These combined \"best of the best\" EDPs from ionosondes and RO are being calibrated to the unbiased values of the F2 layer peak plasma density and can serve as an analogy for incoherent scatter radar-derived \"full profiles\" as references for the altitudinal distribution of ionospheric plasma density. Such EDPs definitely can be used for ionospheric model validation. Within this study framework, we examined this opportunity for the validation of the profile-based empirical International Reference Ionosphere (IRI) model. IRI is an empirical (data-based) model representing primary ionospheric parameters based on a long data record that exists from ground- and space-based observations of the Earth's ionosphere. The core model describes monthly averages of electron density, electron temperature, ion temperature, and ion composition globally in the altitude range from 60 to 2000 km. The IRI simulates the ionospheric vertical electron density profile by dividing it into six sub-regions, including the topside, the F2 bottomside, and the F1 layer. The F2 layer peak serves as a main anchor point for the F2 region. This means that the electron densities in the topside and bottomside areas are normalized to the F2-peak electron density NmF2. The full vertical electron density profile is described by a set of mathematical expressions, each valid in a defined range of altitude [54]. These expressions are described in the following sections starting from the topside down to the D region. All densities are in el/m\({}^{3}\), all frequencies are in MHz, and heights are in km [55,56]. Here, we used the most up-to-date version of the IRI model, IRI-2020, which was released in 2022 and is freely available for the research community ([[http://irimodel.org/](http://irimodel.org/)]([http://irimodel.org/](http://irimodel.org/))). The main input parameters of the IRI model are location, time, sunspot number or solar radio flux, and magnetic Ap/Kp index. We used the URSI coefficients to specify the F2 layer peak parameters and actual sunspot number and solar radio flux indices. The foF2 STORM model option was turned off because this study deals with the quiet geomagnetic conditions. The \"NeQuick topside\" option was used for the topside electron density profile specification. For modeling hmF2, we used the default option of the Shubin-COSMIC model, which was based on hmF2 data derived from ionosonde and COSMIC-1 RO observations [57]. Figure 7 illustrates comparison of the IRI-2020 calculated EDPs with the reference profiles combined from ionosonde and COSMIC-2 RO measurements during the selected night, day, and evening hours for 24-26 October 2022 and 19-20 March 2024. It is important to note that IRI is a climatological model. It provides predictions of the monthly average values of ionospheric electron densities, e.g., in the form of EDPs for a selected location and conditions. These profiles could not reflect day-by-day variability of the ionosphere. For some cases, IRI can show that its profile shape is in excellent agreement with other observations, like in Figure 7a, or with some discrepancies near the F2 peak, like in Figure 7d. Also, reasonable agreement between updated IRI-2020 profiles and real observation data was found during the daytime; IRI tends to slightly underestimate density around the F2 peak. Also, the model overestimates density around the F2 peak during the night hours and in the topside ionosphere.