## 2 Materials and Methods
Figure 2 shows the COSMIC-2 RO tangent point projections over the Ebre and El Arenosillo ionosondes' area of operation in the October 2022 and March 2024 periods, illustrating the RO-ionosonde colocation geometry. Figure 1: Example (from (**left**) to (**right**)) of the ground-based HF (ionosonde) sounding recording with results of the ionogram processing and ionosonde-derived electron density profile Ne(h). Figure 2: COSMIC-2 RO observations with EDP tangent point projections (pink lines) for 26 October 2022 and 19 March 2024. The blue dots corresponded to the F2 layer peak locations derived from COSMIC-2 RO profiles. Red and green triangles pinpoint the Ebre and El Arenosillo ionosonde locations, respectively. Our colocation criteria for the GNSS RO vs. ionosonde measurements are based on the spatial correlation factor of ionospheric plasma density variability. For the quiet-time midlatitude ionosphere, the correlation distance (correlation coefficient r > 0.70) can be considered as approximately 1500-3000 km in an east-west direction and 1000-1800 km in a north-south direction [44; 45]. At low and equatorial latitudes, this correlation distance is shorter due to significantly larger temporal and spatial variability of the equatorial ionosphere, and it can be considered as 1000-1500 km in an east-west direction and 500-1000 km in a north-south direction [45]. It is also necessary to note that ionosonde soundings are not vertical incidences only; they detect echoes from the ionosphere within a wider angle of \(\sim\)15\({}^{\circ}\) or more around the zenith, depending on the transmitting and receiving antenna patterns. This effect is also used for Directograms and Skymap Digisonde products, which estimate echo directions and plasma drift velocities [11]. Since the Ebre and El Arenosillo ionosondes are located close to southern midlatitudes, it is reasonable to select the maximal distance of \(\sim\)1000 km (10\({}^{\circ}\)) as the criterion for a reliable colocation between the two independent ionospheric soundings from the ground and from an LEO. For selected COSMIC GNSS RO EDPs, the tangent point coordinates of the F2 peak point should be within a 10\({}^{\circ}\) radius of the ionosonde location, and a clear ionogram should be available within a \(\pm\)5 min interval of the colocation time. With the most typical ionogram cadence of 15 min, the advantage of these high-rate ionosonde campaigns is that we can obtain more precise colocations in temporal domain. Generally, the COSMIC-2 RO-derived peak parameters, peak density NmF2, and peak height nmF2 are in good agreement with the true values, similar to the ones derived from ground-based measurements. However, some RO profiles can be affected by uncertainties resulting from equatorial ionization anomaly impacts on RO soundings, because the Abel inversion retrieval method relies on a spherical symmetry assumption, which is not always valid at low latitudes [46; 47; 48; 49]. To mitigate such issues, all COSMIC-2 RO electron density profiles were visually inspected in order to exclude potentially distorted or plasma-affected irregularities within the profiles. A couple of sample cases of COSMIC-2 RO electron density profiles, which were filtered out during quality control, are presented in Figure 3.  
Figure 3: Examples of COSMIC-2 RO electron density profiles distorted due to the impact of the equatorial ionization anomaly.