## 4 Discussion
IRI-Plas overestimates vTEC, while NeQuick2 underestimates vTEC during high solar activity and overestimates it during local daytime for low and moderate solar activity (but not as much as IRI-Plas). Our results confirm that IRI-Plas (and Klobuchar model) overestimate TEC, while other studied models underestimate it. Several studies showed that IRI-Plas overestimates electron density in the topside ionosphere [40; 41]. The IRI-Plas significantly overestimate vertical TEC, especially during night-time and winter periods. IRI-2021 reproduced vertical TEC better [40]. Our results agree that IRI-Plas in general overestimate vertical TEC but disagree for the local time dependence; we observed the highest IRI-Plas errors during daytime (both relative and absolute). We should note that the IRI-Plas models were updated during this time and that actually, we used solar cycle 25 data, while [PERSON] et al. [40] considered solar cycle 24. We should note that IRI-2012 and IRI-2016 overperform IRI-Plas (with the parameters we choose) in both TEC and positioning domains. [PERSON] et al. [42] confirm our results and show that IRI-Plas-TEC values are significantly larger than those of daytime GIM-TEC at all locations of Russia and Kazakhstan and for all levels of solar activity. Nevertheless, IRI-Plas could underestimate 30% of TEC during the nighttime equinox [41]. Our results show that at least during nighttime and early morning hours, IRI-Plas performs almost the same way as other climatological empirical models. [PERSON] et al. [43] studied the Neustrelitz Global Model (similarly to GEMTEC the model is based on GIM CODG) and IRI-Plas; they mentioned that the IRI-Plas featured higher errors at low latitudes. We observed an increase in TEC error for all the models at the low latitudes, but the biggest errors are for the IRI-Plas model in the low-latitude region. [PERSON] et al. [44] suggested modifying Neustrelitz Model to use the Galileo broadcast model. Such a model also overperformed the Galileo operational NeQuickG model [44] and is comparable with NeQuickG without Galileo broadcast coefficients [45]. It agrees with our results for a similar model GEMTEC, but deep checking Neustrelitz Model should be studied further. [PERSON] et al. [46] studied the performance of the IRI-2012 model over Chinese mid-latitudes and found that the model underestimated nighttime TEC during both solar minima and solar maxima; it had difficulties in capturing the solar activity component of TEC and it underestimated annual and one-third annual periodic amplitude. We also confirm that when _F10.7_ increases the IRI-2012 and IRI-2016, models exhibit an increase in TEC errors larger than NeQuick2, GEMTEC, or IRI-Plas. The IRI-2012 model agrees with GPS-TEC at equatorial latitudes during nighttime quiet periods, but during the daytime, it underestimates GPS-TEC [47]. This fact could be related to an increase in the observed TEC error during the daytime (Figure 14). The performance of IRI models (at least for IRI-2001 and IRI-2007) decreases almost linearly when _F10.7_ increases and nonlinearly when _Kp_ increases [48]. Here, we showed the same _Kp_-pattern for all the models, but we did not analyze periods with high _Kp_. We should note that there is no significant error dependence on _Kp_ for the IRI-Plas model. While the TEC error increases linearly with _F10.7_ for all the models, for IRI-Plas, at low _F10.7_, we note higher errors. In the positioning domain, we observe a sharp increase in \(G\)3D even during slightly disturbed geomagnetic periods. That can be due to ionospheric irregularity that starts to play an important role [49]. In their pioneering paper, [PERSON] et al. [50] found that IRI models (IRI-90 and IRI-95) overestimated TEC at midlatitudes; however, [PERSON] et al. [51] showed that it is different at low latitudes where the IRI-2007 underestimated TEC. We should note that the IRI topside was improved significantly in the new-generation models [9]. [PERSON] et al.