*4.1. Slant Total Electron Content Analysis*

The computed sTEC, obtained from the NEDM2020 and NeQuick 2 models, serves as the foundational parameter for the subsequent derivations of the relative ionospheric group delay and Doppler shift. The assessment of the sTEC is presented across different grazing elevation ranges: 5◦–10◦ (very low), 10◦–20◦ (low), and 20◦–30◦(mid-low), along with the distinct regions of north, tropics, and south. The outcomes of the NEDM2020 and NeQuick 2 sTEC computations during LSA are depicted in Figures 7 and 8, respectively. While discrepancies of up to ~60 TECUs between the two models are noticeable in the tropics region at very low angles for the direct ray, both models consistently demonstrate similar behavior across all analyzed scenarios.

**Figure 7.** sTEC obtained from NEDM2022 model. Color-coded according to the direct (dr), incident (in), and reflected (re) rays.

**Figure 8.** sTEC obtained from NeQuick 2 model. Color-coded according to the direct (dr), incident (in), and reflected (re) rays.

The highest sTEC is prominently observed at elevation angles ranging from 5◦ to 10◦ within the tropics region, and to a lesser extent in polar regions, but with lower magnitudes, specifically for the direct ray. This behavior occurs because, at such elevation angles, the direct ray traverses a longer path through the ionosphere than the incident and reflected rays. This effect diminishes as the elevation angle increases. At low elevations, the magnitudes of the sTEC are relatively similar for each ray, while at mid-low elevations, the contribution of the incident and reflected rays becomes more prominent in comparison to the direct ray.

Across all scenarios, local time, representing solar radiation, plays a pivotal role in sTEC retrievals. Figures 7 and 8 illustrate how the sTEC values exhibit a progressive increase as the noon-time period approaches, with the highest peaks occurring between 12:00 and 13:00 h. Following sunset, the electron density and consequently the sTEC values gradually decrease accordingly.

Table 3 provides a comparative analysis of both models, presenting the mean and standard deviation values for each ray in the distinct regions. To facilitate interpretation in terms of local time, the events have been categorized into two distinct periods: daytime (DT), spanning from 06:00 to 18:00, and nighttime (NT), encompassing the interval from 18:00 to 06:00. Notably, the range of sTEC magnitude for the direct ray is broader for the NeQuick model computations in the tropics region. However, the NEDM2020 computations consistently yield a higher mean sTEC in most cases except for the direct ray at very low elevations in the tropics during daytime. Particularly higher differences in mean values between the two models are evident in the south region (~6 TECUs), while comparatively smaller differences are observed in the tropics region (~2 TECUs).

**Table 3.** sTEC mean and standard deviation value comparison between NEDM2020 and NeQuick 2 models for F10.7 = 75.

