Distribution of N (left) and ΔN (right) as a function of the altitude from the solar surface. Green and red lines indicate the measured N_e obtained through observations with Grueff at 18.3 GHz (green) and 25.8 GHz (red) during a period of minimum solar activity (2018–2020), respectively (see the legend of the right panel in Fig. 7). The shaded area indicates the 1σ error of the distributions. Blue lines indicate the measured N_e obtained through observations with Metis (dashed line: equatorial component; dotted line: polar component) during the ascending phase of the solar cycle (March 2022). The black line indicates the theoretical N_e associated with the original eSSC model. The y-axis of these plots is shown in logarithmic scale. In the right panel, the difference between ΔN_e referred to Metis data is reported in terms of absolute value.