Left panel: foF1 trend (black) and related models obtained by applying a linear fit (red curve) and second-degree polynomial fit (green). Middle panel: Scatter plot of foF1 trend and related models obtained by applying a linear fit (red curve) and second-degree polynomial fit (green). The R² of the models is reported in the legend, while the black dashed line indicates the bisector. Right panel: Time profile of the ratio between foF1 trend and the regressed models, according to the same colour code of the previous panel. The related dashed lines indicate the linear fit evaluated for the two curves.